Take a look at this technote : http://www.ibm.com/support/docview.wss?uid=swg21426000
It explains a little bit how Maps are used in IBMJS.
To enable Java methods directly in IBMJS you will have to add the to the scriptframework.properties. This line will add support for java.util :
But be aware that casting in the normal Java way is npot supported in IBMJS.