Java addAll(collection) vs new ArrayList(collection)

问题描述

为什么我会出现不同的行为:

Why do i get different behaviors with:

1. Collection col2 = new ArrayList(col);

Collection col2 = new ArrayList();
col2.addAll(col)

我正在与观众一起工作，代码很复杂，我试图解释问题的根源".另一个有趣的事实是下一个......

I'm working with viewers, and the code is complex, and i'm trying to explain the "root" of the problem. Another interesting fact is the next one...

//IF i use this code i have the correct behavior in my app:
public void updateCollection(Collection<Object> col) {
    this.objectCollection.clear();
    this.objectCollection.addAll(col);
}

//IF i use this code i have unexpected behavior in my app:
public void updateCollection(Collection<Object> col) {
    this.objectCollection=new ArrayList(col);
}

推荐答案

此代码有效:

public void updateCollection(Collection<Object> col) {
    this.objectCollection.clear();
    this.objectCollection.addAll(col);
}

但这会带来问题:

public void updateCollection(Collection<Object> col) {
    this.objectCollection = new ArrayList(col);
}

我怀疑您的第一种方法的这种变化会引入相同的问题:

I suspect that this variation on your first method would introduce identical problems:

public void updateCollection(Collection<Object> col) {
    this.objectCollection = new ArrayList();
    this.objectCollection.clear();
    this.objectCollection.addAll(col);
}

为什么?显然，您在某处使用了另一个对 objectCollection 的引用.在您的代码中，另一个对象说(例如):

Why? Evidently you have another reference to objectCollection in use somewhere. Somewhere in your code, another object is saying (for instance):

myCopyOfObjectCollection = theOtherObject.objectCollection;

如果您正在使用 getter，那不会改变基本行为 - 您仍然保留另一个 引用.

If you're using a getter, that doesn't change the underlying behavior - you are still keeping another reference around.

因此，如果在初始分配中，例如，集合包含 {1, 2, 3}，则开始时:

So if on initial assignment, say, the collection contained {1, 2, 3}, you start out with:

• this.objectCollection: {1, 2, 3}
• that.copyOfObjectCollection:{1, 2, 3}

• this.objectCollection: {1, 2, 3}
• that.copyOfObjectCollection: {1, 2, 3}

当您将 new ArrayList 分配给 this.objectCollection，并使用 {4, 5, 6} 填充它时，您会得到:

When you assign a new ArrayList to this.objectCollection, and populate it with, say, {4, 5, 6}, you get this:

• this.objectCollection: {4, 5, 6}
• that.copyOfObjectCollection:{1, 2, 3}

• this.objectCollection: {4, 5, 6}
• that.copyOfObjectCollection: {1, 2, 3}

所以 that 仍然指向原始的 ArrayList.

So that is still pointing to the original ArrayList.

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