将JButton actionListener传递给另一个Java类时出现Nullpointer异常

Question

I have two classes ( Main_Menu and CancelListener ). When i called an instance of my CancelListener to use the proper actionPerformed method, i get a NullPointerException .

In my MainMenu.java class, i declare the button like this:

JButton button = new JButton();
button.addActionListener(new CancelListener());

Here is my CancelListener() class.

class CancelListener implements ActionListener {

    private Main_Menu menu;
    public JPanel mpan;

    public CancelListener() {
    }

    @Override
    public void actionPerformed(ActionEvent ae) {


        System.out.println("worked");
        try {

            CardLayout c1 = (CardLayout) (menu.MainPanel.getLayout());
            c1.show(menu.MainPanel, "AppPanel");
        } catch (Exception e) {

            System.out.println("Exception here " + e);
        }
    }
}

Further Information: MainPanel is a JPanel with a CardLayout structure. On the Cancel Button, i want to show a particular Panel. I wanted to separate my button view class from the button logic? it works when i dont use a separate class.

Answer 1

You need the menu variable to be set:

In CancelListener class:

private Main_Menu menu;

public CancelListener(Main_Menu menu) {
    this.menu = menu;
}

Then, when constructing:

button.addActionListener(new CancelListener(this));