SynchronizationProblemWithStatic.java

package com.codecafe.concurrency._synchronized;

class Sample1 {

  static int a = 5; // static variables are NOT thread-safe
  int b = 10;

  public int getB() {
    return b;
  }

  public void increment() {
    synchronized (Sample1.class) {
      int c = a;
      c++;
      try {
        Thread.sleep(2);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      a = c;
    }

    synchronized (this) {
      b++;
    }
  }

}

class IncTask implements Runnable {

  Sample1 obj;

  public IncTask(Sample1 obj) {
    this.obj = obj;
  }

  @Override
  public void run() {
    obj.increment();
  }

}

public class SynchronizationProblemWithStatic {

  public static void main(String[] args) {
    Sample1 obj1 = new Sample1();
    Sample1 obj2 = new Sample1();

    Thread t1 = new Thread(new IncTask(obj1));
    Thread t2 = new Thread(new IncTask(obj2));

    t1.start();
    t2.start();

    try {
      t1.join();
      t2.join();
    } catch (InterruptedException e) {
      e.printStackTrace();
    }

    System.out.printf("a = %d%n", Sample1.a);
    System.out.printf("obj1 - b = %d%nobj2 - b = %d", obj1.getB(), obj2.getB());
  }

}