Hi Mohan,
Here is the link to the runtime api documentation.
https://apidocs.mendix.com/7/runtime/
What are you trying to accomplish? Any reason why you can't do what Ronald suggested, and use a microflow?
You cannot use arrays/lista as attribute. Instead use associations in the domain model. It is just a one to many association between two entities which is conceptually the right way to model what you want to accomplish. Furthermore only use java as a last resort.