hg: lambda/lambda/jdk: Modify Iterables.uniqueElements to be fully lazy. (original) (raw)

Rémi Forax forax at univ-mlv.fr
Fri Apr 13 16:12:28 PDT 2012

On 04/14/2012 12:23 AM, stuart.marks at oracle.com wrote:

Changeset: 08c12c22393c Author: smarks Date: 2012-04-13 15:24 -0700 URL: http://hg.openjdk.java.net/lambda/lambda/jdk/rev/08c12c22393c

Modify Iterables.uniqueElements to be fully lazy. ! src/share/classes/java/util/Iterables.java ! src/share/classes/java/util/Iterators.java ! test-ng/tests/org/openjdk/tests/java/util/IterableTest.java ! test-ng/tests/org/openjdk/tests/java/util/IterablesTest.java

Hi Stuart, null should be an allowed value. It's really easy to implement it with a null object. Also I think it's better to let the user choose which Set he wants to use (the equals semantics is not always the best).

So the code should be something like this:

private static final Object NULL_OBJECT = new Object();

public static Iterator uniqueElements(final Iterator<? extends T> iterator, final Set<? super T> seen) { Objects.requireNonNull(iterator); Objects.requireNonNull(cache);

     return new Iterator<T>() {
         private T cache = NULL_OBJECT;

         @Override
         public boolean hasNext() {
             if (cache != NULL_OBJECT) {
                 return true;
             }

             // cache is empty so attempt to populate it
             while (iterator.hasNext()) {
                 T tmp = iterator.next();
                 if (seen.add(tmp)) {
                     cache = tmp;
                     return true;
                 }
             }
             return false;
         }

         @Override
         public T next() {
             if (!hasNext()) {
                 throw new NoSuchElementException();
             }
             T r = cache;
             cache = NULL_OBJECT;
             return r;
         }
     };
 }

Rémi

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