Question

我创建了一个程序，它应该显示一个宇宙飞船（基本上是两个矩形），并在每次按下空格键时从中射出一颗子弹。虽然因为很明显，我的程序将一个空的 Ship 引用传递给我的其他类 MoveAction 类，但它并不起作用。我完全不知道为什么会这样，因为在我的 Main 类中，我在 Shoot 类之前实例化我的 Ship 对象（调用使用ship对象的构造函数）。我很抱歉，如果这个问题有点愚蠢，因为我仍然是一个新手程序员，他不知道他大部分时间都在做什么:)这里＆＃39我的代码：

public enum Direction {
    LEFT, RIGHT, SPACE
}

import javax.swing.JFrame;

public class Main {
    public static void main(String[] args) {
        JFrame frame;

        Ship s1;
        Shoot shoot;

        // Set the frame up
        frame = new JFrame();
        frame.setSize(400, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);
        frame.setVisible(true);

        // Get some more necessary objects
        s1 = new Ship();
        shoot = new Shoot(s1);
        frame.getContentPane().add(shoot);
        s1.setShoot(shoot);

        // Threads
        Thread ship = new Thread(s1);
        ship.start();
    }
}

import java.awt.Graphics;
import java.awt.event.KeyEvent;

import javax.swing.Action;
import javax.swing.ActionMap;
import javax.swing.InputMap;
import javax.swing.JPanel;
import javax.swing.KeyStroke;

public class Shoot extends JPanel {

    Ship s1;

    public Shoot(Ship s1) {
        this.s1 = s1;

        addKeyBinding(KeyEvent.VK_LEFT, "left.pressed", new MoveAction(true, s1, Direction.LEFT), true);
        addKeyBinding(KeyEvent.VK_LEFT, "left.released", new MoveAction(false, s1, Direction.LEFT), false);

        addKeyBinding(KeyEvent.VK_RIGHT, "right.pressed", new MoveAction(true, s1, Direction.RIGHT), true);
        addKeyBinding(KeyEvent.VK_RIGHT, "right.released", new MoveAction(false, s1, Direction.RIGHT), false);

        addKeyBinding(KeyEvent.VK_SPACE, "space.pressed", new MoveAction(true, s1, Direction.SPACE), true);
        addKeyBinding(KeyEvent.VK_SPACE, "space.released", new MoveAction(false, s1, Direction.SPACE), false);

        setDoubleBuffered(true);
    }

    @Override
    public void paintComponent(Graphics g) {
        // Draw the ship
        super.paintComponent(g);
        s1.draw(g);
        g.fill3DRect(40, 50, 10, 10, false);
    }

    protected void addKeyBinding(int keyCode, String name, Action action, boolean keyPressed) {
        if (keyPressed) {
            addKeyBinding(KeyStroke.getKeyStroke(keyCode, 0, false), name, action);
        } else {
            addKeyBinding(KeyStroke.getKeyStroke(keyCode, 0, true), name, action);
        }
    }

    protected void addKeyBinding(KeyStroke keyStroke, String name, Action action) {
        InputMap inputMap = getInputMap(WHEN_IN_FOCUSED_WINDOW);
        ActionMap actionMap = getActionMap();
        inputMap.put(keyStroke, name);
        actionMap.put(name, action);
    }

}

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Rectangle;

public class Ship implements Runnable {
    int x, y, xDirection, bx, by;
    boolean readyToFire, shooting = false;
    Rectangle bullet;
    Shoot shoot;

    public Ship() {
        x = 175;
        y = 275;
        bullet = new Rectangle(0, 0, 3, 5);
    }

    public void draw(Graphics g) {
        // System.out.println("draw() called");
        g.setColor(Color.BLUE);
        g.fillRect(x, y, 40, 10);
        g.fillRect(x + 18, y - 7, 4, 7);
        if (shooting) {
            g.setColor(Color.RED);
            g.fillRect(bullet.x, bullet.y, bullet.width, bullet.height);
        }
        shoot.repaint();
    }

    public void move() {
        x += xDirection;
        if (x <= 0)
            x = 0;
        if (x >= 360)
            x = 360;
        shoot.repaint();
    }

    public void shoot() {
        if (shooting) {
            bullet.y--;
            shoot.repaint();
        }
    }

    public void setXDirection(int xdir) {
        xDirection = xdir;
    }

    public void setShoot(Shoot shoot) {
        this.shoot = shoot;
    }

    @Override
    public void run() {
        try {
            while (true) {
                shoot();
                move();
                Thread.sleep(5);
            }
        } catch (Exception e) {
            System.err.println(e.getMessage());
        }

    }
}

import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.util.HashSet;

import javax.swing.AbstractAction;

public class MoveAction extends AbstractAction {

    boolean pressed;
    Ship s1;
    Direction dir;
    private HashSet<Direction> movement;

    public MoveAction(boolean pressed, Ship s1, Direction dir) {
        System.out.println("moveaction class");
        this.pressed = pressed;
        this.s1 = s1;
        this.dir = dir;
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        try {
            if (movement.contains(Direction.LEFT)) {
                if (pressed) {
                    s1.setXDirection(-1);
                } else {
                    s1.setXDirection(0);
                }
            } else if (movement.contains(Direction.RIGHT)) {
                if (pressed) {
                    s1.setXDirection(1);
                } else {
                    s1.setXDirection(0);
                }
            } else if (movement.contains(Direction.SPACE)) {
                if (pressed) {
                    if (s1.bullet == null)
                        s1.readyToFire = true;
                    if (s1.readyToFire) {
                        s1.bullet.x = s1.x + 18;
                        s1.bullet.y = s1.y - 7;
                        s1.shooting = true;
                    }
                } else {
                    s1.readyToFire = false;
                    if (s1.bullet.y <= -7) {
                        s1.bullet = null;
                        s1.shooting = false;
                        s1.bullet = null;
                        s1.bullet = new Rectangle(0, 0, 0, 0);
                        s1.readyToFire = true;
                    }
                }
            }
        } catch (NullPointerException ex) {
            System.out.println("NullPointerException");
        }
    }

Stack Trace：）

at MoveAction.actionPerformed(MoveAction.java:24)
    at javax.swing.SwingUtilities.notifyAction(Unknown Source)
    at javax.swing.JComponent.processKeyBinding(Unknown Source)
    at javax.swing.KeyboardManager.fireBinding(Unknown Source)
    at javax.swing.KeyboardManager.fireKeyboardAction(Unknown Source)
    at javax.swing.JComponent.processKeyBindingsForAllComponents(Unknown Source)
    at javax.swing.SwingUtilities.processKeyBindings(Unknown Source)
    at javax.swing.UIManager$2.postProcessKeyEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.dispatchEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(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.access$500(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$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)

Answer 1

您永远不会初始化您的成员变量HashSet<Direction> movement，这就是您总是获得NPE的原因。

试试这个

private Set<Direction> movement = new HashSet<>();

顺便说一下，将实现类HashSet声明为变量类型是一种不好的做法。您应该使用接口Set代替。

Answer 2

内部您的public Shoot(Ship s1)构造函数，您实例化new MoveAction(true, s1, Direction.LEFT), true);，并使用s1，这也是该字段的名称。

问题是，在构造函数完成运行之前，不允许引用字段。这种情况下的行为是未定义的。可能你的编译器认为你引用的s1是字段。在我的编译器中并非如此，但我不知道您使用的是哪种编译器。

在任何情况下，尝试为构造函数和字段的参数赋予不同的名称 - s1，并将参数传递给MoveAction(..)构造函数。

随机将空引用传递给对象

2 个答案: