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Break the problem down into smaller steps 1) get the user entered value - you can use the getText() method to retrieve this from a JTextField 2) Convert that value to something the method you wish to...

What have you tried? What exactly are the errors?

What do you mean by 'link up'? Do you mean call the methods of Calculator from the action listeners? As ButtonDemo extends Calculator, you can just directly call the method(s), passing entered value...