Observer pattern for UI updates 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]();
}

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Anotify

Bupdate

Crefresh

Dlisten

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names like notify or listen which are not standard in Observer interface.

The Observer interface requires an update method that the Subject calls 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);
    }
}

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AregisterObserver

BaddObserver

Cattach

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Using names like attach which are less common in this context.

The method to add an observer is often called registerObserver to clearly indicate the action.

3fill in blank

hard

Fix the error in the Subject's notify method to call update on all observers.

LLD

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

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

Anotify

Blisten

Crefresh

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using notify or listen which are not defined in the Observer interface.

The Subject calls the update method on each observer to notify them of changes.

4fill in blank

hard

Fill both blanks to complete the Subject's state change and notify flow.

LLD

public void setState(String state) {
    this.state = [1];
    [2]();
}

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Astate

BnotifyObservers

CupdateObservers

Dthis.state

Attempts:

3 left

💡 Hint

Common Mistakes

Using this.state on the right side instead of the parameter state.

Calling a non-existent method like updateObservers.

The state is set to the new value passed in, then notifyObservers() is called to update all observers.

5fill in blank

hard

Fill all three blanks to complete the Observer's update method using the Subject's state.

LLD

class ConcreteObserver implements Observer {
    private Subject subject;
    private String observerState;

    public ConcreteObserver(Subject subject) {
        this.subject = subject;
    }

    public void update() {
        observerState = subject.[1]();
        System.out.println("Observer state updated to: " + [2]);
        [3]();
    }

    private void refreshUI() {
        // Code to refresh UI
    }
}

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

AgetState

BobserverState

CrefreshUI

DsetState

Attempts:

3 left

💡 Hint

Common Mistakes

Using setState instead of getState to retrieve the subject's state.

Printing the wrong variable.

Not calling the UI refresh method.

The observer calls getState() on the subject to get the latest state, updates its own observerState, then calls refreshUI() to update the user interface.