Help me with Java Programming!!!?
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I am using an ActionListener to a TextField so that when the enter key is pushed by the user the TextField is disabled and a button called "Activate Buttons" is enabled. The activateButton is contained in a separate class than the TextField because there are two active windows. When I try using: GameField.activateButton.setEnabled(tr… Multiple errors appear. Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at project1.ScoreBoard$TextField1Listener.a… at javax.swing.JTextField.fireActionPerform… Source) at javax.swing.JTextField.postActionEvent(U… Source) at javax.swing.JTextField$NotifyAction.acti… Source) at javax.swing.SwingUtilities.notifyAction(… Source) at javax.swing.JComponent.processKeyBinding… Source) at javax.swing.JComponent.processKeyBinding… Source) at javax.swing.JComponent.processKeyEvent(U… Source) at java.awt.Component.processEvent(Unknown Source) at java.awt.Container.processEvent(Unknown Source) at java.awt.Component.dispatchEventImpl(Unk… Source) at java.awt.Container.dispatchEventImpl(Unk… Source) at java.awt.Component.dispatchEvent(Unknown Source) at java.awt.KeyboardFocusManager.redispatch… Source) at java.awt.DefaultKeyboardFocusManager.dis… Source) at java.awt.DefaultKeyboardFocusManager.pre… Source) at java.awt.DefaultKeyboardFocusManager.typ… Source) at java.awt.DefaultKeyboardFocusManager.dis… Source) at java.awt.Component.dispatchEventImpl(Unk… Source) at java.awt.Container.dispatchEventImpl(Unk… Source) at java.awt.Window.dispatchEventImpl(Unknow… Source) at java.awt.Component.dispatchEvent(Unknown Source) at java.awt.EventQueue.dispatchEvent(Unknow… Source) at java.awt.EventDispatchThread.pumpOneEven… Source) at java.awt.EventDispatchThread.pumpEventsF… Source) at java.awt.EventDispatchThread.pumpEventsF… Source) at java.awt.EventDispatchThread.pumpEvents(… Source) at java.awt.EventDispatchThread.pumpEvents(… Source) at java.awt.EventDispatchThread.run(Unknown Source)
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Answer:
not sure without seeing your code, but JTextfield was built for FocusListener. When the Enter key (default) is 'heard' the JTextfield goes out of focus and that is the event you want. If JButton is another class and not an instance var, then you would have needed another field in the constructor of that button to reference the method of the unrelated window class. Or, use an interface. Or, put everyting in one code listing, using inline classes -- which I don't really like to do, too much scrolling up and down through the listing while trying to remember the connectivity. Remember, an action bubbles up through the heirarchy until it is consumed.
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Other answers
Looks like you forgot to initialize one or more of your controls. In Java, object variables must be declared and initialized. This will produce an error... a null reference exception to be exact. JButton a; a.setText("Hello"); You need to do this: JButton a = new JButton(); a.setText("Hello"); Make sure you're actually creating (with the keyword new) all your controls.
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