Why is it Necessary to Call setChanged Before Notifying Observers in Java?

Why must I invoke the setChanged method before calling notifyObservers in the Observer pattern?

Observable observable = new Observable();

// Mark observable as changed
observable.setChanged();

// Notify all observers
observable.notifyObservers();

In Java's Observer pattern, the setChanged method serves a crucial role in ensuring that observers are only notified when there is a legitimate change in the state of the Observable object. Failing to invoke setChanged before notifyObservers can lead to confusion because observers would receive notifications for unchanged states or none at all, compromising the integrity of the application.

Observable observable = new Observable();

// Change the state of observable
observable.setData(newData);
// Indicate that the observable has changed
observable.setChanged();
// Notify all observers about the change
observable.notifyObservers();

Causes

• NotifyObservers does not check if the state has changed before notifying observers.
• Without calling setChanged, the state does not reflect that something has changed.

Solutions

• Always invoke setChanged() whenever there is a change in the Mutable state of the Observable object before notifying the observers.
• Review state changes to ensure that setChanged is appropriately invoked.

Mistake: Neglecting to call setChanged before notifyObservers.

Solution: Always ensure that setChanged is called after the state changes.

Mistake: Calling notifyObservers without checking if any state has changed.

Solution: Implement checks to avoid unnecessary notifications and ensure the validity of updates.