Question

Looking at the javadocs it just says

<T> Future<T> submit(Runnable task, T result)

Submits a Runnable task for execution and returns a Future representing that task. The Future’s get method will return the given result upon successful completion.

Parameters:

task – the task to submit

result – the result to return

but what does it do with result? does it store anything there? does it just use the type of result to specify the type of Future<T>?

Answer 1

It doesn’t do anything with the result – just holds it. When the task successfully completes, calling future.get() will return the result you passed in.

Here is the source code of Executors$RunnableAdapter, which shows that after the task has run, the original result is returned:

static final class RunnableAdapter<T> implements Callable<T> {
    final Runnable task;
    final T result;
    RunnableAdapter(Runnable  task, T result) {
        this.task = task;
        this.result = result;
    }
    public T call() {
        task.run();
        return result;
    }
}

And yes, the generic type of the result should match that of the returned Future.