添加到 List 时抛出 java.util.ConcurrentModificationException

问题描述

当我运行这个时，我得到一个 java.util.ConcurrentModificationException 尽管我使用了 iterator.remove();

When I run this, I get a java.util.ConcurrentModificationException despite me using iterator.remove();

显然是我在循环中添加了数字 6.发生这种情况是否因为迭代器不知道"它在那里并且无论如何要修复它?

it's obviously me adding the number 6 in the loop. Does this happen because the iterator "doesn't know" it's there and is there anyway to fix it?

public static void main(String args[]){

    List<String> list = new ArrayList<>();

    list.add("1");
    list.add("2");
    list.add("3");
    list.add("4");
    list.add("5");

    for(Iterator<String> it = list.iterator();it.hasNext();){
        String value = it.next();

        if(value.equals("4")) {
            it.remove();
            list.add("6");
        }

        System.out.println("List Value:"+value);
    }
}

推荐答案

ConcurrentModificationException 在调用 String value = it.next(); 时被抛出.但真正的罪魁祸首是list.add(6");.在直接迭代它时，您不得修改 Collection.您正在使用 it.remove(); 这很好，但不是 list.add("6");.

The ConcurrentModificationException is thrown when calling String value = it.next();. But the actual culprit is list.add("6");. You mustn't modify a Collection while iterating over it directly. You are using it.remove(); which is fine, but not list.add("6");.

虽然你可以用 Streams 解决问题，但我会先用 Iterators 提供一个解决方案，因为这是理解问题的良好第一步.

While you can solve the problem with Streams, I will first offer a solution with Iterators, as this is a good first step for understanding the problem.

您需要一个 ListIterator; 如果你想在迭代过程中添加和删除:

You need a ListIterator<String> if you want to add and remove during iteration:

for (ListIterator<String> it = list.listIterator(); it.hasNext();){
    String value = it.next();

    if (value.equals("4")) {
        it.remove();
        it.add("6");
    }

    System.out.println("List Value: " + value);
}

这应该可以解决问题！

使用Streams的解决方案:

List<String> newList = list.stream()
        .map(s -> s.equals("4") ? "6" : s)
        .collect(Collectors.toList());

这里我们创建了一个从您的 List 流式传输.我们映射所有的值到他们自己，只有4"被映射到6"然后我们把它收集回一个列表.但请注意，newList 是不可变的！

Here we create a Stream from your List. We map all values to themselves, only "4" gets mapped to "6" and then we collect it back into a List. But caution, newList is immutable!

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