What is the best way to filter a Java Collection?

Question

I want to filter a java.util.Collection based on a predicate.

Answer 1

My answer builds on that from Kevin Wong, here as a one-liner using CollectionUtils from spring and a Java 8 lambda expression.

CollectionUtils.filter(list, p -> ((Person) p).getAge() > 16);

This is as concise and readable as any alternative I have seen (without using aspect-based libraries)

Spring CollectionUtils is available from spring version 4.0.2.RELEASE, and remember you need JDK 1.8 and language level 8+.

Answer 2

Use CollectionUtils.filter(Collection,Predicate), from Apache Commons.

Answer 3

Let’s look at how to filter a built-in JDK List and a MutableList using Eclipse Collections (formerly GS Collections).

List<Integer> jdkList = Arrays.asList(1, 2, 3, 4, 5);
MutableList<Integer> ecList = Lists.mutable.with(1, 2, 3, 4, 5);

If you wanted to filter the numbers less than 3, you would expect the following outputs.

List<Integer> selected = Lists.mutable.with(1, 2);
List<Integer> rejected = Lists.mutable.with(3, 4, 5);

Here’s how you can filter using an anonymous inner class as the Predicate.

Predicate<Integer> lessThan3 = new Predicate<Integer>()
{
    public boolean accept(Integer each)
    {
        return each < 3;
    }
};

Assert.assertEquals(selected, Iterate.select(jdkList, lessThan3));

Assert.assertEquals(selected, ecList.select(lessThan3));

Here are some alternatives to filtering JDK lists and Eclipse Collections MutableLists using the Predicates factory.

Assert.assertEquals(selected, Iterate.select(jdkList, Predicates.lessThan(3)));

Assert.assertEquals(selected, ecList.select(Predicates.lessThan(3)));

Here is a version that doesn't allocate an object for the predicate, by using the Predicates2 factory instead with the selectWith method that takes a Predicate2.

Assert.assertEquals(
    selected, ecList.selectWith(Predicates2.<Integer>lessThan(), 3));

Sometimes you want to filter on a negative condition. There is a special method in Eclipse Collections for that called reject.

Assert.assertEquals(rejected, Iterate.reject(jdkList, lessThan3));

Assert.assertEquals(rejected, ecList.reject(lessThan3));

Here’s how you can filter using a Java 8 lambda as the Predicate.

Assert.assertEquals(selected, Iterate.select(jdkList, each -> each < 3));
Assert.assertEquals(rejected, Iterate.reject(jdkList, each -> each < 3));

Assert.assertEquals(selected, gscList.select(each -> each < 3));
Assert.assertEquals(rejected, gscList.reject(each -> each < 3));

The method partition will return two collections, containing the elements selected by and rejected by the Predicate.

PartitionIterable<Integer> jdkPartitioned = Iterate.partition(jdkList, lessThan3);
Assert.assertEquals(selected, jdkPartitioned.getSelected());
Assert.assertEquals(rejected, jdkPartitioned.getRejected());

PartitionList<Integer> ecPartitioned = gscList.partition(lessThan3);
Assert.assertEquals(selected, ecPartitioned.getSelected());
Assert.assertEquals(rejected, ecPartitioned.getRejected());

Note: I am a committer for Eclipse Collections.