Friday, November 30, 2018

In JDK 9 (and well 8) and above everything can be a stream.

In JDK 8 we finally got to use streams and everything was good apart from the times when the API you used couldn't produce a stream. Then you ended up writing a wrapper class method that allowed you to convert an iterator into a Stream, because you missed streams.

public static <T> Stream<T> asStream(Iterator<T> it) {
        Spliterator.IMMUTABLE | Spliterator.ORDERED),false);

Now there are methods to programmatically generate stream in the case of iterate and generate but both of these generate an infinite stream whereas in most cases you really wanted to adapt an existing interface into a finite Stream.

This was resolved nicely in JDK 9 with the introduction of a new form of iterate method that allows you to provide a predicate to signal the end of the stream.

In the examples below I am going to use a predicate that continues until you get a null entry to the stream, I will leave it up to the reader to come up with more imaginative uses for predicate. In this simple example I am using the getCause method of Throwable to move us along a linked list of errors. Note how little code this would take when compared with a pre stream version.

// Simple linked list
Exception e = new Exception("one");
Exception e2 = new Exception("two",e);
Exception e3 = new Exception("three", e2);

Stream.iterate(e3, Objects::nonNull, Throwable::getCause)

    // Output the messages in turn

The second example converts a ReferenceQueue into a Stream so that we can easily drain its contents for processing as required. This code is a little bit different because the container is different from the object be worked on, so we provide the seed and the next value using the same method, This returns null when the queue becomes empty.

ReferenceQueue<Thing> queue = new ReferenceQueue<>();

// Make some things and then collect them
WeakReference one = new WeakReference<Thing>(new Thing(), queue);
WeakReference two = new WeakReference<Thing>(new Thing(), queue);
System.gc(); System.gc(); System.gc(); System.gc(); System.gc();

Stream.<Reference<? extends Thing>>iterate(
    queue.poll(), Objects::nonNull, v -> queue.poll())


The third example shows a walk over a Node tree, note the nested stream iterator to work back up the list when we have worked to the end of a leaf.

Node root = doc.getDocumentElement();

    v -> {
        if (v.getFirstChild()!=null) {
            return v.getFirstChild();

        if (v.getNextSibling()!=null) {
            return v.getNextSibling();

        return Stream.iterate(v, Objects::nonNull, Node::getParentNode)
            .filter(node -> node.getNextSibling()!=null)


So with a little bit of mental gymnastics it is possible to transform most legacy APIs into a nice clean Stream, so you can ignore those nasty old fashioned for loops. And if you are stuck in JDK 8 then it is quite easy to put together a similar function using the asStream from before:

public static<T> Stream<T> iterateFinite(T seed, Predicate<? super T> hasNext, UnaryOperator<T> next) {

    return asStream(new Iterator<>() {

        T current = seed;

        public boolean hasNext() {
            return hasNext.test(current);

        public T next() {
            if (current == null) {
                throw new NoSuchElementException();
            try {
                return current;
            } finally {
                current = next.apply(current);

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