Leonid Leonid - 1 year ago 61
Java Question

Collecting stream back into the same collection type

Suppose I have a collection of the unknown type. What I want to do is stream it, do some stuff on the stream, and collect it back into the same collection type as my original collection. For instance:

Collection<? extends Integer> getBigger(Collection<? extends Integer> col, int value) {
return col.stream().filter(v -> v > value).collect(????);

The idea of this incomplete code example is to return a
is of
class (or any subclass of it), a
is of
class, etc... The method name and actual operations on the stream here are not important, I've specified them just to illustrate my question.
So, is it possible?

Answer Source

It is not possible without violating the principle on which the Java streams framework has been built on. It would completely violate the idea of abstracting the stream from its physical representation.

The sequence of bulk data operations goes in a pipeline, see the following picture: Pipeline: A Sequence of Bulk Data Operations

The stream is somehow similar to the Schrödinger's cat - it is not materialized until you call the terminal operation. The stream handling is completely abstract and detached from the original stream source.

Pipeline as a Black Box

If you want to work so low-level with your original data storage, don't feel ashamed simply avoiding the streams. They are just a tool, not anything sacred. By introducing streams, the Good Old Collections are still as good as they were, with added value of the internal iteration - the new Iterable.forEach() method.

Added to satisfy your curiosity :)

A possible solution follows. I don't like it myself, and I have not been able to solve all the generics issues there, but it works with limitations.

The idea is creating a collector returning the same type as the input collection. However, not all the collections provide a nullary constructor (with no parameters), and without it the Class.newInstance() method does not work. There is also the problem of the awkwardness of checked exceptions within lambda expression. (It is mentioned in this nice answer here: http://stackoverflow.com/a/22919112/2886891)

public Collection<Integer> getBiggerThan(Collection<Integer> col, int value) {
    // Collection below is an example of one of the rare appropriate 
    // uses of raw types. getClass returns the runtime type of col, and 
    // at runtime all type parameters have been erased.
    final Class<? extends Collection> clazz = col.getClass();
    System.out.println("Input collection type: " + clazz);
    final Supplier<Collection<Integer>> supplier = () -> {
        try {
            return clazz.newInstance();
        catch (InstantiationException | IllegalAccessException e) {
            throw new RuntimeException(
                    "A checked exception caught inside lambda", e);
    // After all the ugly preparatory code, enjoy the clean pipeline:
    return col.stream()
            .filter(v -> v > value)
            .collect(supplier, Collection::add, Collection::addAll);

As you can see, it works in general, supposed your original collection provides a nullary constructor.

public void test() {
    final Collection<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

    final Collection<Integer> arrayList = new ArrayList<>(numbers);
    final Collection<Integer> arrayList2 = getBiggerThan(arrayList, 6);

    final Collection<Integer> set = new HashSet<>(arrayList);
    final Collection<Integer> set2 = getBiggerThan(set, 6);

    // This does not work as Arrays.asList() is of a type
    // java.util.Arrays$ArrayList which does not provide a nullary constructor
    final Collection<Integer> numbers2 = getBiggerThan(numbers, 6);
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