This question already has an answer here:
Is there a way to find out what imports a class has?
In this question: Jon Skeet says that you can't do this using reflection, but
If you want to find all the types used within the compiled code, that's a slightly different matter. You may want to look at BCEL as a way of analyzing bytecode.
This is what I want to know how to do.
Here is an old tutorial for ASM bytecode manipulation framework explaining how to read class dependencies with ASM.
You can use a Java Decompiler like this one: Java Decompiler