GUI卡布局-操作侦听器不起作用[重复]

Question

此问题已经在这里有了答案：

What is a NullPointerException, and how do I fix it?（12个答案）

[<11天前关闭。

我只有第一个按钮可以工作。直到我将第一个面板更新为具有其他面板（oneTwo，oneThree等）之前，它一直工作良好。我这样做是因为我必须使程序看起来像这样=> Example

但是现在当我单击第一个按钮时，它不再切换到第二张卡。我不知道。

这是我的代码

public class AddressBook implements ActionListener {
    static CardLayout contentPaneLayout;
    static JButton B1= new JButton("Create a New Address"),  B2= new JButton("Load Contacts"), B3= new JButton("Add New Contacts"),
            B4= new JButton("View/Delete Contacts"), B5= new JButton("Backup Contacts"),  B6= new JButton("Exit"), B7 = new JButton ("Cancel");
    static JTextField TF1 = new JTextField(13), TF2 = new JTextField (13);
    static JLabel L1 = new JLabel ("Use The Buttons Below To Manage Contacts",JLabel.CENTER), L2 = new JLabel ("This is Card 2"), L3 = new JLabel("Username:",JLabel.CENTER),
            L4 = new JLabel("Number of Contacts:",JLabel.CENTER);;
    static Container contentPane;


    public void actionPerformed(ActionEvent e) {
        Object click = e.getSource();
        if (click==B1) contentPaneLayout.show(contentPane,"Card 2");

        if (click==B6) System.exit(0);
        if (click==B7) contentPaneLayout.show(contentPane, "Card 1");




    }

    public static void main(String[] args) {
        JFrame frm = new JFrame("Address Book");
        contentPane = frm.getContentPane();
        contentPane.setLayout(new CardLayout());
        JPanel one = new JPanel (new GridLayout( 5,5 ));
        JPanel two = new JPanel (new GridLayout(4,6));

        JPanel oneTwo = new JPanel(new FlowLayout());
        JPanel oneThree = new JPanel(new GridLayout(2,3,6,5));
        JPanel oneFour = new JPanel(new GridLayout());
        JPanel oneFive = new JPanel(new GridLayout());




        one.add (L1);
        oneTwo.add (L3);
        oneTwo.add (TF1);
        TF1.setEditable(false);
        TF2.setEditable(false);
        oneTwo.add(L4);
        oneTwo.add(TF2);


        one.add(oneFive);
        one.add(oneTwo);
        oneThree.add(B1); oneThree.add(B2);oneThree.add(B3);oneThree.add(B4);oneThree.add(B5);oneThree.add(B6);
        one.add(oneFour);
        one.add(oneThree);

        two.add(L2);
        two.add(B7);

        contentPane.add("Card 1", one);
        contentPane.add("Card 2", two) ;
        //contentPaneLayout.show(contentPane, "Card 1");



        ActionListener AL= new AddressBook();
        B1.addActionListener(AL); B7.addActionListener(AL); B6.addActionListener(AL);
        frm.pack();
        frm.setSize(550,250);
        frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frm.setVisible(true);
    }

}

Answer 1

您有NullPointerException

contentPaneLayout.show(contentPane, "Card 2");

contentPaneLayout从未分配值。
过度使用static会加剧这种情况，>
一点点的返工，您有一个更好的解决方案
import java.awt.CardLayout;
import java.awt.Container;
import java.awt.EventQueue;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class AddressBook implements ActionListener {

    private CardLayout contentPaneLayout;
    private JButton b1 = new JButton("Create a New Address"), b2 = new JButton("Load Contacts"), b3 = new JButton("Add New Contacts"),
            b4 = new JButton("View/Delete Contacts"), b5 = new JButton("Backup Contacts"), b6 = new JButton("Exit"), b7 = new JButton("Cancel");
    private JTextField tf1 = new JTextField(13), tf2 = new JTextField(13);
    private JLabel l1 = new JLabel("Use The Buttons Below To Manage Contacts", JLabel.CENTER), l2 = new JLabel("This is Card 2"), l3 = new JLabel("Username:", JLabel.CENTER),
            l4 = new JLabel("Number of Contacts:", JLabel.CENTER);

    private Container contentPane;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                new AddressBook();
            }
        });
    }

    public AddressBook() {
        JFrame frm = new JFrame("Address Book");
        contentPane = frm.getContentPane();
        contentPaneLayout = new CardLayout();
        contentPane.setLayout(contentPaneLayout);
        JPanel one = new JPanel(new GridLayout(5, 5));
        JPanel two = new JPanel(new GridLayout(4, 6));

        JPanel oneTwo = new JPanel(new FlowLayout());
        JPanel oneThree = new JPanel(new GridLayout(2, 3, 6, 5));
        JPanel oneFour = new JPanel(new GridLayout());
        JPanel oneFive = new JPanel(new GridLayout());

        one.add(l1);
        oneTwo.add(l3);
        oneTwo.add(tf1);
        tf1.setEditable(false);
        tf2.setEditable(false);
        oneTwo.add(l4);
        oneTwo.add(tf2);

        one.add(oneFive);
        one.add(oneTwo);
        oneThree.add(b1);
        oneThree.add(b2);
        oneThree.add(b3);
        oneThree.add(b4);
        oneThree.add(b5);
        oneThree.add(b6);
        one.add(oneFour);
        one.add(oneThree);

        two.add(l2);
        two.add(b7);

        contentPane.add("Card 1", one);
        contentPane.add("Card 2", two);
        //contentPaneLayout.show(contentPane, "Card 1");

        b1.addActionListener(this);
        b7.addActionListener(this);
        b6.addActionListener(this);
        frm.pack();
        frm.setSize(550, 250);
        frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frm.setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        Object click = e.getSource();
        System.out.println("...");
        if (click == b1) {
            contentPaneLayout.show(contentPane, "Card 2");
        }

        if (click == b6) {
            System.exit(0);
        }
        if (click == b7) {
            contentPaneLayout.show(contentPane, "Card 1");
        }

    }

}
重要的变化是在构造函数中...
contentPaneLayout = new CardLayout();
contentPane.setLayout(contentPaneLayout);