ArrayList.add（）在一个循环之后，所有的元素都相同。为什么？ [英] After ArrayList.add() in a loop, all elements are identical. Why?

问题描述

如果我这样做[准基于Java code]：

If I do this [quasi-java-code]:

while (loop)
{
    localObject = getDataForObject();
    globalPublicStaticArrayList<Object>.add(localObject);
}

所有globalPublicStaticArrayList的元素是相同的，等于localObject的最后一个副本，添加。我踩通在调试器中循环，发现只要一个对象被添加，它被复制到globalPublicStaticArrayList的所有元素。

All the elements in globalPublicStaticArrayList are identical, equal to the last copy of localObject added. I stepped thru the loop in the debugger and saw that as soon as an Object is added, it is copied in to all the elements of the globalPublicStaticArrayList.

我找到的解决方法是：

while (loop)
{
    localObject = getDataForObject();
    globalPublicStaticArrayList<Object>.add(new Object(localObject.member1, localObject.member2,...));
}

有它是与通行证通过引用在Java中？为什么元素是在第一种情况相同？谢谢你。

Has it something to do with pass-by-reference in Java? How come the elements are identical in the first case? Thanks.

推荐答案

Java使用按值调用，但在这里，这些值的引用的对象。

Java uses call by value, but here those values are references to objects.

什么要添加到列表中是不是对象的副本，但引用的副本。你的方法每次调用时都会返回同一个对象。它可能应该每次都返回一个新的对象，那么你就不会需要这个解决办法。

What you are adding to the list is not a copy of the object, but a copy of the reference. Your method returned the same object each time you called it. It probably should return a new object each time, then you wouldn't need this workaround.

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