Hi Sandeep,
Unfortunately, there is no API support for custom reports. Custom reports are part of the analytics service. The only API available for the analytics service is LookBack API which can only query artifacts. The reports are a separate section and the Lookback API as well as any other API have no access to that configuration. And so, the Java or any of our tool-kits will have no access either.
The only possible access is using the SDK. This is a Javascript framework that supports developing actual UI extensions to Agile Central (see: Agile Central App SDK 2.1 Docs ), but that's interactive only, not API based.
You are welcome to post this idea (allowing API access to custom reports) for future feature request to our ideas site at: https://ideas.rallydev.com . This site requires a username, if you don't have one then you can let us know here or email at: ac-ideas@ca.com .
Thanks,
Sagi