java - 线程中出现异常 "Thread-2"java.lang.NullPointerException

Question

每当我运行我的程序时，我都会收到此错误:

Exception in thread "Thread-2" java.lang.NullPointerException
    at ca.wamlab.game.entities.Player.tick(Player.java:20)
    at ca.wamlab.game.level.Level.tick(Level.java:38)
    at ca.wamlab.game.Game.tick(Game.java:169)
    at ca.wamlab.game.Game.run(Game.java:112)
    at java.lang.Thread.run(Thread.java:744)

忽略除第一个以外的所有内容，因为所有这些都是因为那个。

我在以下代码中收到该错误:

包 ca.wamlab.game.entities;

import ca.wamlab.game.InputHandler;
import ca.wamlab.game.gfx.Colors;
import ca.wamlab.game.gfx.Screen;
import ca.wamlab.game.level.Level;

public class Player extends Mob{

private InputHandler input;
private int color = Colors.get(-1, 111, 145, 543);

public Player(Level level, int x, int y, InputHandler input) {
    super(level, "Player", x, y, 1);
}

    public void tick() {
        int xa = 0;
        int ya = 0;
        if(input.up.isPressed()){
            ya--;
        }
        if(input.down.isPressed()){
            ya++;
        }
        if(input.right.isPressed()){
            xa++;
        }
        if(input.left.isPressed()){
            xa--;
        }


        if (xa != 0 || ya != 0){
            move(xa, ya);
            isMoving = true;
        }else{
            isMoving = false;
        }
    }

    public void render(Screen screen) {
        int xTile = 0;
        int yTile = 28;

        int modifier =  8*scale;
        int xOffset = x - modifier/2;
        int yOffset = y - modifier /2-4;

        screen.render(xOffset, yOffset, xTile + yTile * 32, color);
        screen.render(xOffset + modifier, yOffset, (xTile + 1) + yTile * 32, color);

        screen.render(xOffset, yOffset + modifier, xTile + (yTile + 1) * 32, color);
        screen.render(xOffset + modifier, yOffset + modifier, (xTile + 1) + (yTile + 1) *      32, color);


    }

    public boolean hasCollided(int xa, int ya) {
        return false;
    }

}

输入类是:

package ca.wamlab.game;

import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

public class InputHandler implements KeyListener{

    public InputHandler(Game game) {
        game.addKeyListener(this);
    }

    public class Key {
        private int numTimesPressed;
        private boolean pressed = false;

        public int getNumTimesPressed(){
            return numTimesPressed;
        }

        public boolean isPressed(){
            return pressed;
        }
        public void toggle(boolean isPressed){
            pressed = isPressed;
            if(isPressed)numTimesPressed++;
        }
    }

    public Key up = new Key();
    public Key down = new Key();
    public Key right = new Key();
    public Key left = new Key();

    public void keyTyped(KeyEvent e) {
    }

    public void keyPressed(KeyEvent e) {
        toggleKey(e.getKeyCode(), true);
    }

    public void keyReleased(KeyEvent e) {
        toggleKey(e.getKeyCode(), false);
    }

    public void toggleKey(int keyCode, boolean isPressed){
        if(keyCode == KeyEvent.VK_W  || keyCode == KeyEvent.VK_UP){ up.toggle(isPressed);}
        if(keyCode == KeyEvent.VK_S || keyCode == KeyEvent.VK_DOWN){ down.toggle(isPressed);}
        if(keyCode == KeyEvent.VK_D || keyCode == KeyEvent.VK_RIGHT){ right.toggle(isPressed);}
        if(keyCode == KeyEvent.VK_A || keyCode == KeyEvent.VK_LEFT){left.toggle(isPressed);}




    }
}

我知道我得到这个是因为代码中的某些内容被设置为null(我认为它是ispressed())，但我不能想办法解决这个问题...

Answer 1

InputHandler类中的实例变量up、down、right、left初始化为

public Key up = new Key();
public Key down = new Key();
public Key right = new Key();
public Key left = new Key();

如果input是InputHandler的引用变量，并且赋值给它一个InputHandler的对象，那么根据上面的代码，上、下、右、左永远不可能为空。因此，根据确认第 20 行是 if(input.up.isPressed()){ 最有可能的 input 为 null。

如果您可以提供代码，其中 InputHander input 被分配了一个对象(如果在任何地方这样做)，这将帮助我们为您提供帮助。使用 addKeyListenter() 方法注册的关键监听器在代码中的哪个位置？提供的 tick() 方法属于哪个类？

从 stacktrace 可以明显看出，所有三个 Player、Level 和 Game 类都有同名的方法 tick()

at ca.wamlab.game.entities.Player.tick(Player.java:20)
    at ca.wamlab.game.level.Level.tick(Level.java:38)
    at ca.wamlab.game.Game.tick(Game.java:169)

许多重要细节尚不清楚，无法帮助解决此问题。更多详细信息将会有所帮助。