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The second execution order involves the same operations, just that t₂ starts and finishes before t₁.

Compliant Solution (cheap read-write lock trick)

It is also permissible to declare flag as volatile to ensure its visibility and while doing so, forgoing to synchronize synchronization of the getFlag() method.

class Flag {
  private volatile boolean flag = true;
 
  public synchronized void toggle() { 
    flag ^= true; // same as flag = !flag; 
  }

  public boolean getFlag() { 
    return flag;
  }
}

The toggle() method still requires synchronization because it performs a non-atomic operation. However, this advanced technique is brittle in most other scenarios, such as, when a getter method performs operations other than just returning the value of the volatile field. The cheap read-write lock trick offers performance advantages since the method to read a value getFlag() is not synchronized. Unless read performance is critical, this method is not recommended.

Compliant Solution (java.util.concurrent.atomic.AtomicBoolean)

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