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If a class uses a private final lock to synchronize shared data, subclasses must also use a private final lock. However, if a class uses intrinsic synchronization over the class object without documenting its locking policy, subclasses may not use intrinsic synchronization over their own class object, unless they explicitly document their locking policy. If the superclass documents its policy by stating that client-side locking is supported, the subclasses have the option of choosing between intrinsic locking over the class object and an internal a private lock. Regardless of which is chosen, subclasses must document their locking policy. Refer to the guideline CON10-J. Do not override thread-safe methods with methods that are not thread-safe for related information.

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public class SomeObject {
  private final Object lock = new Object(); // private final lock object

  public void changeValue() {
    synchronized (lock) { // Locks on the private Object
      // ...
    }
  }
}

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public class SomeObject {
  private static final Object lock = new Object(); // private final lock object

  public static void ChangeValue() {
    synchronized (lock) { // Locks on the private Object
      // ...
    }
  }
}

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EX2: If a superclass of the class documents that it supports client-side locking and synchronizes on its class object, the class should also support client-side locking in the same way and document this policy. If instead the superclass uses an internal a private final lock, the derived class should document its own locking policy.

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