How do I turn a Java Enumeration into a Stream?

Question

I have a third party library that gives me an Enumeration. I want to work with that enumeration lazily as a Java 8 Stream, calling thing

Answer 1

This answer already provides a solution which creates a Stream out of an Enumeration:

 public static  Stream enumerationAsStream(Enumeration e) {
     return StreamSupport.stream(
         Spliterators.spliteratorUnknownSize(
             new Iterator() {
                 public T next() {
                     return e.nextElement();
                 }
                 public boolean hasNext() {
                     return e.hasMoreElements();
                 }
             },
             Spliterator.ORDERED), false);
 }

It should be emphasized that the resulting Stream is as lazy as any other Stream, as it won’t process any items before the terminal action has been commenced and if the terminal operation is short-circuiting, it will iterate only as many items as necessary.

Still, it has room for improvement. I’d always add a forEachRemaining method when there is a straight-forward way to process all elements. Said method will be called by the Stream implementation for most non-short-circuiting operations:

public static  Stream enumerationAsStream(Enumeration e) {
    return StreamSupport.stream(
        Spliterators.spliteratorUnknownSize(
            new Iterator() {
                public T next() {
                    return e.nextElement();
                }
                public boolean hasNext() {
                    return e.hasMoreElements();
                }
                public void forEachRemaining(Consumer action) {
                    while(e.hasMoreElements()) action.accept(e.nextElement());
                }
            },
            Spliterator.ORDERED), false);
}

However, the code above is a victim of the “using Iterator because it’s so familiar” antipattern. The created Iterator will get wrapped into an implementation of the new Spliterator interface and provides no advantage over implementing Spliterator directly:

public static  Stream enumerationAsStream(Enumeration e) {
    return StreamSupport.stream(
        new Spliterators.AbstractSpliterator(Long.MAX_VALUE, Spliterator.ORDERED) {
            public boolean tryAdvance(Consumer action) {
                if(e.hasMoreElements()) {
                    action.accept(e.nextElement());
                    return true;
                }
                return false;
            }
            public void forEachRemaining(Consumer action) {
                while(e.hasMoreElements()) action.accept(e.nextElement());
            }
    }, false);
}

On the source code level, this implementation is as simple as the Iterator-based, but eliminates the delegation from a Spliterator to an Iterator. It only requires its readers to learn about the new API.