java - 在 Java 中访问组合对象

Question

Java 新手。

我有一个下面 Player 类的实例player1。

Player player1 = new Player(0,0);

在 Player 类中，我编写了一个 Coord 类型的对象坐标(定义如下)。当我实例化上面的player1时，“玩家位于坐标0,0”按预期显示。

public class Player extends Entity { 
    public Coord coordinate;
    public Player(int x, int y) {
        Coord coordinate = new Coord(x,y);
        System.out.println(“Player is at coordinate “ + coordinate.getX() + “,” 
        + coordinate.getY());
    }
}

Coord 类定义如下。

public class Coord {
    private int x;
    private int y;

    public Coord(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }
}

当我实例化player1后尝试访问obj及其各自的方法时，就会出现问题。当我尝试访问坐标时，出现 NullPointerException 错误。

Player player1 = new Player(0,0);

System.out.println(“Player is at coordinate “ + player1.coordinate.getX() + 
“,” + player1.coordinate.getY());

我做错了什么？

Answer 1

您没有将 Coord obj; 设为您的类的字段。这可能就像这样简单

public class Player extends Entity { 
    Coord obj;
    public Player(int x, int y) {
        obj = new Coord(x,y);
        System.out.println(“Player is at coordinate “ + obj.getX() + “,” 
        + obj.getY());
    }
}

请注意，obj 是一个糟糕的字段名称，并且它在这里具有默认级别的访问权限。一种改进方法可能是这样的

public class Player extends Entity { 
    private Coord coordinates;
    public Player(int x, int y) {
        coordinates = new Coord(x,y);
        System.out.println(“Player is at coordinate “ + obj.getX() + “,” 
        + obj.getY());
    }
    public Coord getCoordinates() {
        return coordinates;
    }
}

然后你可以像这样使用它

Player player1 = new Player(0,0);

System.out.println(“Player is at coordinate “ 
    + player1.getCoordinates().getX() 
    + “,” + player1.getCoordinates().getY());

您还可以在 Coord 类中重写 toString()，然后您可以说

System.out.println(“Player is at coordinate “ 
    + player1.getCoordinates());