comparison of code performance, threaded versus non-threaded

//code without thread use
public static int getNextPrime(int from) {
    int nextPrime = from+1;
    boolean superPrime = false;
    while(!superPrime) {
        boolean prime = true;
        for(int i = 2;i &lt nextPrime;i++) {
            if(nextPrime % i == 0) {
                prime = false;
            }
        }
        if(prime) {
            superPrime = true;
        } else {
            nextPrime++;
        }
    }
    return nextPrime;
}

public static void main(String[] args) {
   int primeStart = 5;
   ArrayList list = new ArrayList();
   for(int i = 0;i &lt 10000;i++) {
       list.add(primeStart);
       primeStart = getNextPrime(primeStart);
   }
}

public class PrimeRunnable implements Runnable {

    private int from;
    private int lastPrime;

    public PrimeRunnable(int from) {
        this.from = from;
    }

    public boolean isPrime(int number) {
        for(int i = 2;i &lt from;i++) {
            if((number % i) == 0) {
                return false;
            }
        }
        lastPrime = number;
        return true;
    }

    public int getLastPrime() {
        return lastPrime;
    }

    public void run() {
        while(!isPrime(++from))
            ;
    }
}

public static void main(String[] args) {
   int primeStart = 5;
   ArrayList list = new ArrayList();
   for(int i = 0;i &lt 10000;i++) {
     PrimeRunnable pr = new PrimeRunnable(primeStart);
     Thread t = new Thread(pr);
     t.start();
     t.join();
     primeStart = pr.getLastPrime();
     list.add(primeStart);
   }
}

public static void main(String[] args) {
   int primeStart = 5;

   // Make thread-safe list for adding results to
   List list = Collections.synchronizedList(new ArrayList());

   // Pull thread pool count out into a value so you can easily change it
   int threadCount = 10000;
   Thread[] threads = new Thread[threadCount];

   // Start all threads
   for(int i = 0;i < threadCount;i++) {
     // Pass list to each Runnable here
     // Also, I added +i here as I think the intention is
     //    to test 10000 possible numbers>5 for primeness -
     //    was testing 5 in all loops
     PrimeRunnable pr = new PrimeRunnable(primeStart+i, list);
     Thread[i] threads = new Thread(pr);
     threads[i].start();  // thread is now running in parallel
   }

   // All threads now running in parallel

   // Then wait for all threads to complete
   for(int i=0; i<threadCount; i++) {
     threads[i].join();
   }
}

public class PrimeRunnable implements Runnable {
    private int from;
    private List results;   // shared but thread-safe

    public PrimeRunnable(int from, List results) {
        this.from = from;
        this.results = results;
    }

    public void isPrime(int number) {
        for(int i = 2;i < from;i++) {
                if((number % i) == 0) {
                        return;
                }
        }
        // found prime, add to shared results
        this.results.add(number);
    }

    public void run() {
        isPrime(from);      // don't increment, just check one number
    }
}

public static void main(String[] args) {
   int primeStart = 5;

   // Make thread-safe list for adding results to
   List list = Collections.synchronizedList(new ArrayList());

   int threadCount = 16;  // Experiment with this to find best on your machine
   ExecutorService exec = Executors.newFixedThreadPool(threadCount);

   int workCount = 10000;  // See how # of work is now separate from # of threads?
   for(int i = 0;i < workCount;i++) {
     // submit work to the svc for execution across the thread pool
     exec.execute(new PrimeRunnable(primeStart+i, list));
   }

   // Wait for all tasks to be done or timeout to go off
   exec.awaitTermination(1, TimeUnit.DAYS);
}

private static int currentPrime;
public static void main(String[] args) throws InterruptedException {
	for (currentPrime = 0; currentPrime < 10000; currentPrime++) {
		Thread t = new Thread(new Runnable() {
			public void run() {
				getNextPrime(currentPrime);
			}});
		t.run();
		t.join();
	}
}

for(int i = 2;i < nextPrime;i++) {
  if(nextPrime % i == 0) {
    prime = false;
  }
}

for(int i = 2;i < from;i++) {
  if((number % i) == 0) {
    return false;
  }
}

for(int i = 2;i < nextPrime;i++) {
  if(nextPrime % i == 0) {
    prime = false;
    break;
  }
}