修改列表对java中的另一个列表有影响 [英] Modify list has an affect on another list in java

问题描述

我在 java 中修改列表有问题，我想用 foreach 更改 listTempEnemy 中的值，然后根据需要更改值，但它有影响到 listEnemy 不应该改变，这是我的代码谢谢.

I have a problem with modify list in java, i want to change value in listTempEnemy with foreach and then the value change as i want, but it has an affect to listEnemy which should not change, this is my code Thank You.

public void hunt() {
    Enemy enemy = new Enemy();
    enemy.setEnemyName("Poring");
    enemy.setEnemyMinATK(10);
    enemy.setEnemyMaxATK(20);
    listEnemy.add(enemy);

    List<Enemy> listTempEnemy = new ArrayList<Enemy>();
    listTempEnemy.addAll(listEnemy);

    for (Enemy enm : listEnemy) {
        System.out.println("Before "+ enm.getEnemyMinATK() + " | " + enm.getEnemyMaxATK());
    }

    for (Enemy enm : listTempEnemy) {
        enm.setEnemyMinATK((enm.getEnemyMinATK() * 10/100) + enm.getEnemyMinATK());
        enm.setEnemyMaxATK((enm.getEnemyMaxATK() * 10/100) + enm.getEnemyMaxATK());
        System.out.println("Value Changed "+ enm.getEnemyMinATK() + " | " + enm.getEnemyMaxATK());
    }

    for (Enemy enm : listEnemy) {
        System.out.println("After "+ enm.getEnemyMinATK() + " | " + enm.getEnemyMaxATK());
    }
}

结果是

2020-02-18T17:02:33.843+0700|Info: Before 10 | 20
2020-02-18T17:02:33.843+0700|Info: Value Changed 11 | 22
2020-02-18T17:02:33.843+0700|Info: After 11 | 22

然后我改变了我的代码

private List<Enemy> loadEnemy() {
    List<Enemy> listEnemy = new ArrayList<Enemy>();

    Enemy enemy = new Enemy();
    enemy.setEnemyName("Poring");
    enemy.setEnemyHP(100);
    enemy.setEnemyMinATK(10);
    enemy.setEnemyMaxATK(20);
    listEnemy.add(enemy);

    return listEnemy;
}

public void hunt() {
    List<Enemy> listTempEnemy = loadEnemy();

    for (Enemy enm : listEnemy) {
        System.out.println("Before "+ enm.getEnemyMinATK() + " | " + enm.getEnemyMaxATK());
    }

    for (Enemy enm : listTempEnemy) {
        enm.setEnemyMinATK((enm.getEnemyMinATK() * 10/100) + enm.getEnemyMinATK());
        enm.setEnemyMaxATK((enm.getEnemyMaxATK() * 10/100) + enm.getEnemyMaxATK());
        System.out.println("Changed Value " + enm.getEnemyMinATK() + " | " + enm.getEnemyMaxATK());
    }

    for (Enemy enm : listEnemy) {
        System.out.println("After "+ enm.getEnemyMinATK() + " | " + enm.getEnemyMaxATK());
    }
}

这就是我想要的结果

2020-02-18T17:46:36.387+0700|Info: Before 10 | 20
2020-02-18T17:46:36.387+0700|Info: Changed Value 11 | 22
2020-02-18T17:46:36.387+0700|Info: After 10 | 20

推荐答案

您已将相同的对象(指向堆(内存)中相同对象的指针/引用)添加到两个单独的列表中.当您更改一个列表中的对象时，它也会更改另一个列表中的对象，因为它们都是同一个对象.这就是为什么不可变对象更可取，因为这些类型的错误真的很难发现(如果你有很多复杂的逻辑......例如哪个类正在改变对象?).对于列表 1 和列表 2

You've added the same object (pointers/references to the same object in heap (memory)) to the two separate lists. When you change the object in one list it changes it in the other because they are both the same object. This is why immutable objects are preferable as these kind of bugs can be really hard to spot (if you had a lot of complex logic... e.g. which class is mutating the objects?). You'll need 2 copies of your object for list 1 and list 2

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