ArrayList.clear() 和 ArrayList.removeAll() 有什么区别?

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问题描述

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假设 arraylist 被定义为 ArrayList;arraylist，是 arraylist.removeAll(arraylist) 等价于 arraylist.clear()?

Assuming that arraylist is defined as ArrayList<String> arraylist, is arraylist.removeAll(arraylist) equivalent to arraylist.clear()?

如果是这样，我可以假设 clear() 方法更有效地清空数组列表吗?

If so, can I assume that the clear() method is more efficient for emptying the array list?

使用arraylist.removeAll(arraylist)代替arraylist.clear()有什么注意事项吗?

Are there any caveats in using arraylist.removeAll(arraylist) instead of arraylist.clear()?

推荐答案

clear()的源码:

public void clear() {
    modCount++;

    // Let gc do its work
    for (int i = 0; i < size; i++)
        elementData[i] = null;

    size = 0;
}

removeAll()的源代码(在AbstractCollection中定义):

The source code for removeAll()(As defined in AbstractCollection):

public boolean removeAll(Collection<?> c) {
    boolean modified = false;
    Iterator<?> e = iterator();
    while (e.hasNext()) {
        if (c.contains(e.next())) {
            e.remove();
            modified = true;
        }
    }
    return modified;
}

clear() 快得多，因为它不必处理所有这些额外的方法调用.

clear() is much faster since it doesn't have to deal with all those extra method calls.

正如 Atrey 指出的那样，c.contains(..) 将 removeAll 的时间复杂度增加到 O(n2) 作为与 clear 的 O(n) 相反.

And as Atrey points out, c.contains(..) increases the time complexity of removeAll to O(n2) as opposed to clear's O(n).

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