访问 Java 中的组合对象

Question

新 Java。

我在下面有一个 Player class 的实例 player1。

Player player1 = new Player(0,0);

在 Player 中 class 我编写了一个 Coord 类型的对象坐标（定义如下）。当我在上方实例化 player1 时，"Player is at coordinate 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());
    }
}

坐标class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;
    }
}

当我在实例化player1 后尝试访问obj 及其各自的方法时出现问题。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 class 的字段。这可以像

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，它在这里具有默认级别的访问权限。一种改进方法可能是

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;
    }
}

然后你就可以像

一样使用它了

Player player1 = new Player(0,0);

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

您也可以在 Coord class 中覆盖 toString()，然后您可以说

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

访问 Java 中的组合对象

Accessing composed objects in Java

java

composition