从另一个类向JButton添加ActionListener会导致NullPointerException吗？

Question

I want to be able to add an AcitonListener to a JButton from another class just to keep the code neat. The only problem is that it brings up a NullPointerException when trying to add it.

I'm adding the ActionListener through the Handler class which is defined as 'h'.

In my Display class:

public class Display {

    private Handler h; //My handler object

    private JFrame frame;
    private JButton btnCalculate;

    /**
     * Launch the application.
     */
    public static void createDisplay() {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Display window = new Display();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the application.
     */
    public Display() {
        initialize();
    }

    /**
     * Initialize the contents of the frame.
     */
    private void initialize() {
        frame = new JFrame();
        frame.setBounds(100, 100, 450, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(null);

        btnCalculate = new JButton("Calculate");
        btnCalculate.setBounds(66, 208, 89, 23);
        btnCalculate.addActionListener(h.getCalculateListener()); //ActionListener is added here.
        frame.add(btnCalculate);
    }

    //Getter for the JButton
    public JButton getBtnCalculate() {
        return btnCalculate;
    }

}

Here's my Handler class that includes mostly getters. It helps make everything a lot more simple by having all the getters in the Handler class instead of having them spread out in multiple different classes:

public class Handler {
    private Display display; //My Display object
    private CalculateActionListener calculate; //My CalculateActionListener object

    public Handler(Display display, CalculateActionListener calculate) {
        this.display = display;
        this.calculate = calculate;
    }

    public JButton getButton() {
        return display.getBtnCalculate();
    }

    public ActionListener getCalculateListener() {
        return calculate.getCalculateListener();
    }
}

And finally, here's my CalculateActionListener class which contains the actual ActionListener:

public class CalculateActionListener {

    //Here's the actual ActionListener
    private ActionListener calculateListener = new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            JOptionPane.showMessageDialog(null, "WORKING!");
        }
    };

    //Getter for my ActionListener
    public ActionListener getCalculateListener() {
        return calculateListener;
    }
}

Note Imports were removed but are there in the actually code. Imports are not the problem.

Answer 1

You are not creating a new instance of Handler . Before you use h , you need to create an instance of it. Add a line before you create the button in the Display class constructor. Something like this:

h = new Handler(this, new CalculateActionListener());