当使用继承时，你可以创建两个类A和B，它们继承自类C。然后你可以创建一个C数组来存储这两个类中的任何一个--C[]。
但是，当使用组合时，数组必须是什么类型才能存储这两种类型中的任何一种？

class Entity {
    public int id;
    public Entity() {
        this.id = 0;
    }
}
class Player extends Entity {
    public Player() {
        this.id = 1;
    }
}
class Monster extends Entity {
    public Monster() {
        this.id = 2;
    }
}
public class Main {
    public static void main(String[] args) {
        Entity[] entities = new Entity[2];
 
        entities[0] = new Player(); // id == 1
        entities[1] = new Monster(); // id == 2
    }
}

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当使用复合时，你必须将Entity存储为字段：

class Entity {
    public int id;
    public Entity() {
        this.id = 0;
    }
}
class Player {
    Entity entity;
    public Player() {
        this.entity = new Entity();
        this.entity.id = 1;
    }
}
class Monster {
    Entity entity;
    public Monster() {        
        this.entity = new Entity();
        this.entity.id = 2;
    }
}
public class Main {
    public static void main(String[] args) {
        Player player = new Player();
        Monster monster = new Monster();
        
        Entity[] entities = new Entity[2];
        // TODO: won't work! what type?
        entities[0] = player;
        entities[1] = monster;
    }
}

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