The A2A.java class contains some method for creating user and agent messages.

There are additional things that can be configured when creating messages like contextId, taskId, parts, etc.

Take a look at the utility methods for handling Messages from a2a-python and then update/add similar functionality to A2A.java:
https://github.com/a2aproject/a2a-python/blob/main/src/a2a/utils/message.py

Appropriate test cases should be added too. Use the Python tests as a guide:

https://github.com/a2aproject/a2a-python/blob/main/tests/utils/test_message.py