public interface Sequence {

 int getNumber();
}

public class ClientThread extends Thread {

 private Sequence sequence;

 public ClientThread(Sequence sequence) {
  this.sequence = sequence;
 }

 @Override
 public void run() {
  for (int i = 0; i < 3; i++) {
   System.out.println(Thread.currentThread().getName() + " => " + sequence.getNumber());
  }
 }
}

public class SequenceA implements Sequence {

 private static int number = 0;

 public int getNumber() {
  number = number + 1;
  return number;
 }

 public static void main(String[] args) {
  Sequence sequence = new SequenceA();

  ClientThread thread1 = new ClientThread(sequence);
  ClientThread thread2 = new ClientThread(sequence);
  ClientThread thread3 = new ClientThread(sequence);

  thread1.start();
  thread2.start();
  thread3.start();
 }
}

public class SequenceB implements Sequence {

 private static ThreadLocal<Integer> numberContainer = new ThreadLocal<Integer>() {
  @Override
  protected Integer initialValue() {
   return 0;
  }
 };

 public int getNumber() {
  numberContainer.set(numberContainer.get() + 1);
  return numberContainer.get();
 }

 public static void main(String[] args) {
  Sequence sequence = new SequenceB();

  ClientThread thread1 = new ClientThread(sequence);
  ClientThread thread2 = new ClientThread(sequence);
  ClientThread thread3 = new ClientThread(sequence);

  thread1.start();
  thread2.start();
  thread3.start();
 }
}

public class MyThreadLocal<T> {

 private Map<Thread, T> container = Collections.synchronizedMap(new HashMap<Thread, T>());

 public void set(T value) {
  container.put(Thread.currentThread(), value);
 }

 public T get() {
  Thread thread = Thread.currentThread();
  T value = container.get(thread);
  if (value == null && !container.containsKey(thread)) {
   value = initialValue();
   container.put(thread, value);
  }
  return value;
 }

 public void remove() {
  container.remove(Thread.currentThread());
 }

 protected T initialValue() {
  return null;
 }
}

public class SequenceC implements Sequence {

 private static MyThreadLocal<Integer> numberContainer = new MyThreadLocal<Integer>() {
  @Override
  protected Integer initialValue() {
   return 0;
  }
 };

 public int getNumber() {
  numberContainer.set(numberContainer.get() + 1);
  return numberContainer.get();
 }

 public static void main(String[] args) {
  Sequence sequence = new SequenceC();

  ClientThread thread1 = new ClientThread(sequence);
  ClientThread thread2 = new ClientThread(sequence);
  ClientThread thread3 = new ClientThread(sequence);

  thread1.start();
  thread2.start();
  thread3.start();
 }
}