What's the Java equivalent of C++'s accumulate or Groovy's inject?

Question

Essentially, I'd like to do the following as a one-liner:

        int sum = initialValue;
        for (int n : collectionOfInts) {
            sum += n;
        }
        return sum;

I see that there's http://functionaljava.org/examples/1.5/#Array.foldLeft , but I'd rather not have to copy the collection.

Answer 1

I see that there's http://functionaljava.org/examples/1.5/#Array.foldLeft , but I'd rather not have to copy the collection.

If you use the foldLeft from IterableW instead of Array, you won't have to copy anything.

Answer 2

Sorry, it still doesn't exist in Java 7. You'll have to wait for Java 8, where Closures shall be implemented.

In the meantime, you can use FunctionalJava, Guava, or a JVM-compatible, closure-enabled language such as Groovy.

Answer 3

Just for fun - here's how to do it without an external library:

return fold(collectionOfInts, 0, ADD);

Oh, and here's the rest :)

static <X, Y> X fold(final Iterable<? extends Y> gen, final X initial, final Function2<? super X, ? super Y, ? extends X> function) {
  final Iterator<? extends Y> it = gen.iterator();
  if (!it.hasNext()) {
    return initial;
  }
  X acc = initial;
  while (it.hasNext()) {
    acc = function.apply(acc, it.next());
  }
  return acc;
}

static final Function2<Integer, Integer, Integer> ADD = new Function2<Integer, Integer, Integer>() {
  @Override
  public Integer apply(Integer a, Integer b) {
    return a + b;
  }
};

interface Function2<A, B, C> {
  C apply(A a, B b);
}