Mediator pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the Mediator interface method for communication.

LLD

interface Mediator {
    void [1](String message, Colleague colleague);
}

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

Abroadcast

BreceiveMessage

CsendMessage

DnotifyAll

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'receiveMessage' instead of 'sendMessage'.

Using 'notifyAll' which is a Java Object method, not part of Mediator.

The Mediator interface typically defines a method like sendMessage to facilitate communication between colleagues.

2fill in blank

medium

Complete the code to implement the colleague's method to send a message via the mediator.

LLD

class ConcreteColleague extends Colleague {
    void send(String message) {
        mediator.[1](message, this);
    }
}

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

AsendMessage

BreceiveMessage

Cnotify

DbroadcastMessage

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a method named 'receiveMessage' on the mediator.

Using 'notify' which is unrelated here.

The colleague uses the mediator's sendMessage method to send messages.

3fill in blank

hard

Fix the error in the mediator's notify method to correctly notify all colleagues except the sender.

LLD

void notifyColleagues(String message, Colleague sender) {
    for (Colleague colleague : colleagues) {
        if (colleague != [1]) {
            colleague.receive(message);
        }
    }
}

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

Athis

Bsender

Cnull

Dcolleague

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' which refers to the mediator itself.

Using 'colleague' which is the loop variable.

The mediator should skip notifying the sender, so it compares with the sender parameter.

4fill in blank

hard

Fill both blanks to define a colleague class constructor and store the mediator reference.

LLD

class ConcreteColleague extends Colleague {
    ConcreteColleague(Mediator [1]) {
        this.[2] = [1];
    }
}

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

Amediator

Bcolleague

CmediatorRef

Dsender

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for parameter and instance variable causing confusion.

Using unrelated names like 'colleague' or 'sender'.

The constructor parameter and the instance variable are both commonly named mediator to hold the mediator reference.

5fill in blank

hard

Fill all three blanks to implement the mediator's sendMessage method that forwards messages to all colleagues except the sender.

LLD

void [1](String message, Colleague sender) {
    for (Colleague [2] : colleagues) {
        if ([2] != sender) {
            [2].receive(message);
        }
    }
}

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

AsendMessage

Bcolleague

Cnotify

Dbroadcast

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Using incorrect method names like 'notify' or 'broadcast'.

The method name is sendMessage. The loop variable is colleague, used twice to check and call receive.

Practice

1. What is the main purpose of the Mediator pattern in system design?

easy

A. To store data persistently in a database

B. To increase direct communication between all components

C. To replace all components with a single monolithic class

D. To centralize communication between components and reduce dependencies

Solution

1. Step 1: Understand the role of Mediator

   The Mediator pattern acts as a central hub to manage communication between components, avoiding direct links between them.

2. Step 2: Compare options with Mediator's purpose

   To centralize communication between components and reduce dependencies correctly states the purpose: centralizing communication and reducing dependencies. Other options describe unrelated or incorrect behaviors.

3. Final Answer:

   To centralize communication between components and reduce dependencies -> Option D

4. Quick Check:

   Mediator centralizes communication = A [OK]

Hint: Mediator centralizes communication, not direct links [OK]

Common Mistakes:

• Thinking Mediator increases direct component communication
• Confusing Mediator with data storage patterns
• Assuming Mediator merges components into one

2. Which of the following is the correct way to define a Mediator interface in a low-level design?

easy

A. interface Mediator { void notify(Component sender, String event); }

B. class Mediator { void notifyAll(); }

C. interface Mediator { void sendMessage(String message); }

D. class Mediator { void receive(Component sender); }

Solution

1. Step 1: Identify typical Mediator method signature

   The Mediator usually has a method to notify it about events from components, often with sender and event details.

2. Step 2: Match method signatures to this pattern

   interface Mediator { void notify(Component sender, String event); } matches this pattern with notify(Component sender, String event). Others lack sender info or use incorrect method names.

3. Final Answer:

   interface Mediator { void notify(Component sender, String event); } -> Option A

4. Quick Check:

   Notify method with sender and event = B [OK]

Hint: Mediator notify method includes sender and event [OK]

Common Mistakes:

• Omitting sender parameter in notify method
• Using generic sendMessage without context
• Naming methods incorrectly for Mediator role

3. Given the following code snippet, what will be the output?

class Mediator {
  notify(sender, event) {
    if (event === 'A') return 'Handled A';
    if (event === 'B') return 'Handled B';
    return 'Unknown event';
  }
}

const mediator = new Mediator();
console.log(mediator.notify('Component1', 'B'));

medium

A. Handled A

B. Error: notify method missing

C. Handled B

D. Unknown event

Solution

1. Step 1: Analyze notify method logic

   The method returns 'Handled A' if event is 'A', 'Handled B' if event is 'B', else 'Unknown event'.

2. Step 2: Check the call with event 'B'

   The call is mediator.notify('Component1', 'B'), so it matches the second condition and returns 'Handled B'.

3. Final Answer:

   Handled B -> Option C

4. Quick Check:

   Event 'B' returns 'Handled B' [OK]

Hint: Match event string exactly in notify method [OK]

Common Mistakes:

• Confusing event 'B' with 'A'
• Assuming default case triggers for known events
• Expecting error due to missing parameters

4. In the following Mediator implementation, what is the main issue?

class Mediator {
  notify(sender, event) {
    if (event === 'start') {
      sender.start();
    } else if (event === 'stop') {
      sender.stop();
    }
  }
}

class Component {
  start() { console.log('Started'); }
  stop() { console.log('Stopped'); }
}

const mediator = new Mediator();
const comp = new Component();
mediator.notify(comp, 'start');

medium

A. Component class lacks notify method

B. Mediator calls methods on sender directly, creating tight coupling

C. notify method does not handle unknown events

D. Missing constructor in Mediator class

Solution

1. Step 1: Review Mediator's notify method behavior

   The Mediator calls start() or stop() directly on the sender component.

2. Step 2: Identify design issue

   This direct call creates tight coupling between Mediator and Component, defeating the purpose of loose coupling in Mediator pattern.

3. Final Answer:

   Mediator calls methods on sender directly, creating tight coupling -> Option B

4. Quick Check:

   Tight coupling breaks Mediator pattern goal = A [OK]

Hint: Mediator should avoid calling sender methods directly [OK]

Common Mistakes:

• Ignoring tight coupling caused by direct calls
• Thinking missing notify in Component is an error
• Assuming constructor absence causes failure

5. You are designing a chat application where multiple users send messages to each other. Which design using the Mediator pattern best fits this scenario?

hard

A. A ChatRoom mediator receives messages from users and forwards them to the intended recipients.

B. Users store messages locally and synchronize with a database periodically.

C. Each user sends messages directly to all other users without a central controller.

D. Users communicate only through email notifications handled by a separate service.

Solution

1. Step 1: Understand the chat communication needs

   Users need a central place to send and receive messages without direct dependencies on each other.

2. Step 2: Match with Mediator pattern usage

   A ChatRoom mediator receives messages from users and forwards them to the intended recipients. describes a ChatRoom mediator that manages message routing, fitting the Mediator pattern perfectly.

3. Final Answer:

   A ChatRoom mediator receives messages from users and forwards them to the intended recipients. -> Option A

4. Quick Check:

   Central message routing = C [OK]

Hint: Mediator centralizes message routing in chat apps [OK]

Common Mistakes:

• Choosing direct user-to-user messaging (no mediator)
• Confusing data storage with communication pattern
• Selecting unrelated communication methods