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Many IDEs have tools to find matching {}s. Put the cursor on {, press the keys/icon and the cursor jumps to the matching }.

Use this tool starting in the middle and work out, making sure that all...

Copy and pasting can be faster but can be a problem if you forget to change it for the new code.

Sorry, when I searched for "// called when a '-' sign is pressed" it was not found in this post...

Hope you backed up the working version.
I would suggest that you start with the working version and make small changes to it, compile it to test for errors and then make some more small changes,...

Please enclose your posted code in code tags to preserve formatting. Use the # icon above the input area to generate the code tags. Not the # character. See: BB Code List - Java Forums

int 1 =...

Yes that should work. The code for the four operations should be almost the same except for the operation itself.

Since those operations all take two operands, you should be able to use the logic for the add with the other three operations. That would be the simplest way.
Another more OOP technique could...