returning values with Runnable.run()

[Dan Murphy]

Hi,

I noticed the following method defined in ExecutorService

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

This implies to me that you can create a Runnable that returns a result and execute this Runnable (and obtain it's result) using the method above.
But how do you create a Runnable that returns a result when this interface contains only a single run() method with a void return type?
If someone could show a simple example, I'd really appreciate it.

Thanks in advance,
Dan
[ March 05, 2008: Message edited by: Dan Murphy ]

[Henry Wong]

This implies to me that you can create a Runnable that returns a result and execute this Runnable (and obtain it's result) using the method above.
But how do you create a Runnable that returns a result when this interface contains only a single run() method with a void return type?
If someone could show a simple example, I'd really appreciate it.

Did you read the JavaDoc completely? The object that will be returned is the object that is passed to the submit() method. The future object will just wait until the run() method is done and return the result object that it already has. The run() method doesn't generate or return the result object.

Now, to answer your question. Ideally, this resultant object is mutable. And the run() method will be passed a copy of this object. Either via a constructor (of the runnable object), or via a global variable. Once a result is calculated, the run() method just stores the result in the resultant object and return. The future object will then return this shared object via the get() method.

Henry