Null check in an enhanced for loop

user_184730 0
Viewed 203743

What is the best way to guard against null in a for loop in Java?

This seems ugly :

if (someList != null) {
    for (Object object : someList) {
        // do whatever
    }
}

Or

if (someList == null) {
    return; // Or throw ex
}
for (Object object : someList) {
    // do whatever
}

There might not be any other way. Should they have put it in the for construct itself, if it is null then don't run the loop?

for-loop syntax loops java
11 Answers

You should better verify where you get that list from.

An empty list is all you need, because an empty list won't fail.

If you get this list from somewhere else and don't know if it is ok or not you could create a utility method and use it like this:

for( Object o : safe( list ) ) {
   // do whatever 
 }

And of course safe would be:

public static List safe( List other ) {
    return other == null ? Collections.EMPTY_LIST : other;
}
Oscar_Reyes_206540

You could potentially write a helper method which returned an empty sequence if you passed in null:

public static <T> Iterable<T> emptyIfNull(Iterable<T> iterable) {
    return iterable == null ? Collections.<T>emptyList() : iterable;
}

Then use:

for (Object object : emptyIfNull(someList)) {
}

I don't think I'd actually do that though - I'd usually use your second form. In particular, the "or throw ex" is important - if it really shouldn't be null, you should definitely throw an exception. You know that something has gone wrong, but you don't know the extent of the damage. Abort early.

user_226560

If you are getting that List from a method call that you implement, then don't return null, return an empty List.

If you can't change the implementation then you are stuck with the null check. If it should't be null, then throw an exception.

I would not go for the helper method that returns an empty list because it may be useful some times but then you would get used to call it in every loop you make possibly hiding some bugs.

user_2566600

Use, CollectionUtils.isEmpty(Collection coll) method which is Null-safe check if the specified collection is empty.

for this import org.apache.commons.collections.CollectionUtils.

Maven dependency

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.0</version>
</dependency>
user_59508460
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