Library method to partition a collection by a predicate

Question

I have a collection of objects that I would like to partition into two collections, one of which passes a predicate and one of which fails a predicate. I was hoping there wo

Answer 1

If you're using Eclipse Collections (formerly GS Collections), you can use the partition method on all RichIterables.

MutableList integers = FastList.newListWith(-3, -2, -1, 0, 1, 2, 3);
PartitionMutableList result = integers.partition(IntegerPredicates.isEven());
Assert.assertEquals(FastList.newListWith(-2, 0, 2), result.getSelected());
Assert.assertEquals(FastList.newListWith(-3, -1, 1, 3), result.getRejected());

The reason for using a custom type, PartitionMutableList, instead of Pair is to allow covariant return types for getSelected() and getRejected(). For example, partitioning a MutableCollection gives two collections instead of lists.

MutableCollection integers = ...;
PartitionMutableCollection result = integers.partition(IntegerPredicates.isEven());
MutableCollection selected = result.getSelected();

If your collection isn't a RichIterable, you can still use the static utility in Eclipse Collections.

PartitionIterable partitionIterable = Iterate.partition(integers, IntegerPredicates.isEven());
PartitionMutableList partitionList = ListIterate.partition(integers, IntegerPredicates.isEven());

Note: I am a committer for Eclipse Collections.