Producing return values from tasks

Producing return values from tasks
A Runnable is a separate task that performs work, but it doesn!ˉt return a value. If you want
the task to produce a value when it!ˉs done, you can implement theCallable interface rather
than the Runnable interface. Callable, introduced in Java SE5, is a generic with a type
parameter representing the return value from the method call( ) (instead of run( )), and
must be invoked using an ExecutorService submit( ) method. Here!ˉs a simple example:

//: concurrency/CallableDemo.java
import java.util.concurrent.*;
import java.util.*;
 
class TaskWithResult implements Callable {
  private int id;
  public TaskWithResult(int id) {
    this.id = id;
  }
  public String call() {
    return "result of TaskWithResult " + id;
  }
}
 
public class CallableDemo {
  public static void main(String[] args) {
    ExecutorService exec = Executors.newCachedThreadPool();
    ArrayList> results =
      new ArrayList>();
    for(int i = 0; i < 10; i++)
      results.add(exec.submit(new TaskWithResult(i)));
    for(Future fs : results)
      try {
        // get() blocks until completion:
        System.out.println(fs.get());
      } catch(InterruptedException e) {
        System.out.println(e);
        return;
      } catch(ExecutionException e) {
        System.out.println(e);
      } finally {
        exec.shutdown();
      }
  }
} /* Output:
result of TaskWithResult 0
result of TaskWithResult 1
result of TaskWithResult 2
result of TaskWithResult 3
result of TaskWithResult 4
result of TaskWithResult 5
result of TaskWithResult 6

result of TaskWithResult 7
result of TaskWithResult 8
result of TaskWithResult 9
*///:~
The submit( ) method produces a Future object, parameterized for the particular type of
result returned by the Callable. You can query the Future with isDone( ) to see if it has
completed. When the task is completed and has a result, you can call get( ) to fetch the
result. You can simply call get( ) without checking isDone( ), in which case get( ) will block
until the result is ready. You can also call get( ) with a timeout, or isDone( ) to see if the
task has completed, before trying to call get( ) to fetch the result. 
The overloaded Executors.callable( ) method takes a Runnable and produces a
Callable. ExecutorService has some "invoke" methods that run collections of Callable
objects. 

 

copied from Thinking in java