public void optionSelected(String str)
{
    t = new Thread("SEL");
    t.start();
    soundURL ="/sesimulator/res/"+str+".mp3";
    t = new Thread("soundURL");
    t.start();
}

public class JoinExample {
 
    public static void main(String args[]) {
        Thread t1 = new Thread(new FirstTask());
        Thread t2 = new Thread(new SecondTask());
 
        t1.start();
        try {
            // wait for t1 to complete
            t1.join();
        } catch (InterruptedException ex) {
            Thread.currentThread().interrupt();
        }
        // start the next
        t2.start();
    }
 
    static class FirstTask implements Runnable {
        int counter=0;
 
        public void run() {
            try {
                while (counter<10){
                    System.out.println(++counter);
                    Thread.sleep(500);
                }
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }
    }
 
    static class SecondTask implements Runnable {
        int counter=64;
 
        public void run() {
            try {
                while (counter<74){
                    System.out.println(Character.toChars(++counter));
                    Thread.sleep(1000);
                }
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }
    }
}

import java.util.concurrent.*;
import java.net.*;
 
public class ConcurrentExample{
 
	public final ExecutorService es;
	private Future currentFuture = null;
 
	public ConcurrentExample(){
		es = Executors.newFixedThreadPool(1);
	}
 
	public static void main(String... args){
 
		ConcurrentExample ce = new ConcurrentExample();
 
		ce.optionSelected("http://www.google.com"); // invokes the run method - main process takes 5 seconds
		ConcurrentExample.advance(4, false); // silently wait 4 seconds before performing new task
		ce.optionSelected("http://www.yahoo.com"); // this request should fail since current request is still occurring
		ConcurrentExample.advance(6, false); // silently wait 6 seconds before performing new task
		ce.optionSelected("http://www.msn.com"); // this request should succeed since the previous task should have finished
		ce.es.shutdownNow(); // shut down processes so the JVM can exit naturally.
 
	}
 
	public static void advance(int seconds, boolean displayStatus){
			long first = System.currentTimeMillis(); // get current time
			long second = (seconds * 1000) + first; // get time arg seconds ahead of the first
			int count = 0;
 
			// while the first time is not equal to the second, assign first to the current time
			while(first < second){
 
				if((second - first) == 1000 && displayStatus){
					System.out.println( "Second: " + (++count));
				}
 
				first = System.currentTimeMillis();
			}
	}
 
	private class Error242 extends Exception{
		public Error242(String arg){
			super(arg);
		}
 
		public Error242(){
			super("An error occurred with the specified request!\nPlease try again!");
		}
	}
 
	public void optionSelected(String str){
 
		// A new thread should only execute after the previous is finished--
		if(currentFuture == null){
			try{
				currentFuture = es.submit(new Request(str));
			}catch(Exception e){
				System.out.println(e);
			}
		}else System.out.println("There is currently a task in process... please wait.");
 
		/*
		t = new Thread("SEL"); // assuming a seperate repetitive process
		t.start(); // starting that process
		soundURL ="/sesimulator/res/"+str+".mp3"; // process specified by user
		t = new Thread("soundURL"); // assign request
		t.start(); // start request
		*/
	}
 
/*
	// Probably not needed--
 
	private Runnable rt = new Runnable(){
 
		@Override public void run(){
			// task specific to "SEL"
 
 
		}
	};
*/
 
	private class Request implements Runnable{
 
		private URL userRequest = null;
 
		public Request(String userURL) throws Error242{
			try{
				userRequest = new URL(userURL);
			}catch(MalformedURLException me){
				// send error to some log for statistical reasons...
				throw new Error242(); // a more user-friendly error
			}
		}
 
		@Override public void run(){
			// task specific to SEL
			// task to be performed based on the user's request
 
			// dummy task to test the logic--
			ConcurrentExample.advance(5, true); // loudly wait 5 seconds
			// finally set the currentFuture to be null when the process is finished--
			currentFuture = null;
		}
	}
}

import java.util.concurrent.*;
import java.net.*;
 
public class ConcurrentExample{
 
	public final ExecutorService es;
	private volatile Future currentFuture = null; // if multiple threads share access to changing reference,
												  // then [merely guessing] they need to be able to notice changes done to
												  // shared reference.
	private Object lockObject = new Object();
 
	public ConcurrentExample(){
		es = Executors.newFixedThreadPool(1);
	}
 
	public static void main(String... args){
 
		ConcurrentExample ce = new ConcurrentExample();
 
		ce.optionSelected("http://www.google.com"); // invokes the run method - main process takes 5 seconds
		ConcurrentExample.advance(4, false); // silently wait 4 seconds before performing new task
		ce.optionSelected("http://www.yahoo.com"); // this request should fail since current request is still occurring
		ConcurrentExample.advance(6, false); // silently wait 6 seconds before performing new task
		ce.optionSelected("http://www.msn.com"); // this request should succeed since the previous task should have finished
		ce.es.shutdownNow(); // shut down processes so the JVM can exit naturally.
 
	}
 
	public static void advance(int seconds, boolean displayStatus){
			long first = System.currentTimeMillis(); // get current time
			long second = (seconds * 1000) + first; // get time 10 seconds ahead of the first
			int count = 0;
 
			// while the first time is not equal to the second, assign first to the current time
			while(first < second){
 
				if((second - first) == 1000 && displayStatus){
					System.out.println( "Second: " + (++count));
				}
 
				first = System.currentTimeMillis();
			}
	}
 
	private class Error242 extends Exception{
		public Error242(String arg){
			super(arg);
		}
 
		public Error242(){
			super("An error occurred with the specified request!\nPlease try again!");
		}
	}
 
	public void optionSelected(String str){
 
		// A new thread should only execute after the previous is finished--
		if(currentFuture == null){ // first test, but not thread safe - when it isn't null threads will skip this
			synchronized(lockObject){ // locks all threads, but one, out of the statement
				if(currentFuture == null){ // the single thread checks to see if currentFuture is null
					try{ 
						currentFuture = es.submit(new Request(str)); // single thread attempts this task
					}catch(Exception e){
						System.out.println(e);
					}
				}
			} // single thread has exited statement, releasing the lock and allowing a different thread to enter
		}else System.out.println("There is currently a task in process... please wait.");
	}
 
	private class Request implements Runnable{
 
		private URL userRequest = null;
 
		public Request(String userURL) throws Error242{
			try{
				userRequest = new URL(userURL);
			}catch(MalformedURLException me){
				// send error to some log for statistical reasons...
				throw new Error242(); // a more user-friendly error
			}
		}
 
		@Override public void run(){
			// task specific to SEL
			// task to be performed based on the user's request
 
			// dummy task to test the logic--
			ConcurrentExample.advance(5, true); // loudly wait 5 seconds
			// finally set the currentFuture to be null when the process is finished--
			currentFuture = null;
		}
	}
}