Law of Demeter in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to follow the Law of Demeter by accessing only immediate friends.

LLD

class Car {
    Engine engine;
    void start() {
        engine.[1]();
    }
}

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

AgetFuel

BgetPistons

CgetOilLevel

Dignite

Attempts:

3 left

💡 Hint

Common Mistakes

Calling methods on objects returned by other methods (chained calls).

The method ignite is a direct action on the immediate friend engine, following the Law of Demeter.

2fill in blank

medium

Complete the code to avoid violating the Law of Demeter by not accessing nested objects directly.

LLD

class Library {
    Book book;
    void printAuthor() {
        System.out.println(book.[1]());
    }
}

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

AgetPublisher

BgetTitle

CgetAuthor

DgetYear

Attempts:

3 left

💡 Hint

Common Mistakes

Calling methods on objects returned by getPublisher() or deeper.

Accessing getAuthor() is allowed because book is a direct friend. Accessing nested objects beyond that would violate the Law of Demeter.

3fill in blank

hard

Fix the error in the code to comply with the Law of Demeter by removing chained calls.

LLD

class Computer {
    CPU cpu;
    void checkStatus() {
        cpu.[1]();
    }
}

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

Astart

BgetSpeed

CgetFan

DgetCache

Attempts:

3 left

💡 Hint

Common Mistakes

Using chained calls like cpu.getFan().getTemperature().

Calling start() on cpu is allowed as it is a direct friend. Calling getFan() returns another object, leading to chained calls which violate the Law of Demeter.

4fill in blank

hard

Fill both blanks to rewrite the code so it respects the Law of Demeter by delegating calls.

LLD

class House {
    Kitchen kitchen;
    void clean() {
        kitchen.[1]();
        kitchen.[2]();
    }
}

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

AcleanSink

BgetFridge

CcleanFloor

DgetOven

Attempts:

3 left

💡 Hint

Common Mistakes

Calling methods on nested objects like getFridge() or getOven().

Calling cleanSink() and cleanFloor() directly on kitchen respects the Law of Demeter by avoiding chained calls.

5fill in blank

hard

Fill all three blanks to refactor the code to follow the Law of Demeter by avoiding chained calls.

LLD

class School {
    Classroom classroom;
    void startClass() {
        classroom.[1]();
        classroom.[2]().[3]();
    }
}

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

AopenDoor

BgetTeacher

CteachLesson

DgetStudents

Attempts:

3 left

💡 Hint

Common Mistakes

Calling methods on objects returned by getStudents() or chaining deeper than one level.

Calling openDoor() directly on classroom and then calling teachLesson() on the teacher obtained via getTeacher() respects the Law of Demeter by limiting access to immediate friends.

Practice

1.

What is the main idea behind the Law of Demeter in system design?

easy

A. An object should only communicate with its immediate friends.

B. An object can access any part of the system freely.

C. Objects should share all their internal details with others.

D. Objects must avoid any communication with other objects.

Solution

1. Step 1: Understand the Law of Demeter principle

   The Law of Demeter states that an object should only talk to its immediate friends, meaning it should not reach deep into other objects.

2. Step 2: Match the principle to the options

   An object should only communicate with its immediate friends. correctly describes this idea, while others contradict it.

3. Final Answer:

   An object should only communicate with its immediate friends. -> Option A

4. Quick Check:

   Law of Demeter = Talk only to immediate friends [OK]

Hint: Remember: Only talk to your immediate friends [OK]

Common Mistakes:

• Thinking objects can access any part of the system
• Believing objects should share all internal details
• Assuming no communication is allowed between objects

2.

Which of the following code snippets follows the Law of Demeter?

class Car {
  Engine engine;
  void start() {
    engine.start();
  }
}

easy

A. Calling engine.start() directly inside Car is allowed.

B. Accessing engine.fuel.tank.level inside Car is allowed.

C. Calling engine.getFuel().getTank().getLevel() is preferred.

D. Car should access engine.fuel.tank.level directly.

Solution

1. Step 1: Identify allowed communication

   According to the Law of Demeter, Car can call methods on its direct member engine.

2. Step 2: Analyze the options

   Calling engine.start() directly inside Car is allowed. shows Car calling engine.start(), which is allowed. Options A, B, and C involve accessing deeper nested objects, which breaks the law.

3. Final Answer:

   Calling engine.start() directly inside Car is allowed. -> Option A

4. Quick Check:

   Direct member calls = Allowed [OK]

Hint: Only call methods on your direct members [OK]

Common Mistakes:

• Accessing nested objects directly
• Chaining multiple getters in one call
• Ignoring encapsulation principles

3.

Consider this code snippet:

class A {
  B b;
  void doSomething() {
    int x = b.getC().getValue();
    System.out.println(x);
  }
}

Which statement about this code is true regarding the Law of Demeter?

medium

A. It follows the Law of Demeter perfectly.

B. It violates the Law of Demeter by chaining calls.

C. It violates the Law of Demeter by not calling b directly.

D. It follows the Law of Demeter by accessing getValue().

Solution

1. Step 1: Analyze the method call chain

   The code calls b.getC().getValue(), which chains two calls to reach a nested object.

2. Step 2: Compare with Law of Demeter

   The Law of Demeter discourages chaining calls like this; an object should only call methods on its direct members.

3. Final Answer:

   It violates the Law of Demeter by chaining calls. -> Option B

4. Quick Check:

   Chained calls = Violation [OK]

Hint: Watch out for chained method calls [OK]

Common Mistakes:

• Thinking chained calls are allowed
• Confusing direct member calls with nested calls
• Ignoring encapsulation rules

4.

Identify the problem in this code snippet related to the Law of Demeter and suggest a fix:

class Order {
  Customer customer;
  void printCustomerCity() {
    System.out.println(customer.getAddress().getCity());
  }
}

medium

A. No problem; code follows the Law of Demeter.

B. Violation: Order should print city without any method calls.

C. Violation: Order should access Address directly. Fix by removing Customer.

D. Violation: Order accesses nested Address. Fix by adding getCity() in Customer.

Solution

1. Step 1: Identify Law of Demeter violation

   Order calls customer.getAddress().getCity(), chaining calls to nested objects.

2. Step 2: Suggest fix to follow Law of Demeter

   Add a getCity() method in Customer that returns the city, so Order calls only customer.getCity().

3. Final Answer:

   Violation: Order accesses nested Address. Fix by adding getCity() in Customer. -> Option D

4. Quick Check:

   Use direct member methods to avoid chaining [OK]

Hint: Add helper methods to avoid deep calls [OK]

Common Mistakes:

• Ignoring nested call chains
• Removing needed objects instead of fixing access
• Trying to print without method calls

5.

You are designing a messaging system where ChatRoom has User objects, and each User has a Profile. To follow the Law of Demeter, how should ChatRoom get a user's display name?

hard

A. Make Profile public and access displayName directly.

B. chatRoom.getUser().getProfile().getDisplayName() chaining calls is fine.

C. Add a method getDisplayName() in User and call chatRoom.getUser().getDisplayName().

D. Let ChatRoom access Profile objects directly.

Solution

1. Step 1: Identify Law of Demeter violation in chaining

   Chaining calls like getUser().getProfile().getDisplayName() breaks the Law of Demeter.

2. Step 2: Apply Law of Demeter by adding helper method

   Adding getDisplayName() in User lets ChatRoom call only one method on its direct friend User.

3. Final Answer:

   Add a method getDisplayName() in User and call chatRoom.getUser().getDisplayName(). -> Option C

4. Quick Check:

   Use helper methods to avoid deep chains [OK]

Hint: Use helper methods in immediate objects [OK]

Common Mistakes:

• Allowing chained calls
• Making internal objects public
• Accessing nested objects directly