List通过遍历删除数据

最新推荐文章于 2023-08-07 19:12:35 发布

# Never Give up &

最新推荐文章于 2023-08-07 19:12:35 发布

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分类专栏：工作文章标签： java

本文链接：https://blog.youkuaiyun.com/qq_41939020/article/details/108873485

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本文探讨了在Java中遍历ArrayList时错误地使用for循环和foreach循环进行元素删除导致的问题。错误的for循环示例会导致不完整的结果，而foreach循环会抛出`ConcurrentModificationException`。正确的做法是在for循环中使用`i--`来修正索引，或者使用迭代器进行安全删除。了解这些最佳实践对于避免运行时异常和确保代码的健壮性至关重要。

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错误示范

1. for循环

List<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
list.add("3");
list.add("5");

for (int i = 0; i < list.size(); i++) {
	list.remove(i);
}

System.out.println(list);

结果：[2, 5]

2.foreach

List<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
list.add("3");
list.add("5");

for (String string : list) {
	list.remove(string);
}

System.out.println(list);

结果：java.util.ConcurrentModificationException
foreach的本质是迭代器，迭代器有版本控制

private class Itr implements Iterator<E> {
    int cursor;       // index of next element to return
    int lastRet = -1; // index of last element returned; -1 if no such
    int expectedModCount = modCount;

    Itr() {}

    public boolean hasNext() {
        return cursor != size;
    }

    @SuppressWarnings("unchecked")
    public E next() {
        checkForComodification();
        int i = cursor;
        if (i >= size)
            throw new NoSuchElementException();
        Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length)
            throw new ConcurrentModificationException();
        cursor = i + 1;
        return (E) elementData[lastRet = i];
    }

    public void remove() {
        if (lastRet < 0)
            throw new IllegalStateException();
        checkForComodification();

        try {
            ArrayList.this.remove(lastRet);
            cursor = lastRet;
            lastRet = -1;
            expectedModCount = modCount;
        } catch (IndexOutOfBoundsException ex) {
            throw new ConcurrentModificationException();
        }
    }

    @Override
    @SuppressWarnings("unchecked")
    public void forEachRemaining(Consumer<? super E> consumer) {
        Objects.requireNonNull(consumer);
        final int size = ArrayList.this.size;
        int i = cursor;
        if (i >= size) {
            return;
        }
        final Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length) {
            throw new ConcurrentModificationException();
        }
        while (i != size && modCount == expectedModCount) {
            consumer.accept((E) elementData[i++]);
        }
        // update once at end of iteration to reduce heap write traffic
        cursor = i;
        lastRet = i - 1;
        checkForComodification();
    }

    final void checkForComodification() {
    	//如果检测到版本发生变化，则抛出异常
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }
}

正确做法

1. for循环

List<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
list.add("3");
list.add("5");

for (int i = 0; i < list.size(); i++) {
	list.remove(i--);
}

System.out.println(list);

2.迭代器

List<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
list.add("3");
list.add("5");

Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
	iterator.next();
	iterator.remove();
}

System.out.println(list);