随机将空引用传递给对象

Question

我创建了一个程序，它应该显示 spaceship（基本上是两个矩形）并在每次按下 space 按钮时从中发射一颗子弹。它不起作用，因为显然，我的程序正在将空 Ship 引用传递到我的其他 classes 之一，即 MoveAction class。我完全不知道为什么会这样，因为在我的 Main class 中，我在 之前实例化了我的 Ship 对象]Shoot class（调用使用 ship 对象的构造函数）。如果这个问题有点愚蠢，我很抱歉，因为我仍然是一个新手程序员，大部分时间都不知道自己在做什么:)这是我的代码：

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");
        }
    }

堆栈跟踪:)

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.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.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)

Answer 1

在你的 public Shoot(Ship s1) 构造函数中实例化 new MoveAction(true, s1, Direction.LEFT), true);，使用 s1，这也是字段的名称。

问题是在构造函数完成它之前不允许引用字段运行。这种情况下的行为是未定义的。可能您的编译器认为您引用的 s1 是该字段。我的编译器不是这样，但是不知道你用的是哪个编译器

在任何情况下，尝试为构造函数的参数和字段 - s1 指定不同的名称，并将 argument[=24] 传递给 MoveAction(..) 构造函数=].

Answer 2

你永远不会初始化你的成员变量HashSet<Direction> movement，这就是你总是得到 NPE 的原因。

试试这个

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

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

随机将空引用传递给对象

Randomly passing null references into objects

java

oop

user-interface

nullpointerexception

calling-convention