This quick tutorial is going to show how to remove all null elements from a List, using plain Java, Guava, the Apache Commons Collections and the newer Java 8 lambda support.

1. Remove nulls from a List using plain Java

The Java Collections Framework offers a simple solution for removing all null elements in the List – a basic while loop:

@Test
public void givenListContainsNulls_whenRemovingNullsWithPlainJava_thenCorrect() {
    List<Integer> list = Lists.newArrayList(null, 1, null);
    while (list.remove(null));

    assertThat(list, hasSize(1));
}

Note that this approach will modify the source list.

2. Remove nulls from a List using Google Guava

We can also remove nulls using Guava and a more functional approach, via predicates:

@Test
public void givenListContainsNulls_whenRemovingNullsWithGuavaV1_thenCorrect() {
    List<Integer> list = Lists.newArrayList(null, 1, null);
    Iterables.removeIf(list, Predicates.isNull());

    assertThat(list, hasSize(1));
}

Alternatively, if we don’t want to modify the source list, Guava will allow us to create a new, filter list:

@Test
public void givenListContainsNulls_whenRemovingNullsWithGuavaV2_thenCorrect() {
    List<Integer> list = Lists.newArrayList(null, 1, null, 2, 3);
    List<Integer> listWithoutNulls = Lists.newArrayList(
      Iterables.filter(list, Predicates.notNull()));

    assertThat(listWithoutNulls, hasSize(3));
}

3. Remove nulls from a List using Apache Commons Collections

Let’s now look at a simple solution using the Apache Commons Collections library using a similar functional style:

private void removeNullsFromListApache() {
    List<Integer> list = Lists.newArrayList(null, 1, 2, null, 3, null);
    CollectionUtils.filter(list, PredicateUtils.notNullPredicate());
}

Note that this solution will also create a new list and leave the original unchanged.

4. Remove nulls from a List using Lambdas (Java 8)

Finally – let’s look at a Java 8 solution using Lambdas to filter the List; the filtering process can be done in parallel or serial:

public void givenListContainsNulls_whenFilteringParallel_thenCorrect() {
    List<Integer> list = Lists.newArrayList(null, 1, 2, null, 3, null);
    List<Integer> listWithoutNulls = 
      list.parallelStream().filter(i -> i != null).collect(Collectors.toList());
}

public void givenListContainsNulls_whenFilteringSerial_thenCorrect() {
    List<Integer> list = Lists.newArrayList(null, 1, 2, null, 3, null);
    List<Integer> listWithoutNulls = 
      list.stream().filter(i -> i != null).collect(Collectors.toList());
}

And that’s it – some quick and very useful solutions for getting rid of all null elements from a List.