引用Arraylist中的元素

Question

所以我的Player扩展了Mob，它扩展了Entity。在我的Level课程中，我有一个数组列表，并在主游戏课程中加入他。

 public Level level; 
    public Player player; 

    player = new Player(level, 100, 100, input, JOptionPane.showInputDialog(this, "Username:")); 
    level.addEntity(player); 

问题出现在我想参考玩家X和Y的时候，因此我可以将这些插入到我的简单史莱姆怪物AI中！

我总是在这里一个NullPointerException（7号线）：

public void tick() { 
    int xa = 0; 
    int ya = 0; 


    if (randomWalkTime == 0) { 
     int xd = level.player.x - x; 
     int yd = level.player.y - y; 
     if (xd * xd + yd * yd < 50 * 50) { 
      xa = 0; 
      ya = 0; 
      if (xd < 0) xa = -1; 
      if (xd > 0) xa = +1; 
      if (yd < 0) ya = -1; 
      if (yd > 0) ya = +1; 
     } 
    } 
    move(xa, ya); 
    randomWalkTime++; 
    tickCount++; 

} 

在第一level.player.x :(无论我怎样尝试引用它通过game.player.x或其他任何我可以想到

这里是主要问题：在另一个类中有一个新的“玩家”来自阵列列表如何在我的（Entites> Mob>）Sprite中引用他的x和y类？

如果上面的代码isn足够了，这里是其余的重要位：

这里是LEVEL类，我有我的实体arraylist！

public class Level { 

private byte[] tiles; 
public int width; 
public int height; 
public List<Entity> entities = new ArrayList<Entity>(); 
private String imagePath; 
private BufferedImage image; 

实体类！

public abstract class Entity { 

public int x, y; 
protected Level level; 

public Entity(Level level) { 
    init(level); 
} 

public final void init(Level level) { 
    this.level = level; 
} 


public abstract void tick(); 

public abstract void render(Screen screen); 

}

暴民类：

public abstract class Mob extends Entity { 

protected String name; 
protected int speed; 
protected int numSteps = 0; 
protected boolean isMoving; 
protected int movingDir = 1; 
protected int scale = 1; 

public Mob(Level level, String name, int x, int y, int speed) { 
    super(level); 
    this.name = name; 
    this.x = x; 
    this.y = y; 
    this.speed = speed; 
} 

public void move(int xa, int ya) { 
    if (xa != 0 && ya != 0) { 
     move(xa, 0); 
     move(0, ya); 
     numSteps--; 
     return; 
    } 
    numSteps++; 
    if (!hasCollided(xa, ya)) { 
     if (ya < 0) 
      movingDir = 0; 
     if (ya > 0) 
      movingDir = 1; 
     if (xa < 0) 
      movingDir = 2; 
     if (xa > 0) 
      movingDir = 3; 
     x += xa * speed; 
     y += ya * speed; 
    } 
} 

public abstract boolean hasCollided(int xa, int ya); 

protected boolean isSolidTile(int xa, int ya, int x, int y) { 
    if (level == null) { 
     return false; 
    } 
    Tile lastTile = level.getTile((this.x + x) >> 3, (this.y + y) >> 3); 
    Tile newTile = level.getTile((this.x + x + xa) >> 3, (this.y + y + ya) >> 3); 
    if (!lastTile.equals(newTile) && newTile.isSolid()) { 
     return true; 
    } 
    return false; 
} 

public String getName() { 
    return name; 
} 

}

这里是我的泥类的主要部分！

public class Slime extends Mob{ 

int xa, ya; 
private int colour = Colours.get(-1, 111, 145, 543); 

private int randomWalkTime = 0; 
private boolean isSwimming; 
private int tickCount = 0; 


public Slime(Level level, String name, int x, int y, int speed) { 
    super(level, "Slime", x, y, 1); 
    x = this.x; 
    y = this.y; 


} 

public void tick() { 
    int xa = 0; 
    int ya = 0; 


    if (randomWalkTime == 0) { 
     int xd = level.player.x - x; 
     int yd = level.player.y - y; 
     if (xd * xd + yd * yd < 50 * 50) { 
      xa = 0; 
      ya = 0; 
      if (xd < 0) xa = -1; 
      if (xd > 0) xa = +1; 
      if (yd < 0) ya = -1; 
      if (yd > 0) ya = +1; 
     } 
    } 
    move(xa, ya); 
    randomWalkTime++; 
    tickCount++; 

} 

Answer 1

由于level.player.x给出了NPE，你有两个posibilities：eitehr level为空或level.player为空。你有两种方法来确定它是：

1. Preferrably，使用调试器在那一行设置一个断点，并设置监视这两个变量。

2. 添加System.out.println()调用打印出这两个变量的值。