Access an external variable from an internal anonymous Runnable

Question

The following sample code (SSCCE) complains that the local variable a must be final.

public class Foo {

    final List<A> list = new ArrayList() {{ add(new A()); }};

    void foo() {
        A a;
        Thread t = new Thread(new Runnable() {
            public void run() {
                a = list.get(0); // not good !
            }
        });
        t.start();
        t.join(0);
        System.out.println(a);
    }

    class A {}
}

To make everything work, I change the code to this

public class Foo {

    final List<A> list = new ArrayList() {{ add(new A()); }};

    void foo() {

        // A a;
        final ObjectRef x = new ObjectRef();
        Thread t = new Thread(new Runnable() {

            public void run() {
                // a = list.get(0);
                x.set(list.get(0));
            }

        });
        t.start();
        t.join(0);

        // System.out.println(a);
        System.out.println(x.get());
    }

    class A {}

     class ObjectRef<T> {
        T x;

        public ObjectRef() {}

        public ObjectRef(T x) { this.x = x; }

        public void set(T x) {  this.x = x; }

        public T get() { return x; }
    }
}

My questions:

• Is there something wrong with this?
• Does the ObjectRef class exist as a standard class in JSE?
• What is the right way?

Answer 1

Have you used it instead Callable? Callablecan be used when you produce a result that seems to be your business.

   final List<A> list = new ArrayList() {{ add(new A()); }};

   void foo() {

      Callable<A> call = new Callable<A> {
          A call() throws Exception
          {
              // do something with the list
              return list.get(0);
          }
       }

       ExecutorService executor = new ScheduledThreadPoolExecutor(1);
       Future<A> future = executor.submit(call);

       System.out.println( future.get() );
    }

Answer 2

The right way is to use FutureTask and Callable

FutureTask task = new FutureTask(new Callable<A>() {
   public A call() {
      return list.get(0);
   }
});

Executor ex = Executors.newFixedThreadPool(1);
ex.execute(task);

// do something else, code is executing in different thread

A a = task.get(); //get result of execution, will wait if it not finished yet


ex.shutdown();

Answer 3

, Callable FutureTask.

But there is no need to use the Contractor: if you are not going to share this Contractor with another code, the three lines necessary to create it represent the task, and then close it again, look too detailed. You can just use Thread.

FutureTask<A> task = new FutureTask(new Callable<A>() {
   public A call() {
      return list.get(0);
   }
});
new Thread(task).start();
A result = task.get();