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class CountBoxes implements Runnable {
  static volatile int counter;
  // ...

  Object lock = new Object();    

  public void run() {
    synchronized(lock) {
      counter++; 
      // ... 
    } 
  }

  public static void main(String[] args) {
    Runnable r1 = new CountBoxes();
    Thread t1 = new Thread(r1);
    Runnable r2 = new CountBoxes();
    Thread t2 = new Thread(r2);
    t1.start();
    t2.start();
  }
}

Compliant Solution (1) (static lock object)

This compliant solution declares the lock object as static and consequently, ensures the atomicity of the increment operation.

class CountBoxes implements Runnable {
  static volatile int counter;
  // ...

  static final Object lock = new Object();    
  // ...
}

There is no requirement of declaring the counter variable as volatile when synchronization is used.

Compliant Solution (2)(intrinsic lock of class)

This compliant solution uses the intrinsic lock of the class to synchronize the increment operation.

class CountBoxes implements Runnable {
  static int counter;
  // ...

  public void run() {
    synchronized(CountBoxes.class) {
      counter++; 
      // ... 
    } 
  }
  // ...
}

Noncompliant Code Example (ReentrantLock lock object)

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