Is it possible to make jar file to .java ?

It depends.

A jar file is just an archive. A normal distribution would only contain class files and may also include the javadoc.
When building a jar for a solution it is possible to include the java files as well, for debugging purposes. You'll find this is the case with most, if not all, of the classes in the core api. As an example in netbeans I can navigate to the source for the String class; try this yourself.

If the jar only contains the class files then the process is much harder, but not impossible. Using reflection you can extract the signatures for all the methods but the hard part is decoding the byte code. That I will leave as an exercise for the reader.

The above answer is right, you can extract JAR file but it only contains class files but you can convert it to .java if you want using Java Decompiler.