Observer pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the Observer interface method.

LLD

interface Observer {
    void [1]();
}

Drag options to blanks, or click blank then click option'

Aupdate

Bnotify

Clisten

Dalert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'notify' which is a keyword in some languages.

Using 'listen' or 'alert' which are not standard method names here.

The Observer interface defines an update method that subjects call to notify observers of changes.

2fill in blank

medium

Complete the code to add an observer to the subject's list.

LLD

class Subject {
    private List<Observer> observers = new ArrayList<>();

    public void [1](Observer o) {
        observers.add(o);
    }
}

Drag options to blanks, or click blank then click option'

Aattach

BregisterObserver

CaddObserver

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'attach' which is less common in Java-style code.

Using 'subscribe' which is more common in event-driven systems.

The method addObserver is commonly used to add observers to the subject's list.

3fill in blank

hard

Fix the error in the notifyObservers method to correctly call update on each observer.

LLD

public void notifyObservers() {
    for (Observer o : observers) {
        o.[1]();
    }
}

Drag options to blanks, or click blank then click option'

Aalert

Bnotify

Crefresh

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'notify' which is not defined in Observer interface.

Using 'refresh' or 'alert' which are not standard method names.

The update method is the standard method observers implement to receive notifications.

4fill in blank

hard

Fill both blanks to complete the Subject class with observer management methods.

LLD

class Subject {
    private List<Observer> observers = new ArrayList<>();

    public void [1](Observer o) {
        observers.add(o);
    }

    public void [2](Observer o) {
        observers.remove(o);
    }
}

Drag options to blanks, or click blank then click option'

AaddObserver

BremoveObserver

CregisterObserver

DunregisterObserver

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing method names like 'registerObserver' with 'unregisterObserver' inconsistently.

Using 'subscribe' or 'unsubscribe' which are less common here.

The addObserver method adds an observer, and removeObserver removes one from the list.

5fill in blank

hard

Fill all three blanks to complete the Observer pattern update flow.

LLD

interface Observer {
    void [1]();
}

class Subject {
    private List<Observer> observers = new ArrayList<>();

    public void [2](Observer o) {
        observers.add(o);
    }

    public void notifyObservers() {
        for (Observer o : observers) {
            o.[3]();
        }
    }
}

Drag options to blanks, or click blank then click option'

Aupdate

BaddObserver

DregisterObserver

Attempts:

3 left

💡 Hint

Common Mistakes

Using different method names for update in interface and notify call.

Confusing addObserver with registerObserver.

The Observer interface has the update method. The Subject adds observers with addObserver. When notifying, it calls update on each observer.