How to collect to a List using streams returning my own List implementation?

Question

How to collect a stream into a list that is a subtype that I specify?

In other words, I'd like this test to pass. What should I do on the commented line to convert a stream to a MyList instance?

import org.junit.*;
import java.util.*;
import static java.util.stream.Collectors.*;
import static junit.framework.Assert.*;

@Test
public void collectUsingDifferentListType() {
    List<String> aList = new ArrayList<>();
    aList.add("A");
    aList.add("B");
    List<String> list1 = aList.stream().collect(toList());
    MyList<String> list2 = aList.stream().collect(toList(MyList::new));  // this doesn't exist, but I wish it did

    assertEquals(aList, list1);
    assertEquals(ArrayList.class, list1.getClass());
    assertEquals(aList, list2);
    assertEquals(MyList.class, list1.getClass());
}

The first assertion is wrong. The returned list of `toList()` happens to be an `ArrayList` in Oracle’s current implementation, but there is no specification guaranteeing you this. On the other hand, the second assertion is not of much use, as the operation fails with a `ClassCastException` if `list2` is not of `MyType` much earlier (unless you consider the possibility of a spurious subclass creation)… and you have a copy&paste error there. — Holger, Oct 23 '15 at 11:17

score 11 · Accepted Answer · answered Oct 23 '15 at 10:49

11

Assuming the MyList type is a Collection, you can use Collectors.toCollection:

MyList<String> list2 = aList.stream().collect(toCollection(MyList::new));

answered Oct 23 '15 at 10:49

Tunaki

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This also assumes MyList is mutable. – bowmore Oct 23 '15 at 11:17
1

@bowmore Well, if the `add` method throws an `UnsupportedOperationException`, there's not much sense in collecting a Stream into it, right? – Tunaki Oct 23 '15 at 11:19
1

@bowmore: so does the OP in the question, apparently. – Holger Oct 23 '15 at 11:20
1

I'm just adding to the answer, it may be obvious to you, it may not be to the OP. – bowmore Oct 23 '15 at 11:22

How to collect to a List using streams returning my own List implementation?

1 Answers1