Java Swing Action 执行

Question

您好，我正在一个项目中工作，我必须创建一个 CUI 游戏，现在将其转换为 GUI，我正在向与游戏交互的按钮添加功能，但遇到了麻烦。

这是我尝试访问的方法。它属于一个名为 Pet 的类。

public double feedPet() {

    if(this.hunger < MAX_HUNGER) {
        this.hunger += food;
            if(this.hunger > MAX_HUNGER) {
                this.hunger = MAX_HUNGER;
            }
        System.out.println(this.petName + " enjoyed the meal!");
        System.out.println("Hunger increased by " + food + " to a total of " + this.hunger);
        spacing();
    }
    else {
        System.out.println(this.petName + " is currently full!");
        spacing();
    }

    return this.hunger;
}

这是尝试访问该方法的类。

public GamePanel(PetWorld petWorld, Pet pet, Game game) {
    initComponents();
    genComponents(pet);
}

private void genComponents(final Pet pet) {


    this.setSize(600, 600);

    graphicsPanel.setSize(600, 400);
    graphicsPanel.setBackground(Color.red);

    buttonPanel.setSize(600, 200);

    buttonPanel.setBorder(BorderFactory.createTitledBorder("Pet Options"));

    JButton feedBtn = new JButton("Feed");
    JButton playBtn = new JButton("Play");
    JButton sickBtn = new JButton("Medicine");
    JButton trainBtn = new JButton("Train");
    JButton sleepBtn = new JButton("Sleep");

    //// error occurring here I believe \\\\
    feedBtn.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            pet.feedPet();
        }
    });

    buttonPanel.setLayout(new GridBagLayout());

    GridBagConstraints gc = new GridBagConstraints();

    //// Pet Options \\\\
    gc.weightx = 0.5;
    gc.weighty = 0.5;

    gc.gridx = 0;
    gc.gridy = 1;
    buttonPanel.add(feedBtn, gc);

    gc.gridx = 1;
    gc.gridy = 1;
    buttonPanel.add(playBtn, gc);

    gc.gridx = 2;
    gc.gridy = 1;
    buttonPanel.add(sickBtn, gc);

    gc.gridx = 3;
    gc.gridy = 1;
    buttonPanel.add(trainBtn, gc);

    gc.gridx = 4;
    gc.gridy = 1;
    buttonPanel.add(sleepBtn, gc);        

}

我必须把宠物做成最终的，我相信这很糟糕，因为我改变了宠物本身的值(value)观，例如幸福，饥饿等

按下按钮时出现的错误

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at pdcmainGUI.GamePanel$1.actionPerformed(GamePanel.java:63)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at     javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6505)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
    at java.awt.Component.processEvent(Component.java:6270)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4861)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2719)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at     java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at     java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:708)
    at java.awt.EventQueue$4.run(EventQueue.java:706)
    at java.security.AccessController.doPrivileged(Native Method)
    at     java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

有更好的方法来实现我想要做的事情吗？我猜这是由于尝试更改变量值但在 gui 类中设置为 Final 的方法引起的？任何建议表示赞赏。

Answer 1

问题与您对 Action 监听器的声明有关。当您在 genComponents(final Pet pet) 过程中创建 ActionListener() 时，这不是它在运行时执行的位置。

但首先，我们还需要明确将变量调用为 final。这意味着 pet 无法重新分配给不同的 pet，但这并不意味着您无法访问 pet 的方法可能会改变，例如 isHungry()、setFed(boolean) 等。

第二，让你的宠物进入范围。当您定义genComponents(final Pet pet)过程时，pet在作用域内，因此java编译器在编译时不会出现问题。但在运行时，genComponents(final Pet pet) 过程会在 pet 已知的情况下执行，并在此执行期间创建 ActionListener，指向参数的变量位置。 genComponents(...) 过程完成后，参数超出范围。当按下按钮触发 ActionListener 时，它会尝试引用超出范围的变量的内存位置。

private void genComponents(final Pet pet) {

    ....

    feedBtn.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            pet.feedPet();   <-- pet refers to the parameter, but it is out of scope when this gets executed.
        }
    });

    ....
}

解决方案空间:要解决此问题，需要从范围内的某个位置访问 pet，您只需将 pet 存储为私有(private)类变量即可做到这一点.

public class GamePanel {

    private final Pet pet;

    public GamePanel(PetWorld petWorld, Pet pet, Game game) {
        this.pet = pet;
        initComponents();
        genComponents(); <-- remove the pet parameter
    }

    private void genComponents() {
        ....

        feedBtn.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                pet.feedPet();    <-- this pet now refers to the class variable, which is still in scope.
            }
        });

        ....
    }
}