Why Qt When java is there?

[high_flyer]

Qt is a a frame work, a tool kit.
Java is a language.
Therefore provided bindings, you can use Qt in Java as well, and other languages too.

Besides, your question is very wide in scope, certainly not for a forum post.
Read about Qt and its strengths and weaknesses.
Read about the same regarding Java.
Then choose either or both depending on the requirements of your project.
Took kits and languages are tools, and you should use your tools according to the tasks you need to accomplish.

You wont use a hammer to drive a screw right?
Its not different with programming.