Java

Question

作为 Java 的初学者，我尝试在 windowsbuilder（eclipse 插件）的帮助下创建一个 Window 应用程序 "Game"。现在，当我尝试更改外部函数 (allblack()) 中的标签时，当我按下特定按钮时，控制台发出以下错误（window 正确显示）：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Leiterspiel.allblack(Leiterspiel.java:81)
    at Leiterspiel.mouseClicked(Leiterspiel.java:234)
    at java.awt.AWTEventMulticaster.mouseClicked(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access0(Unknown Source)
    at java.awt.EventQueue.run(Unknown Source)
    at java.awt.EventQueue.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.run(Unknown Source)
    at java.awt.EventQueue.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Leiterspiel.allblack(Leiterspiel.java:81)
    at Leiterspiel.mouseClicked(Leiterspiel.java:234)
    at java.awt.AWTEventMulticaster.mouseClicked(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access0(Unknown Source)
    at java.awt.EventQueue.run(Unknown Source)
    at java.awt.EventQueue.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.run(Unknown Source)
    at java.awt.EventQueue.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

如果有人能向我解释它，因为 "allblack()" 中的相同代码在 "inizialize()" 中有效，我将不胜感激。谢谢

[Pre-sampled import]
public void allblack()
 {
    l1.setForeground(Color.black);
    l2.setForeground(Color.black);
 }
 public void initialize() {

    frmDasLeiterspiel = new JFrame();
    frmDasLeiterspiel.setResizable(false);
    frmDasLeiterspiel.setTitle("Das Leiterspiel");
    frmDasLeiterspiel.setBounds(100, 100, 420, 420);
    frmDasLeiterspiel.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frmDasLeiterspiel.getContentPane().setLayout(null);

    JLabel l1 = new JLabel("1");
    l1.setBackground(Color.LIGHT_GRAY);
    l1.setBounds(340, 190, 50, 50);
    l1.setHorizontalAlignment(SwingConstants.CENTER);
    frmDasLeiterspiel.getContentPane().add(l1, BorderLayout.CENTER);

    JLabel l2 = new JLabel("2");
    l2.setBackground(Color.LIGHT_GRAY);
    l2.setHorizontalAlignment(SwingConstants.CENTER);
    l2.setBounds(278, 190, 50, 50);
    frmDasLeiterspiel.getContentPane().add(l2);
    [....]
    JButton btnNewButton = new JButton("Roll the dice!");
    btnNewButton.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent arg0) {
            allblack();
        }
    });
    btnNewButton.setBounds(12, 294, 378, 25);
    frmDasLeiterspiel.getContentPane().add(btnNewButton);
    [...]
    }

Answer 1

看起来变量l1和l2只存在于initialize方法中。试试这个：

public void allblack() {
    l1.setForeground(Color.black);
    l2.setForeground(Color.black);
}

private JLabel l1,l2;
public void initialize() {

    frmDasLeiterspiel = new JFrame();
    frmDasLeiterspiel.setResizable(false);
    frmDasLeiterspiel.setTitle("Das Leiterspiel");
    frmDasLeiterspiel.setBounds(100, 100, 420, 420);
    frmDasLeiterspiel.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frmDasLeiterspiel.getContentPane().setLayout(null);

    l1 = new JLabel("1");
    l1.setBackground(Color.LIGHT_GRAY);
    l1.setBounds(340, 190, 50, 50);
    l1.setHorizontalAlignment(SwingConstants.CENTER);
    frmDasLeiterspiel.getContentPane().add(l1, BorderLayout.CENTER);

    l2 = new JLabel("2");
    l2.setBackground(Color.LIGHT_GRAY);
    l2.setHorizontalAlignment(SwingConstants.CENTER);
    l2.setBounds(278, 190, 50, 50);
    frmDasLeiterspiel.getContentPane().add(l2);
    [....]
    JButton btnNewButton = new JButton("Roll the dice!");
    btnNewButton.addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent arg0) {
            allblack();
        }
    });
    btnNewButton.setBounds(12, 294, 378, 25);
    frmDasLeiterspiel.getContentPane().add(btnNewButton);
    [...]
    }

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