yield-java

Implementation of c#'s Yield in Java. https://msdn.microsoft.com/en-us/library/9k7k7cf0.aspx

Based on Jim Blackler's implementation using Threads. http://jimblackler.net/blog/?p=61

Updated to take advantage of Java 8 features.

@Test public void example() {
    ArrayList<Integer> results = new ArrayList<>();
    for (Integer number : oneToFive()) results.add(number);
    assertEquals(asList(1,2,3,4,5), results);
}

public static Yielderable<Integer> oneToFive() {
    return yield -> {
        for (int i = 1; i < 10; i++) {
            if (i == 6) yield.breaking();
            yield.returning(i);
        }
    };
}

public static Yielderable<String> fooBar(SideEffects sideEffects) {
   return yield -> {
       sideEffects.sideEffect(1);
       yield.returning("foo");
       sideEffects.sideEffect(2);
       yield.returning("bar");
       sideEffects.sideEffect(3);
   };
}

@Test public void example() {
   SideEffects sideEffects = mock(SideEffects.class);

   Iterable<String> strings = fooBar(sideEffects);

   verifyZeroInteractions(sideEffects);

   assertEquals("foo", strings.iterator().next());

   verify(sideEffects).sideEffect(1);
   verifyNoMoreInteractions(sideEffects);
}

You can also use for infinite iterables, but you should close the iterator when you're done with it to free up the thread.

@Test public void example() {
   try (ClosableIterator<Integer> positiveIntegers = positiveIntegers().iterator()) {
       assertEquals(Integer.valueOf(1), positiveIntegers.next());
       assertEquals(Integer.valueOf(2), positiveIntegers.next());
       assertEquals(Integer.valueOf(3), positiveIntegers.next());
   }
}

public static Yielderable<Integer> positiveIntegers() {
   return yield -> {
       int i = 0;
       while (true) yield.returning(++i);
   };
}