How to Get a Return Value from a Thread in Java

If we need to get a return value from a thread in Java, prior to Java 5, we can do something like this.

public class ReturnValueWithThread {
    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.start();

        // busy doing something.....

        while (!t.isDone()) {
            Thread.yield();
        }
        System.out.println(t.getMessage());
    }
    
    static class MyThread extends Thread {
        private boolean done;
        private String message;
        
        /** 
         * {@inheritDoc}
         */
        @Override
        public void run() {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            done = true;
            message = "Hello World";
        }
        
        public boolean isDone() {
            return done;
        }
        
        public String getMessage() {
            return message;
        }
    }
}

In Java 5 onwards, this task can be easily achieved by using java.util.concurrent.ExecutorService and java.util.concurrent.Future.

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class ReturnValueWithExecutor {
    public static void main(String[] args) {
        ExecutorService e = Executors.newFixedThreadPool(1);
        Future<String> f = e.submit(new Callable<String>() {
            @Override
            public String call() throws Exception {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                return "Hello World";
            }
        });

        // busy doing something...

        try {
            String message = f.get();
            System.out.println(message);
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        } catch (ExecutionException e1) {
            e1.printStackTrace();
        }
    }
}