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Yes, what you want is a Java decompiler.
There are quite a few available if you search for them on the internet.
here is one: "http://java.decompiler.free.fr"

There are some limitations, so you...

I think you need to understand how the compiler works, it creates .class files from source files .java, you actually cannot call a method from a .java file, but you can from the compiled file .class....