Access outer variable from inner anonymous Runnable

Question

The following example code (SSCCE) complains that 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 things working i change the code to that one

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:

1. Is there something wrong with this ?
2. The ObjectRef class exists as standard class in JSE ?
3. What is the right way ?

Answer 1

Right way is using 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's not finished yet


ex.shutdown();

Answer 2

Did you consider using Callable instead? Callable can be used when you produce a result, which seem to be your case.

   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() );
    }