Java 检查列表是否为空的两种方法 - 差异？
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Two ways to check if a list is empty - differences?
提问by dziki
I have a short question.
我有一个简短的问题。
Lets assume we have a Listwhich is an ArrayListcalled list. We want to check if the list is empty.
让我们假设我们有一个List被ArrayList调用的list。我们想检查列表是否为空。
What is the difference (if there is any) between:
之间有什么区别（如果有的话）：
if (list == null) { do something }
and
和
if (list.isEmpty()) { do something }
I'm working on an ancient code (written around 2007 by someone else) and it is using the list == nullconstruction. But why use this construction when we have list.isEmpty()method...
我正在编写一个古老的代码（由其他人在 2007 年左右编写）并且它正在使用该list == null结构。但是当我们有list.isEmpty()方法时为什么要使用这种结构......
采纳答案by Eran
The first tells you whether the listvariable has been assigned a List instance or not.
第一个告诉您list变量是否已分配给 List 实例。
The second tells you if the List referenced by the listvariable is empty.
If listis null, the second line will throw a NullPointerException.
第二个告诉您list变量引用的 List是否为空。如果list为空，第二行将抛出一个NullPointerException.
If you want to so something only when the list is empty, it is safer to write :
如果您只想在列表为空时才这样做，那么编写更安全：
if (list != null && list.isEmpty()) { do something }
If you want to do something if the list is either null or empty, you can write :
如果您想在列表为空或为空时执行某些操作，您可以编写：
if (list == null || list.isEmpty()) { do something }
If you want to do something if the list is not empty, you can write :
如果你想在列表不为空的情况下做一些事情，你可以写：
if (list != null && !list.isEmpty()) { do something }
回答by atish shimpi
if (list == null)checking is list is nullor not.
if (list == null)检查列表是否null存在。
if (list.isEmpty())checking is list is empty or not, If list is nulland you call isEmpty()it will give you NullPointerException.
if (list.isEmpty())检查列表是否为空，如果列表是null，您调用isEmpty()它会给您NullPointerException。
Its better to check whether list is nullor not first and then check is empty or not.
最好先检查列表null是否为空，然后再检查是否为空。
if(list !=null && ! list.isEmpty()){
// do your code here
}
回答by josivan
Another approach is to use Apache Commons Collections.
另一种方法是使用Apache Commons Collections。
Take a look in method CollectionUtils.isEmpty(). It is more concise.
看看方法CollectionUtils.isEmpty()。它更简洁。
