How do you cast a List of supertypes to a List of subtypes?

Question

For example, lets say you have two classes:

public class TestA {}
public class TestB extends TestA{}

I have a method that returns a L

Answer 1

You can use the selectInstances method in Eclipse Collections. This will involved creating a new collection however so will not be as efficient as the accepted solution which uses casting.

List parent =
        Arrays.asList("1","2","3", new StringBuffer("4"));
List strings =
        Lists.adapt(parent).selectInstancesOf(String.class);
Assert.assertEquals(Arrays.asList("1","2","3"), strings);

I included StringBuffer in the example to show that selectInstances not only downcasts the type, but will also filter if the collection contains mixed types.

Note: I am a committer for Eclipse Collections.