Java addAll（collection）vs new ArrayList（collection）

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问题描述

为什么我得到不同的行为：

Why do i get different behaviors with:

集合col2 = new ArrayList ;

集合col2 = new ArrayList（）;

col2.addAll（col）

我正在与观众，代码是复杂的，我试图解释问题的根。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

This code works:

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

但这会带来一些问题：

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

我怀疑你的第一个方法的这个变化会引入相同的问题： p>

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？显然你有另一个引用objectCollection在某处使用。在你的代码中，另一个对象在（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;

如果你使用的getter，这不会改变底层的行为 - 你仍然保持另一个引用。

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

所以如果在初始赋值，说，集合包含{1，2，3}，您首先使用：

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}

ArrayList to this.objectCollection，然后使用{4，5，6}填充它，即可得到：

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}

that仍指向原始的ArrayList。

"that" is still pointing to the original ArrayList.

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