blocks|key|3590809|text|ArrayList.clear()算法的时间复杂度为O(n)，removeAll算法的时间复杂度为O(n%5E2)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3590810|所以，是的，ArrayList.clear的速度要快得多。|3590811|entityMap^0|0|H|Q|4|V|9|1D|6|0|6|F|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|I|$]]

The time complexity of <code>ArrayList.clear()</code> is <code>O(n)</code> and of <code>removeAll</code> is <code>O(n^2)</code>.

So yes, <code>ArrayList.clear</code> is much faster.

blocks|key|3590833|text|clear()方法删除单个ArrayList的所有元素。这是一个快速的操作，因为它只是将数组元素设置为null。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3590834|从AbstractCollection继承的removeAll(Collection)方法从调用该方法的集合中移除参数集合中的所有元素。这是一个相对较慢的操作，因为它必须搜索其中一个涉及的集合。|3590835|entityMap^0|0|7|D|9|1F|4|0|1|I|M|L|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|I|$]]

The <code>clear()</code> method removes all the elements of a single <code>ArrayList</code>. It's a fast operation, as it just sets the array elements to <code>null</code>.

The <code>removeAll(Collection)</code> method, which is inherited from <code>AbstractCollection</code>, removes all the elements that are in the argument collection from the collection you call the method on. It's a relatively slow operation, as it has to search through one of the collections involved.

blocks|key|3590735|text|除非有一个特定的优化来检查传递给removeAll()的参数是否是集合本身(我非常怀疑这样的优化是否存在)，否则它将比简单的.clear()慢得多。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3590736|除此之外(至少同样重要)：arraylist.removeAll(arraylist)只是晦涩、令人困惑的代码。这是一种非常倒退的方式，意思是“清除这个集合”。与非常容易理解的arraylist.clear()相比，它有什么优势？|3590737|entityMap^0|G|B|1Q|8|0|D|U|2G|H|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|I|$]]

Unless there is a specific optimization that checks if the argument passed to <code>removeAll()</code> is the collection itself (and I highly doubt that such an optimization is there) it will be significantly slower than a simple <code>.clear()</code>.

Apart from that (and at least equally important): <code>arraylist.removeAll(arraylist)</code> is just obtuse, confusing code. It is a very backwards way of saying "clear this collection". What advantage would it have over the very understandable <code>arraylist.clear()</code>?

blocks|key|3590711|text|它们服务于不同的目的。clear()清除类的实例，removeAll()删除所有给定的对象并返回操作的状态。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3590712|entityMap^0|B|7|P|B|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

They serve different purposes. <code>clear()</code> clears an instance of the class, <code>removeAll()</code> removes all the given objects and returns the state of the operation.

blocks|key|220773|text|clear()将遍历底层数组，并将每个条目设置为空；|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|220774|removeAll(collection)将对集合进行ArrayList检查，如果存在，则对其执行remove(Object)。|220775|我会想像clear()比removeAll快得多，因为它不是比较，等等。|220776|entityMap^0|0|7|0|0|L|1D|E|0|4|7|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|T|8|@$9|U|A|V|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|K|$]]

<code>clear()</code> will go through the underlying Array and set each entry to null;

<code>removeAll(collection)</code> will go through the ArrayList checking for collection and <code>remove(Object)</code> it if it exists. 

I would imagine that <code>clear()</code> is way faster then removeAll because it's not comparing, etc.

blocks|key|1976471|text|Clear更快，因为它不会循环要删除的元素。此方法可以假设所有元素都可以删除。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1976472|Remove+all不一定意味着删除列表中的所有元素，只删除那些作为参数提供的元素。因此，需要更多的努力来保留那些不应该删除的内容。|offset|length|style|CODE|1976473|澄清|BOLD|1976474|所谓“循环”，我的意思是它不需要检查元素是否应该保留。它可以将引用设置为null，而无需搜索提供的要删除的元素列表。|1976475|Clear比deleteall更快。|1976476|entityMap^0|0|0|A|0|0|2|0|10|4|0|0|5|6|9|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@$D|S|E|T|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|U|8|@$D|V|E|W|F|J]]|9|@]|A|$]]|$1|K|3|L|5|6|7|X|8|@$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|M|3|N|5|6|7|10|8|@$D|11|E|12|F|G]|$D|13|E|14|F|G]]|9|@]|A|$]]|$1|O|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|P|$]]

Clear is faster because it does not loop over elements to delete. This method can assume that ALL elements can be deleted.

<code>Remove all</code> does not necessarily mean delete all elements in the list, only those provided as parameters SHOULD be delete. Hence, more effort is required to keep those which should not be deleted.

CLARIFICATION

By 'loop', I mean it does not have to check whether the element should be kept or not. It can set the reference to <code>null</code> without searching through the provided lists of elements to delete.

<code>Clear</code> IS faster than <code>deleteall</code>.

blocks|key|3876089|text|clear()将会更加高效。它将简单地删除每一项。使用removeAll(+arraylist+)将需要更多的工作，因为在删除它之前，它会检查arraylist中的每一项，看看它是否存在于arraylist中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3876090|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

clear() will be much more efficient. It will simply remove each and every item. Using removeAll(arraylist) will take a lot more work because it will check every item in arraylist to see if it exists in arraylist before removing it.

blocks|key|220209|text|数组=>一旦在运行时为数组变量分配了空间，就不能扩展或删除所分配的空间。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|220210|arraylist+=>在ArrayList中不是这样的。ArrayList可以在运行时增长和缩小。分配的空间可以在运行时最小化或最大化。|220211|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Array => once the space is allocated for an Array variable at the run time, the allocated space can not be extended or removed.

ArrayList => This is not the case in arraylist. ArrayList can grow and shrink at the run time.
 The allocated space can be minimized or maximized at the run time.

Assuming that <code>arraylist</code> is defined as <code>ArrayList&lt;String&gt; arraylist</code>, is <a href="http://developer.android.com/reference/java/util/AbstractCollection.html#removeAll%28java.util.Collection%3C?%3E%29" rel="noreferrer"><code>arraylist.removeAll(arraylist)</code></a> equivalent to <a href="http://developer.android.com/reference/java/util/ArrayList.html#clear%28%29" rel="noreferrer"><code>arraylist.clear()</code></a>?

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

Are there any caveats in using <code>arraylist.removeAll(arraylist)</code> instead of <code>arraylist.clear()</code>?

What is the difference between ArrayList.clear() and ArrayList.removeAll()?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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假设arraylist定义为ArrayList<String> arraylist，是否等同于如果是这样，我可以假设clear()方法在清空数组列表方面更有效吗？使用arraylist.removeAll(arraylist)而不是arraylist.clear()有什么注意事项吗？

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