Java继承->无法在构造函数错误中引用实例字段？

Question

我希望c为随机颜色，因此我可以产生许多不同颜色的敌人，但不确定如何做。如果我把Color.cyan放在里面的话，效果很好。但是，如果我想在super（）中放入一个变量，它就不会那样。我是Java新手，所以我只是想弄清楚。是否可以在不将变量设为静态的情况下进行？

package first.Game;

import java.awt.Color;
import java.util.Random;

public class MenuEnemy extends Enemy{

    private Random r = new Random();
    private Color c = new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255));

    public MenuEnemy(float x, float y, Handler handler)
    {
        super(x, y, 16, 16, 2, 8, c, ID.Enemy, handler);
    }
}

Answer 1

问题是字段尚未初始化，因为在super之后执行了子类非静态字段初始化。

这里有几个选项供您选择，有两种可能：

• 您可以添加静态方法来获得随机颜色，并在构造函数中使用该方法代替c。

public static Color getRandomColor() {
    Random r = new Random();
    return new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255));
}

• 您也可以在调用super之后定义颜色，尽管我建议您不要这样做，就像您在超类构造函数中使用c来做某件事一样，因为我们将其设置为null，将其设置为null随机的静态颜色也可能会产生有害的副作用。

    super(..., null, ...);

    Random r = new Random();
    this.c = new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255));

---

如果使用第一个建议，这将是最终结果。

public class Enemy {

    protected float x, y;
    protected int i1, i2, i3, i4;
    protected Color c;

    protected String id;
    protected Handler handler;

    public Enemy(float x, float y, int i1, int i2, int i3, int i4, Color c, String id, Handler handler) {
        this.x = x;
        this.y = y;
        this.i1 = i1;
        this.i2 = i2;
        this.i3 = i3;
        this.i4 = i4;
        this.c = c;
        this.handler = handler;
    }

    public static Color getRandomColor() {
        Random r = new Random(); // TODO possibly reuse a static random somewhere, depends on use-case
        return new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255));
    }
}

public class MenuEnemy extends Enemy {

    public MenuEnemy(float x, float y, Handler handler) {
        super(x, y, 16, 16, 2, 8, Enemy.getRandomColor(), ID.Enemy, handler);
    }

}

Answer 2

您应使用Builder Design Pattern

package first.Game;

import java.awt.Color;
import java.util.Random;

class ID {
    public static final String Enemy = "e";
}

class Handler {
}

class Enemy {

    public Enemy(float x, float y, int p1, int p2, int p3, int p4, Color c, String id, Handler handler) {
    }

}

public class MenuEnemy extends Enemy {

    private Random r = new Random();
    private Color c = new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255));

    protected MenuEnemy(Builder builder) {
        super(builder.x, builder.y, 16, 16, 2, 8, builder.c, ID.Enemy, builder.handler);
    }

    public static class Builder {

        private float x;
        private float y;
        private Handler handler;
        private Random r = new Random();
        private Color c;

        public Builder x(float x) {this.x = x; return this;};
        public Builder y(float y) {this.y = y; return this;};
        public Builder handler(Handler handler) {this.handler = handler; return this;};
        public Builder r(Random r) {this.r = r; return this;};
        public Builder c(Color c) {this.c = c; return this;};

        public MenuEnemy build() {
            if(c == null)
                c = new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255));
            return new MenuEnemy(this);
        }

    }

    public static void main(String[] args) {
        MenuEnemy enemy = new Builder()
                .x(10)
                .y(10)
                .handler(new Handler())
                .r(new Random())
                .build();
        System.out.println(enemy);
    }
}

Answer 3

可以有另一个构造函数。

package first.Game;

import java.awt.Color;
import java.util.Random;

public class MenuEnemy extends Enemy {

    private Random r = new Random();
    protected Color c;

    public MenuEnemy(float x, float y, Handler handler)
    {
        this(x, y, handler, new Color((new Random()).nextInt(255), (new Random()).nextInt(255), (new Random()).nextInt(255)));
    }

    private MenuEnemy(float x, float y, Handler handler, Color c)
    {
        super(x, y, 16, 16, 2, 8, c, ID.Enemy, handler);
        this.c = c;
    }
}