Introduction to Java programming, Part 1: Java language basics

Object-oriented programming on the Java platform

This two-part tutorial introduces the structure, syntax, and programming paradigm of the Java™ language and platform. You'll learn the Java syntax you are most likely to encounter professionally and Java programming idioms you can use to build robust, maintainable Java applications. In Part 1, J. Steven Perry guides you through the essentials of object-oriented programming on the Java platform, including fundamental Java syntax and its use. You'll get started with creating Java objects and adding behavior to them, and conclude with an introduction to the Java Collections Framework, with considerable ground covered in between.

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J Steven Perry (steve.perry@makotoconsulting.com), Principal Consultant, Makoto Consulting Group, Inc.

Photo of J Steven PerryJ. Steven Perry is a software developer, architect, and general Java nut who has been developing software professionally since 1991. His professional interests range from the inner workings of the JVM to UML modeling and everything in between. Steve has a passion for writing and mentoring; he is the author of Java Management Extensions (O'Reilly), Log4j (O'Reilly), and the IBM developerWorks articles "Joda-Time" and OpenID for Java Web applications." In his spare time, he hangs out with his three kids, rides his bike, and teaches yoga.



19 August 2010

Also available in Portuguese Spanish

Before you begin

Find out what to expect from this tutorial and how to get the most out of it.

About this tutorial

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This content is part of a progressive knowledge path for advancing your skills. See Become a Java developer

The two-part "Introduction to Java programming" tutorial is intended to get software developers who are new to Java technology up and running with object-oriented programming (OOP) and real-world application development using the Java language and platform.

This first part is a step-by-step introduction to OOP using the Java language. The tutorial begins with an overview of the Java platform and language and is followed by instructions for setting up a development environment consisting of a Java Development Kit (JDK) and the Eclipse IDE. Once you have been introduced to your development environment's components, you will begin learning basic Java syntax hands-on.

Part 2 covers more-advanced language features, including regular expressions, generics, I/O, and serialization. Programming examples in Part 2 build on the Person object that you begin developing in Part 1.

Objectives

When you've finished Part 1, you will be familiar with basic Java language syntax and able to write simple Java programs. You should follow up with "Introduction to Java programming, Part 2: Constructs for real-world applications" to build on this foundation.

Prerequisites

This tutorial is for software developers who are not yet experienced with Java code or the Java platform. The tutorial includes an overview of OOP concepts.

System requirements

To complete the exercises in this tutorial, install and set up a development environment consisting of:

  • JDK 6 from Sun/Oracle.
  • Eclipse IDE for Java Developers.

Download and installation instructions for both are included in the tutorial.

The recommended system configuration is:

  • A system supporting Java SE 6 with at least 1GB of main memory. Java 6 is supported on Linux®, Windows®, and Solaris®.
  • At least 20MB of disk space to install the software components and examples.

Java platform overview

Java technology is used to develop applications for a wide range of environments, from consumer devices to heterogeneous enterprise systems. In this section, get a high-level view of the Java platform and its components. See Resources to learn more about the Java platform components discussed in this section.

The Java language

Get to know the Java APIs

Most Java developers constantly reference the official online Java API documentation — also called the Javadoc (see Resources). By default, you see three frames in the Javadoc. The top-left frame shows all of the packages in the API, and beneath that are the classes in each package. The main frame (to the right) shows details for the currently selected package or class. For example, if you select the java.util package in the top-left frame and then select the ArrayList class listed below it, in the right-hand frame you will see details about ArrayList, including a description of what it does, how to use it, and its methods.

Like any programming language, the Java language has its own structure, syntax rules, and programming paradigm. The Java language's programming paradigm is based on the concept of object-oriented programming (OOP), which the language's features support.

The Java language is a C-language derivative, so its syntax rules look much like C's: for example, code blocks are modularized into methods and delimited by braces ({ and }), and variables are declared before they are used.

Structurally, the Java language starts with packages. A package is the Java language's namespace mechanism. Within packages are classes, and within classes are methods, variables, constants, and so on. You'll learn about the parts of the Java language in this tutorial.

The Java compiler

When you program for the Java platform, you write source code in .java files and then compile them. The compiler checks your code against the language's syntax rules, then writes out bytecodes in .class files. Bytecodes are standard instructions targeted to run on a Java virtual machine (JVM). In adding this level of abstraction, the Java compiler differs from other language compilers, which write out instructions suitable for the CPU chipset the program will run on.

The JVM

At run time, the JVM reads and interprets .class files and executes the program's instructions on the native hardware platform for which the JVM was written. The JVM interprets the bytecodes just as a CPU would interpret assembly-language instructions. The difference is that the JVM is a piece of software written specifically for a particular platform. The JVM is the heart of the Java language's "write-once, run-anywhere" principle. Your code can run on any chipset for which a suitable JVM implementation is available. JVMs are available for major platforms like Linux and Windows, and subsets of the Java language have been implemented in JVMs for mobile phones and hobbyist chips.

The garbage collector

Rather than forcing you to keep up with memory allocation (or use a third-party library to do this), the Java platform provides memory management out of the box. When your Java application creates an object instance at run time, the JVM automatically allocates memory space for that object from the heap, which is a pool of memory set aside for your program to use. The Java garbage collector runs in the background, keeping track of which objects the application no longer needs and reclaiming memory from them. This approach to memory handling is called implicit memory management because it doesn't require you to write any memory-handling code. Garbage collection is one of the essential features of Java platform performance.

The Java Development Kit

When you download a Java Development Kit (JDK), you get — in addition to the compiler and other tools — a complete class library of prebuilt utilities that help you accomplish just about any task common to application development. The best way to get an idea of the scope of the JDK packages and libraries is to check out the JDK API documentation (see Resources).

The Java Runtime Environment

The Java Runtime Environment (JRE; also known as the Java runtime) includes the JVM, code libraries, and components that are necessary for running programs written in the Java language. It is available for multiple platforms. You can freely redistribute the JRE with your applications, according to the terms of the JRE license, to give the application's users a platform on which to run your software. The JRE is included in the JDK.


Setting up your Java development environment

In this section, you'll get instructions for downloading and installing JDK 6 and the current release of the Eclipse IDE, and for setting up your Eclipse development environment.

If you already have the JDK and Eclipse IDE installed, you may want to skip to the Getting started with Eclipse section or to the one after that, Object-oriented programming concepts.

Your development environment

The JDK includes a set of command-line tools for compiling and running your Java code, including a complete copy of the JRE. Although you certainly can use these tools to develop your applications, most developers appreciate the additional functionality, task management, and visual interface of an IDE.

Eclipse is a popular open source IDE for Java development. It handles basic tasks, such as code compilation and setting up a debugging environment, so that you can focus on writing and testing code. In addition, you can use Eclipse to organize source code files into projects, compile and test those projects, and store project files in any number of source repositories. You need an installed JDK in order to use Eclipse for Java development.

Install JDK 6

Follow these steps to download and install JDK 6:

  1. Browse to Java SE Downloads and click the Java Platform (JDK) box to display the download page for the latest version of the JDK (JDK 6, update 21 at the time of this writing).
  2. Click the Download button.
  3. Select the operating system platform you need.
  4. You will be asked for your account username and password. Enter them if you have an account, sign up if you don't, or you can click Continue to skip this step and proceed to the download.
  5. Save the file to your hard drive when prompted.
  6. When the download is complete, run the install program. (The file you've just downloaded is a self-extracting ZIP file that is also the install program.) Install the JDK to your hard drive in an easy-to-remember location (such as C:\home\jdk1.6.0_20 on Windows or ~/jdk1.6.0_20 on Linux). It's a good idea to encode the update number in the name of the install directory you choose.

You now have a Java environment on your machine. Next, you will install the Eclipse IDE.

Install Eclipse

To download and install Eclipse, follow these steps:

  1. Browse to Eclipse Galileo Sr2 Packages.
  2. Click on Eclipse IDE for Java Developers.
  3. Under Download Links on the right-hand side, select your platform.
  4. Select the mirror you want to download from, then save the file to your hard drive.
  5. Extract the contents of the .zip file to a location on your hard drive that you'll be able to remember easily (such as C:\home\eclipse on Windows or ~/eclipse on Linux).

Set up Eclipse

The Eclipse IDE sits atop the JDK as a useful abstraction, but it still needs to access the JDK and its various tools. Before you can use Eclipse to write Java code, you have to tell it where the JDK is located.

To set up your Eclipse development environment:

  1. Launch Eclipse by double-clicking on eclipse.exe (or the equivalent executable for your platform).
  2. The Workspace Launcher will appear, allowing you to select a root folder for your Eclipse projects. Choose a folder you will easily remember, such as C:\home\workspace on Windows or ~/workspace on Linux.
  3. Dismiss the Welcome to Eclipse screen.
  4. Click Window > Preferences > Java > Installed JREs. Figure 1 shows the setup screen for the JRE:
    Figure 1. Configuring the JDK used by Eclipse
    Correct JDK 6 JRE setup in Eclipse.
  5. Eclipse will point to an installed JRE. You need to make sure you use the one you downloaded with JDK 6. If Eclipse does not automatically detect the JDK you installed, click Add... and in the next dialog Standard VM, then click Next.
  6. Specify the JDK's home directory (such as C:\home\jdk1.6.0_20 on Windows), then click Finish.
  7. Confirm that the JDK you want to use is selected and click OK.

Eclipse is now set up and ready for you to create projects and compile and run Java code. The next section will familiarize you with Eclipse.


Getting started with Eclipse

Eclipse is not just an IDE, it is an entire development ecosystem. This section is a brief hands-on introduction to using Eclipse for Java development. See Resources if you want to learn more about Eclipse.

The Eclipse development environment

The Eclipse development environment has four main components:

  • Workspace
  • Projects
  • Perspectives
  • Views

The primary unit of organization in Eclipse is the workspace. A workspace contains all of your projects. A perspective is a way of looking at each project (hence the name), and within a perspective are one or more views.

The Java perspective

Figure 2 shows the Java perspective, which is the default perspective for Eclipse. You should see this perspective when you start up Eclipse.

Figure 2. Eclipse Java perspective
The Eclipse IDE startup screen shows a default Java perspective.

The Java perspective contains the tools you need to begin writing Java applications. Each tab shown in Figure 2 is a view for the Java perspective. Package Explorer and Outline are two particularly useful views.

The Eclipse environment is highly configurable. Each view is dockable, so you can move it around in the Java perspective and place it where you want it. For now, though, stick with the default perspective and view setup.

Create a project

Follow these steps to create a new Java project:

  1. Click on File > New > Java Project ... and you will see a dialog box open like the one shown in Figure 3:
    Figure 3. New Java Project wizard
    New Project wizard dialog box.
  2. Enter Intro as the project name and click Finish.
  3. If you want to modify the default project settings, click Next. (This is recommended only if you have experience with the Eclipse IDE.)
  4. Click Finish to accept the project setup and create the project.

You have now created a new Eclipse Java project and source folder. Your development environment is ready for action. However, an understanding of the OOP paradigm — covered in this tutorial's next two sections — is essential. If you are familiar with OOP concepts and principles, you might want to skip to Getting started with the Java language.


Object-oriented programming concepts

The Java language is (mostly) object-oriented. If you haven't used an object-oriented language before, its concepts might seem strange at first. This section is a short introduction to OOP language concepts, using structured programming as a point of contrast.

What is an object?

Structured programming languages like C and COBOL follow a very different programming paradigm from object-oriented ones. The structured-programming paradigm is highly data-oriented, which means that you have data structures on one hand, and then program instructions that act on that data. Object-oriented languages like the Java language combine data and program instructions into objects.

An object is a self-contained entity that contains attributes and behavior, and nothing more. Rather than having a data structure with fields (attributes) and passing that structure around to all of the program logic that acts on it (behavior), in an object-oriented language, data and program logic are combined. This combination can occur at vastly different levels of granularity, from fine-grained objects like a Number, to coarse-grained objects such as a FundsTransfer service in a large banking application.

Parent and child objects

A parent object is one that serves as the structural basis for deriving more-complex child objects. A child object looks like its parent but is more specialized. The object-oriented paradigm allows you to reuse the common attributes and behavior of the parent object, adding to its child objects attributes and behavior that differ. (You'll learn more about inheritance in the next section of this tutorial.)

Object communication and coordination

Objects talk to other objects by sending messages (method calls in the Java language). Furthermore, in an object-oriented application, program code coordinates the activities among objects to perform tasks within the context of the given application domain. (In the Model-View-Controller paradigm, this coordinating program code is the Controller. See Resources to learn more about MVC.)

Object summary

A well-written object:

  • Has crisp boundaries
  • Does a finite set of activities
  • Knows only about its data and any other objects that it needs to accomplish its activities

In essence, an object is a discrete entity that has only the necessary dependencies on other objects to perform its tasks.

Now you'll see what an object looks like.

The Person object

I'll start with an example that is based on a common application-development scenario: an individual being represented by a Person object.

Going back to the definition of an object, you know that an object has two primary elements: attributes and behavior. You'll see how these apply to the Person object.

Attributes

What attributes can a person have? Some common ones include:

  • Name
  • Age
  • Height
  • Weight
  • Eye color
  • Gender

You can probably think of more (and you can always add more attributes later), but this list is a good start.

Behavior

An actual person can do all sorts of things, but object behaviors usually relate to some kind of application context. In a business-application context, for instance, you might want to ask your Person object, "What is your age?" In response, Person would tell you the value of its Age attribute.

More-complex logic could be hidden inside of the Person object, but for now suppose that Person has the behavior of answering these questions:

  • What is your name?
  • What is your age?
  • What is your height?
  • What is your weight?
  • What is your eye color?
  • What is your gender?

State and string

State is an important concept in OOP. An object's state is represented at any moment in time by the value of its attributes.

In the case of Person, its state is defined by attributes such as name, age, height, and weight. If you wanted to present a list of several of those attributes, you might do so using a String class, which I'll talk more about later in the tutorial.

Together, the concepts of state and string allow you to say to Person: tell me who you are by giving me a listing (or String) of your attributes.


Principles of OOP

If you come from a structured-programming background, the OOP value proposition might not be clear yet. After all, the attributes of a person and any logic to retrieve (and convert) their values could be written in C or COBOL. This section clarifies the benefits of the OOP paradigm by explaining its defining principles: encapsulation, inheritance, and polymorphism.

Encapsulation

Recall that an object is above all discrete, or self-contained. This is the principle of encapsulation at work. Hiding is another term that is sometimes used to express the self-contained, protected nature of objects.

Regardless of terminology, what's important is that the object maintains a boundary between its state and behavior, and the outside world. Like objects in the real world, objects used in computer programming have various types of relationships with different categories of objects in the applications that use them.

On the Java platform, you can use access specifiers (which I'll introduce later in the tutorial) to vary the nature of object relationships from public to private. Public access is wide open, whereas private access means the object's attributes are accessible only within the object itself.

The public/private boundary enforces the object-oriented principle of encapsulation. On the Java platform, you can vary the strength of that boundary on an object-by-object basis, depending on a system of trust. Encapsulation is a powerful feature of the Java language.

Inheritance

In structured programming, it is common to copy a structure, give it a new name, and add or modify the attributes that make the new entity (such as an Account record) different from its original source. Over time, this approach generates a great deal of duplicated code, which can create maintenance issues.

OOP introduces the concept of inheritance, whereby specialized objects — without additional code — can "copy" the attributes and behavior of the source objects they specialize. If some of those attributes or behaviors need to change, then you simply override them. You only change what you need to change in order to create specialized objects. As you know from the Object-oriented programming concepts section, the source object is called the parent, and the new specialization is called the child.

Inheritance at work

Suppose you are writing a human-resources application and want to use the Person object as the basis for a new object called Employee. Being the child of Person, Employee would have all of the attributes of a Person object, along with additional ones, such as:

  • Taxpayer identification number
  • Hire date
  • Salary

Inheritance makes it easy to create the new Employee class of the object without needing to copy all of the Person code manually or maintain it.

You'll see plenty of examples of inheritance in Java programming later in the tutorial, especially in Part 2.

Polymorphism

Polymorphism is a harder concept to grasp than encapsulation and inheritance. In essence, it means that objects that belong to the same branch of a hierarchy, when sent the same message (that is, when told to do the same thing), can manifest that behavior differently.

To understand how polymorphism applies to a business-application context, return to the Person example. Remember telling Person to format its attributes into a String? Polymorphism makes it possible for Person to represent its attributes in a variety of ways depending on the type of Person it is.

Polymorphism is one of the more complex concepts you'll encounter in OOP on the Java platform and not within the scope of an introductory tutorial. See Resources if you want to learn more about polymorphism.


Getting started with the Java language

The Java language: Not purely object-oriented

The Java language lets you create first-class objects, but not everything in the language is an object. Two qualities differentiate the Java language from purely object-oriented languages such as Smalltalk. First, the Java language is a mixture of objects and primitive types. Second, it allows you to write code that exposes the inner workings of one object to any other object that uses it.

The Java language does give you the tools necessary to follow sound OOP principles and produce sound object-oriented code. Because Java is not purely object-oriented, you must exercise some discipline in how you write code — the language doesn't force you to do the right thing, so you must do it yourself. (This tutorial's last section, Writing good Java code, provides tips.)

It would be impossible to introduce the entire Java language syntax in a single tutorial. The remainder of Part 1 focuses on the basics of the language, leaving you with enough knowledge and practice to write simple programs. OOP is all about objects, so this section starts with two topics specifically related to how the Java language handles them: reserved words and the structure of a Java object.

Reserved words

Like any programming language, the Java language designates certain words that the compiler recognizes as special, and as such you are not allowed to use them for naming your Java constructs. The list of reserved words is surprisingly short:

  • abstract
  • assert
  • boolean
  • break
  • byte
  • case
  • catch
  • char
  • class
  • const
  • continue
  • default
  • do
  • double
  • else
  • enum
  • extends
  • final
  • finally
  • float
  • for
  • goto
  • if
  • implements
  • import
  • instanceof
  • int
  • interface
  • long
  • native
  • new
  • package
  • private
  • protected
  • public
  • return
  • short
  • static
  • strictfp
  • super
  • switch
  • synchronized
  • this
  • throw
  • throws
  • transient
  • try
  • void
  • volatile
  • while

Note that true, false, and null are technically not reserved words. Although they are literals, I included them in this list because you can't use them to name Java constructs.

One advantage of programming with an IDE is that it can use syntax coloring for reserved words, as you'll see later in this tutorial.

Structure of a Java object

Remember that an object is a discrete entity that contains attributes and behavior. That means that it has a crisp boundary and a state and can do things when correctly asked. Every object-oriented language has rules about how to define an object.

In the Java language, objects are defined as demonstrated in Listing 1:

Listing 1. Object definition
package packageName;
import ClassNameToImport;
accessSpecifier class ClassName {
  accessSpecifier dataType variableName [= initialValue];
  accessSpecifier ClassName([argumentList]) {
    constructorStatement(s)
  }
  accessSpecifier returnType methodName([argumentList]) {
    methodStatement(s)
  }
  // This is a comment
  /* This is a comment too */
  /* This is a
     multiline
     comment */
}

Listing 1 contains various types of constructs, which I've differentiated with font formatting. The constructs shown in bold (which you'll find in the list of reserved words) are literals; in any object definition, they must be exactly what they are here. The names that I've given the other constructs describe the concepts they represent. I'll explain all of the constructs in detail in the rest of this section.

Note: In Listing 1 and some other code examples in this section, square brackets indicate that the constructs within them are not required. The brackets themselves (unlike { and }) are not part of the Java syntax.

Comments in code

Notice that Listing 1 also includes some comment lines:

  // This is a comment
  /* This is a comment too */
  /* This is a
     multiline
     comment */

Just about every programming language allows the programmer to add comments to help document the code. Java syntax allows for both single-line and multiline comments. A single-line comment must be contained on one line, although you can use adjacent single-line comments to form a block. A multiline comment begins with /*, must be terminated with */, and can span any number of lines.

You'll learn more about comments when you get to this tutorial's Writing good Java code section.

Packaging objects

The Java language lets you choose the names for your objects, such as Account, Person, or LizardMan. At times, you may end up using the same name to express two slightly different concepts. This is called a name collision, and it happens frequently. The Java language uses packages to resolve these conflicts.

A Java package is a mechanism for providing a namespace: an encapsulated area in which names are unique, but outside of which they might not be. To identify a construct uniquely, you must fully qualify it by including its namespace.

Packages also give you a nice way to build more complex applications into discrete units of functionality.

Package definition

To define a package, you use the package keyword followed by a legal package name, terminated with a semicolon. Often package names are separated by dots, and follow this de facto scheme:

package orgType.orgName.appName.compName;

This package definition breaks down like so:

  • orgType is the organization type such as com, org, or net.
  • orgName is the name of the organization's domain, such as makotogroup, sun, or ibm.
  • appName is the name of the application, abbreviated.
  • compName is the name of the component.

The Java language doesn't force you to follow this package convention. In fact, you don't need to specify a package at all, in which case all of your objects must have unique class names and will reside in the default package. As a best practice, I recommend that you define all of your Java classes in packages. You'll follow that convention throughout this tutorial.

Import statements

Up next in the object definition (referring back to Listing 1) is the import statement. An import statement tells the Java compiler where to find classes you reference inside of your code. Any nontrivial object uses other objects for some functionality, and the import statement is how you tell the Java compiler about them.

An import statement usually looks like this:

import ClassNameToImport;

You specify the import keyword followed by the class that you want to import followed by a semicolon. The class name should be fully qualified, meaning it should include its package.

To import all classes within a package, you can put .* after the package name. For example, this statement imports every class in the com.makotogroup package:

import com.makotogroup.*;

Importing an entire package can make your code less readable, however, so I recommend that you import just the classes you need.

Eclipse simplifies imports

When writing code in the Eclipse editor, you can type the name of a class you want to use, followed by Ctrl+Shift+O. Eclipse figures out which imports you need and adds them automatically. If Eclipse finds two classes with the same name, it displays a dialog box asking you which class you want to add imports for.

Class declaration

In order to define an object in the Java language, you must declare a class. Think of a class as a template for an object, like a cookie cutter. The class defines the object's basic structure, and at run time your application creates an instance of the object. The word object is often used synonymously with the word class. Strictly speaking, a class defines the structure of a thing of which the object is an instance.

Listing 1 includes this class declaration:

accessSpecifier class ClassName {
  accessSpecifier dataType variableName [= initialValue];
  accessSpecifier ClassName([argumentList]) {
    constructorStatement(s)
  }
  accessSpecifier returnType methodName([argumentList]) {
    methodStatement(s)
  }
}

A class's accessSpecifier could have several values, but most of the time it is public. You'll look at other values of accessSpecifier soon.

Class-naming conventions

You can name classes pretty much however you want, but the convention is to use CamelCase: start with a capital letter, capitalize the first letter of each concatenated word, and make all the other letters lowercase. Class names should contain only letters and numbers. Sticking to these guidelines will ensure that your code is more accessible to other developers following the same conventions.

Classes can have two types of members: variables and methods.

Variables

The values of a given class's variables distinguish each instance of that class and define its state. These values are often referred to as instance variables. A variable has:

  • An accessSpecifier
  • A dataType
  • A variableName
  • Optionally, an initialValue

The possible accessSpecifier values are:

Public variables

It's never a good idea to use public variables, but in extremely rare cases it could be necessary, so the option exists. The Java platform doesn't constrain your use cases, so it's up to you to be disciplined about using good coding conventions, even if tempted to do otherwise.

  • public: Any object in any package can see the variable. (Don't ever use this value.)
  • protected: Any object defined in the same package, or a subclass (defined in any package), can see the variable.
  • No specifier (also called friendly or package private access): Only objects whose classes are defined in the same package can see the variable.
  • private: Only the class containing the variable can see it.

A variable's dataType depends on what the variable is — it could be a primitive type or another class type (again, more about this later).

The variableName is up to you, but by convention variable names use the CamelCase convention I described earlier, except that they begin with a lowercase letter. (This style is sometimes called lowerCamelCase.)

Don't worry about the initialValue for now; just know that you can initialize an instance variable when you declare it. (Otherwise, the compiler generates a default for you that will be set when the class is instantiated.)

Example: Class definition for Person

Before moving on to methods, here's an example that summarizes what you've learned so far. Listing 2 is a class definition for Person:

Listing 2. Basic class definition for Person
package com.makotogroup.intro;

public class Person {
  private String name;
  private int age;
  private int height;
  private int weight;
  private String eyeColor;
  private String gender;
}

The basic class definition for Person isn't very useful at this point because it defines only its attributes (and private ones at that).

To be more interesting, the Person class needs behavior — and that means methods.

Methods

A class's methods define its behavior. Sometimes this behavior is nothing more than to return the current value of an attribute. Other times, the behavior can be quite complex.

There are essentially two categories of methods: constructors and all other methods — of which there are many types. A constructor method is used only to create an instance of a class. Other types of methods can be used for virtually any application behavior.

Looking back at Listing 1, it shows the way to define the structure of a method, which includes things like:

  • accessSpecifier
  • returnType
  • methodName
  • argumentList

The combination of these structural elements in a method's definition is called its signature.

Next you'll look in more detail at the two types of methods, starting with constructors.

Constructor methods

Constructors let you specify how to instantiate a class. Listing 1 shows the constructor declaration syntax in abstract form; here it is again in Listing 3:

Listing 3. Constructor declaration syntax
accessSpecifier ClassName([argumentList]) {
  constructorStatement(s)
}

Constructors are optional

If you don't provide a constructor, the compiler will provide one for you, called the default (or no-argument) constructor. If you provide a constructor other than a no-argument (or no-arg) constructor, the compiler will not generate one for you.

A constructor's accessSpecifier is the same as for variables. The name of the constructor must match the name of the class. So if you call your class Person, then the name of the constructor must also be Person.

For any constructor other than the default constructor, you pass an argumentList, which is one or more of:

argumentType argumentName

Arguments in an argumentList are separated by commas, and no two arguments can have the same name. argumentType is either a primitive type or another class type (the same as with variable types).

Class definition with a constructor

Now you'll see what happens when you add the capability to create a Person object in two ways: by using a no-arg constructor and by initializing a partial list of attributes.

Listing 4 shows how to create constructors and also how to use argumentList:

Listing 4. Person class definition with a constructor
package com.makotogroup.intro;
public class Person {
  private String name;
  private int age;
  private int height;
  private int weight;
  private String eyeColor;
  private String gender;
  public Person() {
    // Nothing to do...
  }
  
public Person(String name, int age, int height, String eyeColor, String gender) {
    this.name = name;
    this.age = age;
    this.height = height;
    this.weight = weight;
    this.eyeColor = eyeColor;
    this.gender = gender;
  }
}

Note the use of the this keyword in making the variable assignments in Listing 4. This is Java shorthand for "this object" and must be used when referencing two variables with the same name (as in this case where age, for example, is both a constructor parameter and a class variable), and helps the compiler disambiguate the reference.

The Person object is getting more interesting, but it needs more behavior. And for that, you need more methods.

Other methods

A constructor is a particular kind of method with a particular function. Similarly, many other types of methods perform particular functions in Java programs. Exploration of other methods begins in this section and continues throughout the tutorial.

Back in Listing 1, I showed you how to declare a method:

accessSpecifier returnType methodName([argumentList]) {
  methodStatement(s)
}

Other methods look much like constructors, with a couple of exceptions. First, you can name other methods whatever you like (though, of course, there are rules). I recommend the following conventions:

  • Start with a lowercase letter.
  • Avoid numbers unless absolutely necessary.
  • Use only alphabetic characters.

Second, unlike constructors, other methods have an optional return type.

Person's other methods

Armed with this basic information, you can see in Listing 5 see what happens when you add a few more methods to the Person object. (I've omitted constructors for brevity.)

Listing 5. Person with a few new methods
package com.makotogroup.intro;

public class Person {
  private String name;
  private int age;
  private int height;
  private int weight;
  private String eyeColor;
  private String gender;

  public String getName() { return name; }
  public void setName(String value) { name = value; }
  // Other getter/setter combinations...
}

Notice the comment in Listing 5 about "getter/setter combinations." You''ll work more with getters and setters later in the tutorial. For now, all you need to know is that a getter is a method for retrieving the value of an attribute, and a setter is a method for modifying that value. I've shown only one getter/setter combination (for the Name attribute), but you could define more in a similar fashion.

Note in Listing 5 that if a method doesn't return a value, you must tell the compiler by specifying the void return type in its signature.

Static and instance methods

There are generally two types of (nonconstructor) methods: instance methods and static methods. Instance methods are dependent on the state of a specific object instance for their behavior. Static methods are also sometimes called class methods, because their behavior is not dependent on any single object's state. A static method's behavior happens at the class level.

Static methods are used largely for utility; you can think of them as a way of having global methods (à la C) while keeping the code itself grouped with the class that needs it.

For example, throughout this tutorial you'll use the JDK Logger class to output information to the console. To create a Logger class instance, you don't instantiate a Logger class; instead, you invoke a static method called getLogger().

The syntax for invoking a static method is different from the syntax used to invoke a method on an object instance. You also use the name of the class that contains the static method, as shown in this invocation:

Logger l = Logger.getLogger("NewLogger");

So to invoke a static method, you don't need an object instance, just the name of the class.


Your first Java object

It's time to pull together what you've learned in the previous sections and start writing some code. This section walks you through declaring a class and adding variables and methods to it using the Eclipse Package Explorer. You'll learn how to use the Logger class to keep an eye on your application's behavior, and also how to use a main() method as a test harness.

Creating a package

If you're not already there, get to the Package Explorer perspective in Eclipse. You're going to get set up to create your first Java class. The first step is to create a place for the class to live. Packages are namespace constructs, but they conveniently map directly to the file system's directory structure as well.

Rather than use the default package (almost always a bad idea), you'll create one specifically for the code you'll be writing. Click File > New > Package to bring up the Java Package wizard, shown in Figure 4:

Figure 4. The Eclipse Java Package wizard
The Eclipse Package Wizard.

Type com.makotogroup.intro into the Name text box and click Finish. You will see the new package created in the Package Explorer.

Declaring the class

There's more than one way to create a class from the Package Explorer, but the easiest way is to right-click on the package you just created and choose New > Class.... You will see the New Class dialog box.

In the Name text box, type Person. Under Which method stubs would you like to create?, check public static void main(String[] args). (You'll see why shortly.) Next, click Finish.

The new class appears in your edit window. I recommend closing a few of the views from their default appearance to make it easier to see your source code, as shown in Figure 5:

Figure 5. A well-ordered workspace
The Eclipse Package wizard's edit window.

Eclipse generates a shell class for you and includes the package statement at the top, along with the main() method you asked for and the comments you see. You just need to flesh out the class now. You can configure how Eclipse generates new classes via Window > Preferences > Java > Code Style > Code Templates. For simplicity, you'll go with Eclipse's out-of-the-box code generation.

In Figure 5, notice the asterisk (*) next to the new source-code file, indicating that I've made a modification. And notice that the code is unsaved. Next, notice that I made a mistake when declaring the Name attribute: I declared Name's type to be Strin. The compiler could not find a reference to such a class and flagged it as a compile error (that's the wavy red line underneath Strin). Of course, I can fix my mistake by adding a g to the end of Strin. This is just a small demonstration of the power of an IDE over using command-line tools for software development.

Adding class variables

In Listing 4, you began to flesh out the Person class, but I didn't explain much of the syntax. Now I'll formally define how to add class variables.

Recall that a variable has an accessSpecifier, a dataType, a variableName, and, optionally, an initialValue. Earlier, you looked briefly at how to define the accessSpecifier and variableName. Now you'll see the dataType that a variable can have.

A dataType can be either a primitive type or a reference to another object. For example, notice that Age is an int (a primitive type), and Name is a String (an object). The JDK comes packed full of useful classes like java.lang.String, and those in the java.lang package do not need to be imported (a shorthand courtesy of the Java compiler). But whether the dataType is a JDK class such as String or a user-defined class, the syntax is essentially the same.

Table 1 shows the eight primitive data types you're likely to see on a regular basis, including the default values that primitives take on if you do not explicitly initialize a member variable's value:

Table 1. Primitive data types
TypeSizeDefault valueRange of values
booleann/afalsetrue or false
byte8 bits0-128 to 127
char16 bits(unsigned)\u0000' \u0000' to \uffff' or 0 to 65535
short16 bits0-32768 to 32767
int32 bits0-2147483648 to 2147483647
long64 bits0-9223372036854775808 to 9223372036854775807
float32 bits0.01.17549435e-38 to 3.4028235e+38
double64 bits0.04.9e-324 to 1.7976931348623157e+308

Built-in logging

Before going further into coding, you need to know how your programs tell you what they are doing.

The Java platform includes the java.util.logging package, a built-in logging mechanism for gathering program information in a readable form. Loggers are named entities that you create through a static method call to the Logger class, like so:

import java.util.logging.Logger;
//. . .
Logger l = Logger.getLogger(getClass().getName());

When calling the getLogger() method, you pass it a String. For now, just get in the habit of passing the name of the class the code you're writing is located in. From any regular (that is, nonstatic) method, the code above will always reference the name of the class and pass that to the Logger.

If you are making a Logger call inside of a static method, just reference the name of the class you're inside of:

Logger l = Logger.getLogger(Person.class.getName());

In this example, the code you're inside of is the Person class, so you reference a special literal called class that retrieves the Class object (more on this later) and gets its Name attribute.

This tutorial's Writing good Java code section includes a tip on how not to do logging.

Using main() as a test harness

main() is a special method that you can include in any class so that the JRE can execute its code. A class is not required to have a main() method — in fact, most never will — and a class can have at most one main() method.

main() is a handy method to have because it gives you a quick test harness for the class. In enterprise development, you would use test libraries, but for the purpose of this tutorial, you will use main() as your test harness.

Go into the Eclipse source code editor for Person and add code to make it look like Listing 4. Eclipse has a handy code generator to generate getters and setters (among other things). To try it out, put your mouse caret on the Person class definition (that is, on the word Person in the class definition), and go to Source > Generate Getters and Setters.... When the dialog box opens, click Select All, as shown in Figure 6:

Figure 6. Eclipse generating getters and setters
Eclipse generating getters and setters.

For the insertion point, choose Last member and click OK. Notice that the getters and setters appear after the main() method.

There's more to main()

Now you'll add some code to main() to allow you to instantiate a Person, set a few attributes, and then print those to the console.

Start by adding a constructor to Person. Type the code in Listing 6 into your source window just below the top part of the class definition (the line immediately beneath public class Person ()):

Listing 6. Person constructor
public Person(String name, int age, int height, int weight, String eyeColor, 
                String gender) {
    this.name = name;
    this.age = age;
    this.height = height;
    this.weight = weight;
    this.eyeColor = eyeColor;
    this.gender = gender;
  }

Make sure you have no wavy lines indicating compile errors.

Next, go into the main() method and make it look like Listing 7:

Listing 7. The main() method
public static void main(String[] args) {
  Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
  Logger l = Logger.getLogger(Person.class.getName());
  l.info("Name: " + p.getName());
  l.info("Age:" + p.getAge());
  l.info("Height (cm):" + p.getHeight());
  l.info("Weight (kg):" + p.getWeight());
  l.info("Eye Color:" + p.getEyeColor());
  l.info("Gender:" + p.getGender());
}

Don't worry about the Logger class for now. Just enter the code as you see it in Listing 7. You're now ready to run your first Java program.

Executing code in Eclipse

To run a Java application from inside Eclipse, select the class you want to run, which must have a main() method. Currently, you have only one class — and it does have a main() method — so select Person, then click the Run icon (which is green and has a small triangular arrow pointing to the right). When asked, select to run Person as a Java application, then sit back and watch it go. You should see something like the screenshot in Figure 7:

Figure 7. See Person run
Eclipse running Person as a Java application.

Notice that the Console view opens automatically and shows Logger output. I have also selected the Outline view in the left-hand pane, which reveals the basic structure of the Person class at a glance.


Adding behavior to a Java object

Person is looking good so far, but it could use some additional behavior to make it more interesting. As you've already learned, creating behavior means adding methods. This section looks more closely at accessor methods — namely, the getters and setters you've already seen in action. You'll also learn syntax for calling methods.

Accessor methods

In order to encapsulate a class's data from other objects, you declare its variables to be private and then provide accessor methods. As you know, a getter is an accessor method for retrieving the value of an attribute; a setter is an accessor method for modifying that value. The naming of accessors follows a strict convention known as the JavaBeans pattern, whereby any attribute Foo has a getter called getFoo() and a setter called setFoo().

The JavaBeans pattern is so common that support for it is built right into the Eclipse IDE. You've even already seen it in action — when you generated getters and setters for Person in the preceding section.

Accessors follow these guidelines:

  • The attribute itself is always declared with private access.
  • The access specifier for getters and setters is public.
  • Getters don't take any parameters and return a value whose type is the same as the attribute it accesses.
  • Settings only take one parameter, of the type of the attribute, and do not return a value.

Declaring accessors

By far the easiest way to declare accessors is to let Eclipse do it for you, as shown back in Figure 6. But you should also know how to hand-code a getter-and-setter pair. Suppose you have an attribute, Foo, whose type is java.lang.String. A complete declaration for it (following the accessor guidelines) would be:

private String foo;
public String getFoo() {
  return foo;
}
public void setFoo(String value) {
  foo = value;
}

You may notice right away that the parameter value passed to the setter is named differently than if it had been generated by Eclipse. This is my own convention, and one I recommend to other developers. On the rare occasion that I do hand-code a setter, I always use the name value as the parameter value to the setter. This eye-catcher reminds that I've hand-coded the setter. Because I usually allow Eclipse to generate getters and setters for me, when I don't there's a good reason for it. Using value as the setter's parameter value reminds me that this setter is special. (Code comments could also do that.)

Calling methods

Invoking, or calling, methods is easy. You saw in Listing 7 how to invoke the various getters of Person to return their values. Now I'll formalize the mechanics of making method calls.

Method invocation with and without parameters

To invoke a method on an object, you need a reference to that object. Method-invocation syntax comprises the object reference, a literal dot, the method name, and any parameters that need to be passed:

objectReference.someMethod();
objectReference.someOtherMethod(parameter);

Here is a method invocation without parameters:

Person p = /*obtain somehow */;
p.getName();

And here is a method invocation with parameters (accessing the Name attribute of Person):

Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");

Remember that constructors are methods, too. And you can separate the parameters with spaces and newlines. The Java compiler doesn't care. These next two method invocations are identical:

new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
new Person("Joe Q Author", 
        42, 
        173, 
        82, 
        "Brown", 
        "MALE");

Nested method invocation

Method invocations can also be nested:

Logger l = Logger.getLogger(Person.class.getName());
l.info("Name: " + p.getName());

Here you are passing the return value of Person.class.getName() to the getLogger() method. Remember that the getLogger() method call is a static method call, so its syntax differs slightly. (You don't need a Logger reference to make the invocation; instead, you just use the name of the class itself as the left-hand side of the invocation.)

That's really all there is to method invocation.


Strings and operators

The tutorial has so far introduced several variables of type String, but without much explanation. Learn more about strings in this section, and also find out when and how to use operators.

Strings

Handling strings in C is labor-intensive because they're null-terminated arrays of 8-bit characters that you have to manipulate. In the Java language, strings are first-class objects of type String, with methods that help you manipulate them. (The closest Java code gets to the C world with regard to strings is the char primitive data type, which can hold a single Unicode character, such as a.)

You've already seen how to instantiate a String object and set its value (back in Listing 5), but there are several other ways to do that. Here are a couple of ways to create a String instance with a value of hello:

String greeting = "hello";
greeting = new String("hello");

Because Strings are first-class objects in the Java language, you can use new to instantiate them. Setting a variable of type String has the same result, because the Java language creates a String object to hold the literal, then assigns that object to the instance variable.

Concatenating strings

You can do many things with String, and the class has many helpful methods. Without even using a method, you've already done something interesting with two Strings by concatenating, or combining, them:

l.info("Name: " + p.getName());

The plus (+) sign is shorthand for concatenating Strings in the Java language. (There is a performance penalty for doing this type of concatenation inside a loop, but for now you don't need to worry about it.)

Concatenation example

Let's try concatenating Strings inside of the Person class. At this point, you have a name instance variable, but it would be nice to have a firstName and lastName. You could then concatenate them when another object requests Person's full name.

The first thing you need to do is add the new instance variables (at the same location in the source code where name is currently defined):

//private String name;
private String firstName;
private String lastName;

You don't need name anymore; you've replaced it with firstName and lastName.

Chaining method calls

Now you can generate getters and setters for firstName and lastName (as shown back in Figure 6), remove the setName() method, and change getName() to look like this:

public String getName() {
  return firstName.concat(" ").concat(lastName);
}

This code illustrates chaining of method calls. This is a technique commonly used with immutable objects like String, where a modification to an immutable object always returns the modification (but doesn't change the original). You then operate on the returned, changed value.

Operators

As you might expect, the Java language can do arithmetic, and you've already seen how to assign variables. Now I'll give you a brief look at some of the Java language operators you'll need as your skills improve. The Java language uses two types of operators:

  • Unary: Only one operand is needed.
  • Binary: Two operands are needed.

The Java language's arithmetic operators are summarized in Table 2:

Table 2. Java language's arithmetic operators
OperatorUsageDescription
+a + bAdds a and b
++aPromotes a to int if it's a byte, short, or char
-a - bSubtracts b from a
--aArithmetically negates a
*a * bMultiplies a and b
/a / bDivides a by b
%a % bReturns the remainder of dividing a by b (the modulus operator)
++a++Increments a by 1; computes the value of a before incrementing
++++aIncrements a by 1; computes the value of a after incrementing
--a--Decrements a by 1; computes the value of a before decrementing
----aDecrements a by 1; computes the value of a after decrementing
+=a += bShorthand for a = a + b
-=a -= bShorthand for a = a - b
*=a *= bShorthand for a = a * b
%=a %= bShorthand for a = a % b

Additional operators

In addition to the operators in Table 2, you've seen several other symbols that are called operators in the Java language. For instance:

  • Period (.), which qualifies names of packages and invokes methods
  • Parentheses (()), which delimit a comma-separated list of parameters to a method
  • new, which (when followed by a constructor name) instantiates an object

The Java language syntax also includes a number of operators that are used specifically for conditional programming; that is, programs that respond differently based on different input. You'll look at those in the next section.


Conditional operators and control statements

In this section, learn about the various statements and operators you'll use to tell your Java programs how you want them to act based on different input.

Relational and conditional operators

The Java language gives you operators and control statements that let you make decisions in your code. Most often, a decision in code starts with a boolean expression (that is, one that evaluates to either true or false). Such expressions use relational operators, which compare one operand or expression to another, and conditional operators.

Table 3 lists the relational and conditional operators of the Java language:

Table 3. Relational and conditional operators
OperatorUsageReturns true if ...
>a > ba is greater than b
>=a >= ba is greater than or equal to b
<a < ba is less than b
<=a <= ba is less than or equal to b
==a == ba is equal to b
!=a != ba is not equal to b
&&a && ba and b are both true, conditionally evaluates b (if a is false, b is not evaluated)
||a || ba or b is true, conditionally evaluates b (if a is true, b is not evaluated)
!!aa is false
&a & ba and b are both true, always evaluates b
|a | ba or b is true, always evaluates b
^a ^ ba and b are different

The if statement

Now that you have a bunch of operators, it's time to use them. This code shows what happens when you add some logic to the Person object's getHeight() accessor:

public int getHeight() {
  int ret = height;
  // If locale of the machine this code is running on is U.S.,
  if (Locale.getDefault().equals(Locale.US))
    ret /= 2.54;// convert from cm to inches
  return ret;
}

If the current locale is in the United States (where the metric system is not in use), then it might make sense to convert the internal value of height (in centimeters) to inches. This example illustrates the use of the if statement, which evaluates a boolean expression inside parentheses. If that expression evaluates to true, it executes the next statement.

In this case, you only need to execute one statement if the Locale of the machine the code is running on is Locale.US. If you need to execute more than one statement, you can use curly braces to form a compound statement. A compound statement groups many statements into one — and compound statements can also contain other compound statements.

Variable scope

Every variable in a Java application has scope, or localized namespace, where you can access it by name within the code. Outside that space the variable is out of scope, and you will get a compile error if you try to access it. Scope levels in the Java language are defined by where a variable is declared, as shown in Listing 8:

Listing 8. Variable scope
public class SomeClass {

  private String someClassVariable;
  public void someMethod(String someParameter) {
    String someLocalVariable = "Hello";
    if (true) {
      String someOtherLocalVariable = "Howdy";
    }
    someClassVariable = someParameter; // legal
    someLocalVariable = someClassVariable; // also legal
    someOtherLocalVariable = someLocalVariable;// Variable out of scope!
  }
  public void someOtherMethod() {
    someLocalVariable = "Hello there";// That variable is out of scope!
  }
}

Within SomeClass, someClassVariable is accessible by all instance (that is, nonstatic) methods. Within someMethod, someParameter is visible, but outside of that method it is not, and the same is true for someLocalVariable. Within the if block, someOtherLocalVariable is declared, and outside of that if block it is out of scope.

Scope has many rules, but Listing 8 shows the most common ones. Take a few minutes to familiarize yourself with them.

The else statement

There are times in a program's control flow when you want to take action only if a particular expression fails to evaluate to true. That's when else comes in handy:

public int getHeight() {
  int ret;
  if (gender.equals("MALE"))
    ret = height + 2;
  else {
    ret = height;
    Logger.getLogger("Person").info("Being honest about height...");
  }
  return ret;
}

The else statement works the same way as if, in that it executes only the next statement it runs across. In this case, two statements are grouped into a compound statement (notice the curly braces), which the program then executes.

You can also use else to perform an additional if check, like so:

if (conditional) {
  // Block 1
} else if (conditional2) {
  // Block 2
} else if (conditional3) {
  // Block 3
} else {

  // Block 4
} // End

If conditional evaluates to true, then Block 1 is executed and the program jumps to the next statement after the final curly brace (which is indicated by // End). If conditional does not evaluate to true, then conditional2 is evaluated. If it is true, then Block 2 is executed, and the program jumps to the next statement after the final curly brace. If conditional2 is not true, then the program moves on to conditional3, and so on. Only if all three conditionals failed would Block 4 be executed.

The ternary operator

The Java language provides a handy operator for doing simple if/else statement checks. Its syntax is:

(conditional) ? statementIfTrue : statementIfFalse;

If conditional evaluates to true, then statementIfTrue is executed; otherwise, statementIfFalse is executed. Compound statements are not allowed for either statement.

The ternary operator comes in handy when you know you will need to execute one statement as the result of the conditional evaluating to true, and another if it does not. Ternary operators are most often used to initialize a variable (like a return value), like so:

public int getHeight() {
  return (gender.equals("MALE")) ? (height + 2) : height;
}

The parentheses following the question mark above aren't strictly required, but they do make the code more readable.


Loops

In addition to being able to apply conditions to your programs and see different outcomes based on various if/then scenarios, you sometimes want your code just to do the same thing over and over again until the job is done. In this section, learn about two constructs used to iterate over code or execute it more than once: for loops and while loops.

What is a loop?

A loop is a programming construct that executes repeatedly while some condition (or set of conditions) is met. For instance, you might ask a program to read all records until the end of a file, or loop over all the elements of an array, processing each one. (You'll learn about arrays in this tutorial's Java Collections section.)

for loops

The basic loop construct in the Java language is the for statement, which lets you iterate over a range of values to determine how many times to execute a loop. The abstract syntax for a for loop is:

for (initialization; loopWhileTrue; executeAtBottomOfEachLoop) {
  statementsToExecute
}

At the beginning of the loop, the initialization statement is executed (multiple initialization statements can be separated by commas). So long as loopWhileTrue (a Java conditional expression that must evaluate to either true or false) is true, the loop will be executed. At the bottom of the loop, executeAtBottomOfEachLoop is executed.

Example of a for loop

If you wanted to change a main() method to execute three times, you could use a for loop, as shown in Listing 9:

Listing 9. A for loop
public static void main(String[] args) {
  Logger l = Logger.getLogger(Person.class.getName());
  for (int aa = 0; aa < 3; aa++) {
    Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
    l.info("Loop executing iteration# " + aa);
    l.info("Name: " + p.getName());
    l.info("Age:" + p.getAge());
    l.info("Height (cm):" + p.getHeight());
    l.info("Weight (kg):" + p.getWeight());
    l.info("Eye Color:" + p.getEyeColor());
    l.info("Gender:" + p.getGender());
  }
}

The local variable aa is initialized to zero at the beginning of the listing. This statement executes only once, when the loop is initialized. The loop then continues three times, and each time aa is incremented by one.

As you will see later, an alternate for loop syntax is available for looping over constructs that implement the Iterable interface (such as arrays and other Java utility classes). For now, just note the use of the for loop syntax in Listing 9.

while loops

The syntax for a while loop is:

while (loopWhileTrue) {
  statementsToExecute
}

As you might suspect, while loopWhileTrue evaluates to true, so the loop will execute. At the top of each iteration (that is, before any statements execute), the condition is evaluated. If true, the loop executes. So it is possible that a while loop will never execute if its conditional expression is not true at least once.

Look again at the for loop in Listing 9. For comparison, Listing 10 codes it using a while loop:

Listing 10. A while loop
public static void main(String[] args) {
  Logger l = Logger.getLogger(Person.class.getName());
  int aa = 0;
  while (aa < 3) {
    Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
    l.info("Loop executing iteration# " + aa);
    l.info("Name: " + p.getName());
    l.info("Age:" + p.getAge());
    l.info("Height (cm):" + p.getHeight());
    l.info("Weight (kg):" + p.getWeight());
    l.info("Eye Color:" + p.getEyeColor());
    l.info("Gender:" + p.getGender());
    aa++;
  }

As you can see, a while loop requires a bit more housekeeping than a for loop. You must initialize the aa variable and also remember to increment it at the bottom of the loop.

do...while loops

If you want a loop that will always execute once and then check its conditional expression, try using a do...while loop, as shown in Listing 11:

Listing 11. A do...while loop
int aa = 0;
do {
  Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
  l.info("Loop executing iteration# " + aa);
  l.info("Name: " + p.getName());
  l.info("Age:" + p.getAge());
  l.info("Height (cm):" + p.getHeight());
  l.info("Weight (kg):" + p.getWeight());
  l.info("Eye Color:" + p.getEyeColor());
  l.info("Gender:" + p.getGender());
  aa++;
} while (aa < 3);

The conditional expression (aa < 3) is not checked until the end of the loop.

Loop branching

There are times when you need to bail out of a loop before the conditional expression evaluates to false. This could happen if you were searching an array of Strings for a particular value, and once you found it, you didn't care about the other elements of the array. For those times when you just want to bail, the Java language provides the break statement, as shown in Listing 12:

Listing 12. A break statement
public static void main(String[] args) {
  Logger l = Logger.getLogger(Person.class.getName());
  int aa = 0;
  while (aa < 3) {
    if (aa == 1)
      break;
    Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
    l.info("Loop executing iteration# " + aa);
    l.info("Name: " + p.getName());
    l.info("Age:" + p.getAge());
    l.info("Height (cm):" + p.getHeight());
    l.info("Weight (kg):" + p.getWeight());
    l.info("Eye Color:" + p.getEyeColor());
    l.info("Gender:" + p.getGender());
    aa++;
  }

The break statement takes you to the very next executable statement outside of the loop in which it's located.

Loop continuation

In the (simplistic) example in Listing 12, you only want to execute the loop once and bail. You can also skip a single iteration of a loop but continue executing the loop. For that, you need the continue statement, shown in Listing 13:

Listing 13. A continue statement
public static void main(String[] args) {
Logger l = Logger.getLogger(Person.class.getName());
int aa = 0;
while (aa < 3) {
  if (aa == 1)
    continue;
  else
    aa++;
  Person p = new Person("Joe Q Author", 42, 173, 82, "Brown", "MALE");
  l.info("Loop executing iteration# " + aa);
  l.info("Name: " + p.getName());
  l.info("Age:" + p.getAge());
  l.info("Height (cm):" + p.getHeight());
  l.info("Weight (kg):" + p.getWeight());
  l.info("Eye Color:" + p.getEyeColor());
  l.info("Gender:" + p.getGender());
}

In Listing 13, you skip the second iteration of a loop but continued to the third. continue comes in handy when you are, say, processing records and come across a record you definitely don't want to process. Just skip that record and move on to the next one.


Java Collections

Most real-world applications deal with collections of things: files, variables, records from files, database result sets, and so forth. The Java language has a sophisticated Collections Framework that allows you to create and manage collections of objects of various types. This section won't teach you everything about Java Collections, but it will introduce the most commonly used collection classes and get you started with using them.

Arrays

Most programming languages include the concept of an array to hold a collection of things, and the Java language is no exception. An array is nothing more than a collection of elements of the same type.

Note: The square brackets in this section's code examples are part of the required syntax for Java Collections, not indicators of optional elements.

You can declare an array in one of two ways:

  • Create it with a certain size, which is fixed for the life of the array.
  • Create it with a certain set of initial values. The size of this set determines the size of the array — it will be exactly large enough to hold all of those values, and its size is fixed for the life of the array.

Declaring an array

In general, you declare an array like this:

new elementType [arraySize]

There are two ways to create an integer array of elements. This statement creates an array with space for five elements, but it is empty:

// creates an empty array of 5 elements:
int[] integers = new int[5];

This statement creates the array and initializes it all at once:

// creates an array of 5 elements with values:
int[] integers = new int[] { 1, 2, 3, 4, 5 };

The initial values go between the curly braces and are separated by commas.

Arrays the hard way

A harder way to create an array would be to create it and then code a loop to initialize it:

int[] integers = new int[5];
for (int aa = 0; aa < integers.length; aa++) {
  integers[aa] = aa;
}

This code declares the integer array of five elements. If you try to put more than five elements in the array, the Java runtime will complain and throw an exception. You'll learn about exceptions and how to handle them in Part 2.

Loading an array

To load the array, you loop through the integers from 1 through the length of the array (which you get by calling .length on the array — more about that in a minute). In this case, you stop when you hit 5.

Once the array is loaded, you can access it as before:

Logger l = Logger.getLogger("Test");
for (int aa = 0; aa < integers.length; aa++) {
  l.info("This little integer's value is: " + integers[aa]);
}

This syntax (new since JDK 5) also works:

Logger l = Logger.getLogger("Test");
for (int i : integers) {
  l.info("This little integer's value is: " + i);
}

I find the newer syntax simpler to work with, and I'll use it throughout this section.

The element index

Think of an array as a series of buckets, and into each bucket goes an element of a certain type. Access to each bucket is gained using an index:

element = arrayName [elementIndex];

To access an element, you need the reference to the array (its name) and the index where the element you want resides.

The length method

A handy method, as you've already seen, is length. It's a built-in method, so its syntax doesn't include the usual parentheses. Just type the word length and it will return — as you would expect — the size of the array.

Arrays in the Java language are zero-based. So, for some array named array, the first element in the array always resides at array[0], and the last resides at array[array.length - 1].

An array of objects

You've seen how arrays can hold primitive types, but it's worth mentioning that they can also hold objects. In that sense, the array is the Java language's most utilitarian collection.

Creating an array of java.lang.Integer objects isn't much different from creating an array of primitive types. Once again, you have two ways to do it:

// creates an empty array of 5 elements:
Integer[] integers = new Integer[5];
// creates an array of 5 elements with values:
Integer[] integers = new Integer[] { Integer.valueOf(1),
                                     Integer.valueOf(2)
                                     Integer.valueOf(3)
                                     Integer.valueOf(4)
                                     Integer.valueOf(5));

Boxing and unboxing

Every primitive type in the Java language has a JDK counterpart class, which you can see in Table 4:

Table 4. Primitives and JDK counterparts
PrimitiveJDK counterpart
booleanjava.lang.Boolean
bytejava.lang.Byte
charjava.lang.Character
shortjava.lang.Short
intjava.lang.Integer
longjava.lang.Long
floatjava.lang.Float
doublejava.lang.Double

Each JDK class provides methods to parse and convert from its internal representation to a corresponding primitive type. For example, this code converts the decimal value 238 to an Integer:

int value = 238;
Integer boxedValue = Integer.valueOf(value);

This technique is known as boxing, because you're putting the primitive into a wrapper, or box.

Similarly, to convert the Integer representation back to its int counterpart, you would unbox it, like so:

Integer boxedValue = Integer.valueOf(238);
int intValue = boxedValue.intValue();

Autoboxing and auto-unboxing

Strictly speaking, you don't need to box and unbox primitives explicitly. Instead, you could use the Java language's autoboxing and auto-unboxing features, like so:

int intValue = 238;
Integer boxedValue = intValue;
//
intValue = boxedValue;

I recommend that you avoid autoboxing and auto-unboxing, however, because it can lead to code problems. The code in the boxing and unboxing snippets is more obvious, and thus more readable, than the autoboxed code, and I believe that's worth the extra effort.

Parsing and converting boxed types

You've seen how to obtain a boxed type, but what about parsing a String you suspect has a boxed type into its proper box? The JDK wrapper classes have methods for that, too:

String characterNumeric = "238";
Integer convertedValue = Integer.parseInt(characterNumeric);

You can also convert the contents of a JDK wrapper type to a String:

Integer boxedValue = Integer.valueOf(238);
String characterNumeric = boxedValue.toString();

Note that when you use the concatenation operator in a String expression (you've already seen this in calls to Logger), the primitive type is autoboxed, and wrapper types automatically have toString() invoked on them. Pretty handy.

Lists

A List is a collection construct that is by definition an ordered collection, also known as a sequence. Because a List is ordered, you have complete control over where in the List items go. A Java List collection can only hold objects, and it defines a strict contract about how it behaves.

List is an interface, so you can't instantiate it directly. You'll work with its most commonly used implementation, ArrayList:

List<Object> listOfObjects = new ArrayList<Object>();

Note that we have assigned the ArrayList object to a variable of type List. Java programming allows you to assign a variable of one type to another so long as the variable being assigned to is a superclass or interface implemented by the variable being assigned from. We will look more at how variable assignments are affected in Part 2 in the Inheritance section.

Formal type

What's with the <Object> in the above code snip? It's called the formal type, and it tells the compiler that this List contains a collection of type Object, which means you can pretty much put whatever you like in the List.

If you wanted to tighten up the constraints on what could or could not go into the List, you'd define it differently:

List<Person> listOfPersons = new ArrayList<Person>();

Now your List can only hold Person instances.

Using Lists

Using Lists is super easy, like Java collections in general. Here are some of the things you will want to do with Lists:

  • Put something in the List.
  • Ask the List how big it currently is.
  • Get something out of the List.

Let's try some of these out. You've already seen how to create an instance of List by instantiating its ArrayList implementation type, so you'll start from there.

To put something in a List, call the add() method:

List<Integer> listOfIntegers = new ArrayList<Integer>();
listOfIntegers.add(Integer.valueOf(238));

The add() method adds the element to the end of the List.

To ask the List how big it is, call size():

List<Integer> listOfIntegers = new ArrayList<Integer>();
listOfIntegers.add(Integer.valueOf(238));
Logger l = Logger.getLogger("Test");
l.info("Current List size: " + listOfIntegers.size());

To retrieve an item from the List, call get() and pass it the index of the item you want:

List<Integer> listOfIntegers = new ArrayList<Integer>();
listOfIntegers.add(Integer.valueOf(238));
Logger l = Logger.getLogger("Test");
l.info("Item at index 0 is: " listOfIntegers.get(0));

In a real-world application, a List would contain records, or business objects, and you would possibly want to look over them all as part of your processing. How do you do that in a generic fashion? You want to iterate over the collection, which you can do because List implements the java.lang.Iterable interface. (You'll learn about interfaces in Part 2.)

Iterable

If a collection implements java.lang.Iterable, it is called an iterable collection. That means you can start at one end and walk through the collection item-by-item until you run out of items.

You've already seen the special syntax for iterating over collections that implement the Iterable interface, in the Loops section. Here it is again:

for (objectType varName : collectionReference) {
  // Start using objectType (via varName) right away...
}

Iterating over a List

That previous example was abstract; now here's a more realistic one:

List<Integer> listOfIntegers = obtainSomehow();
Logger l = Logger.getLogger("Test");
for (Integer i : listOfIntegers) {
  l.info("Integer value is : " + i);
}

That little code snip does the same thing as this longer one:

List<Integer> listOfIntegers = obtainSomehow();
Logger l = Logger.getLogger("Test");
for (int aa = 0; aa < listOfIntegers.size(); aa++) {
  Integer I = listOfIntegers.get(aa);
  l.info("Integer value is : " + i);
}

The first snip uses shorthand syntax: there is no index variable (aa in this case) to initialize, and no call to the List's get() method.

Because List extends java.util.Collection, which implements Iterable, you can use the shorthand syntax to iterate over any List.

Sets

A Set is a collections construct that by definition contains unique elements — that is, no duplicates. Whereas a List can contain the same object hundreds of times, a Set can only contain a given instance once. A Java Set collection can only hold objects, and it defines a strict contract about how it behaves.

Because Set is an interface, you can't instantiate it directly, so I'll show you one of my favorite implementations: HashSet. HashSet is easy to use and is similar to List.

Here are some things you will want to do with a Set:

  • Put something in the Set.
  • Ask the Set how big it currently is.
  • Get something out of the Set.

Using Sets

A Set's distinguishing attribute is that it guarantees uniqueness among its elements, but doesn't care about the order of the elements. Consider the following code:

Set<Integer> setOfIntegers = new HashSet<Integer>();
setOfIntegers.add(Integer.valueOf(10));
setOfIntegers.add(Integer.valueOf(11));
setOfIntegers.add(Integer.valueOf(10));
for (Integer i : setOfIntegers) {
  l.info("Integer value is: " + i);
}

You might expect that the Set would have three elements in it, but in fact it only has two because the Integer object that contains the value 10 will only be added once.

Keep this behavior in mind when iterating over a Set, like so:

Set<Integer> setOfIntegers = new HashSet();
setOfIntegers.add(Integer.valueOf(10));
setOfIntegers.add(Integer.valueOf(20));
setOfIntegers.add(Integer.valueOf(30));
setOfIntegers.add(Integer.valueOf(40));
setOfIntegers.add(Integer.valueOf(50));
Logger l = Logger.getLogger("Test");
for (Integer i : setOfIntegers) {
  l.info("Integer value is : " + i);
}

Chances are the objects will print out in a different order than you added them in because a Set guarantees uniqueness, not order. You'll see this for yourself if you paste the code above into the main() method of your Person class and run it.

Maps

A Map is a handy collection construct because it lets you associate one object (the key) with another (the value). As you might imagine, the key to the Map must be unique, and it's used to retrieve the value at a later time. A Java Map collection can only hold objects, and it defines a strict contract about how it behaves.

Because Map is an interface, you can't instantiate it directly, so I'll show you one of my favorite implementations: HashMap.

Here are some of the things you will want to do with Maps:

  • Put something in the Map.
  • Get something out of the Map.
  • Get a Set of keys to the Map— for iterating over it.

Using Maps

To put something into a Map, you need to have an object that represents its key and an object that represents its value:

public Map<String, Integer> createMapOfIntegers() {
  Map<String, Integer> mapOfIntegers = new HashMap<String, Integer>();
  mapOfIntegers.put("1", Integer.valueOf(1));
  mapOfIntegers.put("2", Integer.valueOf(2));
  mapOfIntegers.put("3", Integer.valueOf(3));
  // . . .
  mapOfIntegers.put("168", Integer.valueOf(168));
}

In this example, Map contains Integers, keyed by a String, which happens to be their String representation. To retrieve a particular Integer value, you need its String representation:

mapOfIntegers = createMapOfIntegers();
Integer oneHundred68 = mapOfIntegers.get("168");

Using Set with Map

On occasion, you may find yourself with a reference to a Map, and you simply want to walk over its entire set of contents. In this case, you will need a Set of the keys to the Map:

Set<String> keys = mapOfIntegers.keySet();
Logger l = Logger.getLogger("Test");
for (String key : keys) {
  Integer value = mapOfIntegers.get(key);
  l.info("Value keyed by '" + key + "' is '" + value + "'");
}

Note that the toString() method of the Integer retrieved from the Map is automatically called when used in the Logger call. Map doesn't return a List of its keys because the Map is keyed, and each key is unique; uniqueness is the distinguishing characteristic of a Set.


Archiving Java code

Now that you've learned a bit about writing Java applications, you may be wondering how to package them up so that other developers can use them, or how to import code from other developers into your applications. This section shows you how.

JARs

The JDK ships with a tool called JAR, which stands for Java Archive. You use this tool to create JAR files. Once you've packaged your code into a JAR file, other developers can simply drop the JAR file into their projects and configure their projects to use your code.

Creating a JAR file in Eclipse is a snap. In your workspace, right-click the com.makotogroup.intro package and select Export. You'll see the dialog shown in Figure 8. Choose Java > JAR file.

Figure 8. Export dialog box.
Eclipse's export dialog box.

When the next dialog box opens, browse to the location where you want to store your JAR file and name the file whatever you'd like. The .jar extension is the default, which I recommend using. Click Finish.

You'll see your JAR file in the location you selected. You can use the classes in it from your code if you put it in your build path in Eclipse. Doing that is a snap, too, as you'll see next.

Using third-party applications

As you grow more comfortable with writing Java applications, you will want to use more and more third-party applications to support your code. For the sake of example, let's say that you wanted to use joda-time, a JDK replacement library for doing date/time handling, manipulations, and calculations.

Let's assume you've already downloaded joda-time, which is stored in a JAR file. To use the classes, your first step is to create a lib directory in your project and drop the JAR file into it:

  1. Right-click the Intro root folder in Project Explorer view.
  2. Choose New > Folder and call the folder lib.
  3. Click Finish.

The new folder shows up at the same level as src. Now copy the joda-time .jar file into your new lib directory. For this example, the file is called joda-time-1.6.jar. (It's common in naming a JAR file to include the version number.)

Now all you need to do is tell Eclipse to include the classes in the joda-time-1.6.jar file into your project:

  1. Right-click the Intro project in your workspace, then select Properties.
  2. In the Properties dialog box, select the Libraries tab, as shown in Figure 9:
    Figure 9. Properties > Java Build Path
    Path to select the Properties dialog box and Libraries tab.
  3. Click the Add External JARs button, then browse to the project's lib directory, select the joda-time-1.6.jar file, and click OK.

Once the code (that is, the class files) in the JAR file have been processed by Eclipse, they are available to reference (import) from your Java code. Notice in Project Explorer that there is a new Folder called Referenced Libraries that contains the joda-time-1.6.jar file.


Writing good Java code

You've got enough Java syntax under your belt to write basic Java programs, which means the first half of this tutorial is about to conclude. This final section lays out a few best practices that will help you write cleaner, more maintainable Java code.

Keep classes small

You created a few classes in this tutorial. After generating getter/setter pairs for even the small number (by the standards of a real-world Java class) of attributes, the Person class has 150 lines of code. This is a small class. It's not uncommon to see classes with 50 or 100 methods and a thousand lines of source (or more). The point to methods is to keep just the ones you need. If you need several helper methods that do essentially the same thing but take different parameters (such as the printAudit() method), that's a fine choice. Just be sure to limit the list of methods to what you need, and no more.

In general, classes represent some conceptual entity in your application, and their size should reflect only the functionality to do whatever that entity needs to do. They should stay tightly focused to do a small number of things and do them well.

Name methods carefully

A good coding pattern when it comes to method names is the intention-revealing method-names pattern. This pattern is easiest to understand with a simple example. Which of the following method names is easier to decipher at a glance?

  • a()
  • computeInterest()

The answer should be obvious, yet for some reason, programmers have a tendency to give methods (and variables, for that matter) small, abbreviated names. Certainly, a ridiculously long name can be inconvenient, but a name that conveys what a method does needn't be ridiculously long. Six months after you write a bunch of code, you may not remember what you meant to do with a method called a(), but it's obvious that a method called computeInterest(), well, probably computes interest.

Keep methods small

Small methods are just as preferable as small classes, and for similar reasons. One idiom I try to follow is to keep the size of a method to one page as I look at it on my screen. This makes my application classes more maintainable.

If a method grows beyond one page, I refactor it. Refactoring means changing the design of existing code without changing its results. Eclipse has a wonderful set of refactoring tools. Usually a long method contains subgroups of functionality bunched together. Take this functionality and move it to another method (naming it accordingly) and pass in parameters as needed.

Limit each method to a single job. I've found that a method doing only one thing well doesn't usually take more than about 30 lines of code.

Use comments

Please, use comments. The people who follow along behind you (or even you, yourself, six months down the road) will thank you. You may have heard the old adage Well-written code is self-documenting, so who needs comments? I'll give you two reasons why this is false:

  • Most code is not well written.
  • Your code probably isn't as well written as you'd like to think.

So, comment your code. Period.

Use a consistent style

Coding style is really a matter of personal preference, but I would like to offer this piece of advice: use standard Java syntax for braces:

public static void main(String[] args) {
}

Don't use this style:

public static void main(String[] args) 
{
}

Or this one:

public static void main(String[] args) 
  {
  }

Why? Well, it's standard, so most code you run across (as in, code you didn't write, but might be paid to maintain) will most likely be written that way. Having said that, Eclipse does allow you to define code styles and format your code any way you like. The main thing is that you pick a style and stick with it.

Use built-in logging

Before Java 1.4 introduced built-in logging, the canonical way to find out what your program was doing was to make a system call like this one:

public void someMethod() {
  // Do some stuff...
  // Now tell all about it
  System.out.println("Telling you all about it:");
  // Etc...
}

The Java language's built-in logging facility (refer back to Your first Java object) is a better alternative. I never use System.out.println() in my code, and I suggest you don't use it either.

In the footsteps of Fowler

The best book in the industry (in my opinion, and I'm not alone) is Refactoring: Improving the Design of Existing Code, by Martin Fowler et al. (see Resources). It's even fun to read. Fowler and his co-authors talk about "code smells" that beg for refactoring, and they go into great detail about the various techniques for fixing them.

In my opinion, refactoring and the ability to write test-first code are the most important skills for new programmers to learn. If everybody were good at both, it would revolutionize the industry. If you become good at both, you will ultimately produce cleaner code and more functional applications than many of your peers.


Conclusion to Part 1

In this tutorial, you've learned about object-oriented programming, discovered Java syntax that lets you create useful objects, and familiarized yourself with an IDE that helps you control your development environment. You know how to create and run Java objects that can do a good number of things, including doing different things based on different input. You also know how to JAR up your applications for other developers to use in their programs, and you've got some basic best Java programming practices under your belt.

What's next

In the second half of this tutorial, you will begin learning about some of the more advanced constructs of Java programming, although the overall discussion will still be introductory in scope. Java programming topics covered in that tutorial include:

  • Inheritance and abstraction
  • Interfaces
  • Nested classes
  • Regular expressions
  • Generics
  • Enum types
  • I/O
  • Serialization

Read "Introduction to Java programming, Part 2: Constructs for real-world applications."

Resources

Learn

Get products and technologies

  • JDK 6: Download JDK 6 from Sun (Oracle).
  • Eclipse: Download the Eclipse IDE for Java Developers.
  • IBM developer kits: IBM provides a number of Java developer kits for use on popular platforms.

Discuss

  • Get involved in the My developerWorks community. Connect with other developerWorks users while exploring the developer-driven blogs, forums, groups, and wikis.

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ArticleTitle=Introduction to Java programming, Part 1: Java language basics
publish-date=08192010