在 Java 中检查集合是否为空: 哪种方法最好？

isEmpty()


Returns true if this list contains no elements.

http://docs.oracle.com/javase/1.4.2/docs/api/java/util/List.html

I would use the first one. It is clear to see right away what it does. I dont think the null check is necessary here.

Unless you are already using CollectionUtils I would go for List.isEmpty(), less dependencies.

Performance wise CollectionUtils will be a tad slower. Because it basically follows the same logic but has additional overhead.

So it would be readability vs. performance vs. dependencies. Not much of a big difference though.

CollectionUtils.isNotEmpty checks if your collection is not null and not empty. This is better comparing to double check but only if you have this Apache library in your project. If you don't then use:

if(list != null && !list.isEmpty())

You should absolutely use isEmpty(). Computing the size() of an arbitrary list could be expensive. Even validating whether it has any elements can be expensive, of course, but there's no optimization for size() which can't also make isEmpty() faster, whereas the reverse is not the case.

For example, suppose you had a linked list structure which didn't cache the size (whereas LinkedList<E> does). Then size() would become an O(N) operation, whereas isEmpty() would still be O(1).

Additionally of course, using isEmpty() states what you're actually interested in more clearly.

If you have the Apache common utilities in your project rather use the first one. Because its shorter and does exactly the same as the latter one. There won't be any difference between both methods but how it looks inside the source code.

Also a empty check using

listName.size() != 0

Is discouraged because all collection implementations have the

listName.isEmpty()

function that does exactly the same.

So all in all, if you have the Apache common utils in your classpath anyway, use

if (CollectionUtils.isNotEmpty(listName))

in any other case use

if(listName != null && listName.isEmpty())

You will not notice any performance difference. Both lines do exactly the same.

Apache Commons' CollectionUtils.isNotEmpty(Collection) is a NULL-SAFE check

Returns TRUE is the Collection/List is not-empty and not-null Returns FALSE if the Collection is Null

Example:

List<String> properties = new ArrayList();
...
if (CollectionUtils.isNotEmpty(properties)) {
// process the list
} else {
// list is null or empty
}

Refer: https://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/CollectionUtils.html#isNotEmpty(java.util.Collection)

A good example of where this matters in practice is the ConcurrentSkipListSet implementation in the JDK, which states:

Beware that, unlike in most collections, the size method is not a constant-time operation.

This is a clear case where isEmpty() is much more efficient than checking whether size()==0.

You can see why, intuitively, this might be the case in some collections. If it's the sort of structure where you have to traverse the whole thing to count the elements, then if all you want to know is whether it's empty, you can stop as soon as you've found the first one.

if (CollectionUtils.isNotEmpty(listName))

Is the same as:

if(listName != null && !listName.isEmpty())

In first approach listName can be null and null pointer exception will not be thrown. In second approach you have to check for null manually. First approach is better because it requires less work from you. Using .size() != 0 is something unnecessary at all, also i learned that it is slower than using .isEmpty()

To Check collection is empty, you can use method: .count(). Example:

DBCollection collection = mMongoOperation.getCollection("sequence");
if(collection.count() == 0) {
SequenceId sequenceId = new SequenceId("id", 0);
mMongoOperation.save(sequenceId);
}

Use CollectionUtils.isEmpty(Collection coll)

Null-safe check if the specified collection is empty. Null returns true.

Parameters: coll - the collection to check, may be null

true if empty or null