Null check in an enhanced for loop

Question

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) {
            


        

Answer 1

For anyone uninterested in writing their own static null safety method you can use: commons-lang's org.apache.commons.lang.ObjectUtils.defaultIfNull(Object, Object). For example:

    for (final String item : 
    (List<String>)ObjectUtils.defaultIfNull(items, Collections.emptyList())) { ... }

ObjectUtils.defaultIfNull JavaDoc

Answer 2

Another way to effectively guard against a null in a for loop is to wrap your collection with Google Guava's Optional<T> as this, one hopes, makes the possibility of an effectively empty collection clear since the client would be expected to check if the collection is present with Optional.isPresent().

Answer 3

It's already 2017, and you can now use Apache Commons Collections4

The usage:

for(Object obj : ListUtils.emptyIfNull(list1)){
    // Do your stuff
}

You can do the same null-safe check to other Collection classes with CollectionUtils.emptyIfNull.

Answer 4

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.

Answer 5

Use ArrayUtils.nullToEmpty from the commons-lang library for Arrays

for( Object o : ArrayUtils.nullToEmpty(list) ) {
   // do whatever 
}

This functionality exists in the commons-lang library, which is included in most Java projects.

// ArrayUtils.nullToEmpty source code 
public static Object[] nullToEmpty(final Object[] array) {
    if (isEmpty(array)) {
        return EMPTY_OBJECT_ARRAY;
    }
    return array;
}

// ArrayUtils.isEmpty source code
public static boolean isEmpty(final Object[] array) {
    return array == null || array.length == 0;
}

This is the same as the answer given by @OscarRyz, but for the sake of the DRY mantra, I believe it is worth noting. See the commons-lang project page. Here is the nullToEmpty API documentation and source

Maven entry to include commons-lang in your project if it is not already.

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.4</version>
</dependency>

Unfortunately, commons-lang doesn't provide this functionality for List types. In this case you would have to use a helper method as previously mentioned.

public static <E> List<E> nullToEmpty(List<E> list)
{
    if(list == null || list.isEmpty())
    {
        return Collections.emptyList();
    }
    return list;
}

Answer 6

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;
}