Confused with CountedCompleter's sample code fragment in java8 doc

Question

I read java8 doc about CountedCompleter, which gives sample usage code as follow:

 class MyOperation<E> { void apply(E e) { ... }  }

 class ForEach<E> extends CountedCompleter<Void> {

   public static <E> void forEach(E[] array, MyOperation<E> op) {
     new ForEach<E>(null, array, op, 0, array.length).invoke();
   }

   final E[] array; final MyOperation<E> op; final int lo, hi;
   ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
     super(p);
     this.array = array; this.op = op; this.lo = lo; this.hi = hi;
   }

   public void compute() { // version 1
     if (hi - lo >= 2) {
       int mid = (lo + hi) >>> 1;
       setPendingCount(2); // must set pending count before fork
       new ForEach(this, array, op, mid, hi).fork(); // right child
       new ForEach(this, array, op, lo, mid).fork(); // left child
     }
     else if (hi > lo)
       op.apply(array[lo]);
     tryComplete();
   }
 }

In compute method, Every ForEach object will fork two subtasks and set pending count to 2, but in the end of compute method, tryComplete can only decrement one in pending count, how does the rest one??

Answer 1

After reading source code of ForkJoinPool and CountedCompleter, I finally understand it.

All CountedCompleters derived from root CountedCompleter will be organized like a tree. When you call tryComplete, if current pending count is positive, it will decrement 1. Otherwise it will call onCompletion of current CountedCompleter, then recursively call tryComplete on parent CountedCompleter. If parent CountedCompleter is null, it means it is already root CountedCompleter, then the whole task complete.

So we know that:

• CountedCompleter task will not end after compute() method is done, it will wait until pending count decrements to 0
• CountedCompleter task isn't always ended by root task itself (very different from RecursiveTask and RecursiveAction), it can be ended by childs' tryComplete

Then let's see the code fragment in java 8 doc (note sequence number 0: 1: 2: 3: in code, it is one possible execute order):

 class MyOperation<E> { void apply(E e) { ... }  }

 class ForEach<E> extends CountedCompleter<Void> {

   public static <E> void forEach(E[] array, MyOperation<E> op) {
     new ForEach<E>(null, array, op, 0, array.length).invoke();
   }

   final E[] array; final MyOperation<E> op; final int lo, hi;
   ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
     super(p);
     this.array = array; this.op = op; this.lo = lo; this.hi = hi;
   }

   public void compute() { // version 1
     if (hi - lo >= 2) {
       int mid = (lo + hi) >>> 1;
       setPendingCount(2); // 0: +2
       new ForEach(this, array, op, mid, hi).fork(); // 2: -1
       new ForEach(this, array, op, lo, mid).fork(); // 3: pending count == 0 complete
     }
     else if (hi > lo)
       op.apply(array[lo]);
     tryComplete();  // 1: -1
   }
 }