Only use what I gave within the method. Delete the field above the constructor (with no replacement there).

I deleted the field above the constructor (line 19 of button) and then put on line 19 of button. Am I supposed to put this line of code in methods instead of above the constructor? Because the code is causing an error...

Put it in any and all methods where needed.

It works without throwing an exception in the terminal, but there's still the issue of currentIndex not increasing at all within Calculate class, and a new issue of ... Never mind, I made a couple small changes while typing this message which fixed these issues. All I had to do was change '//world.updateDisplay();' from a comment to code (in button class) and add 'Calculator world = ((Calculator)getWorld());' a couple lines above it. For some reason, hitting an operator throws an exception, but I haven't tried resolving that issue so I'll post another reply if I'm having difficulties with it. Should likely be a simple fix (hopefully). Edit: It's because the add method in calculate is unfinished. Easy enough to solve, I'll get working on that.