Memento pattern in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of the Memento Pattern

What is the primary purpose of the Memento pattern in software design?

A To create a single instance of a class and provide a global point of access to it.

B To capture and externalize an object's internal state so it can be restored later without violating encapsulation.

C To define a family of algorithms, encapsulate each one, and make them interchangeable.

D To separate the construction of a complex object from its representation so the same construction process can create different representations.

💡 Hint

Think about saving and restoring an object's state without exposing its details.

The Memento pattern is used to save and restore the internal state of an object without exposing its implementation details, preserving encapsulation.

❓ Architecture

intermediate

Components of the Memento Pattern

Which of the following correctly identifies the three main components of the Memento pattern?

A Originator, Memento, Caretaker

B Subject, Observer, ConcreteObserver

C Builder, Director, Product

D Component, Decorator, ConcreteDecorator

💡 Hint

Think about who creates the state, who stores it, and who manages it.

The Memento pattern involves the Originator (creates and restores state), the Memento (stores state), and the Caretaker (manages mementos).

❓ scaling

advanced

Scaling Memento Pattern for Large State Objects

When using the Memento pattern for objects with large internal states, what is the best approach to optimize memory usage?

A Use incremental snapshots or delta changes to store only differences between states.

B Avoid using the Memento pattern and instead serialize the entire object to disk after each change.

C Store full copies of the state every time a change occurs to ensure complete rollback capability.

D Keep all mementos in memory indefinitely to allow unlimited undo operations.

💡 Hint

Think about storing only what changes instead of full copies.

Using incremental snapshots or storing only the differences (deltas) between states reduces memory usage and improves scalability.

❓ tradeoff

advanced

Trade-offs of Using the Memento Pattern

What is a common trade-off when implementing the Memento pattern in a system?

A It increases coupling between objects, making the system harder to maintain.

B It violates encapsulation by exposing internal state details to other objects.

C It can lead to high memory consumption due to storing multiple object states.

D It makes the system slower by requiring synchronous communication between components.

💡 Hint

Consider the cost of saving many states over time.

The Memento pattern can consume significant memory because it stores copies of object states, especially if many states are saved.

❓ component

expert

Request Flow in Memento Pattern

In a system using the Memento pattern, what is the correct sequence of interactions when saving and restoring an object's state?

A 3,1,2,4

B 2,1,3,4

C 1,3,2,4

D 1,2,3,4

💡 Hint

Think about who initiates the save and restore actions and the order of storing and restoring.

The Caretaker asks the Originator for a memento, the Originator creates it, the Caretaker stores it, and later the Originator restores state from it.

Practice

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

easy

A. To create multiple instances of an object efficiently

B. To convert one interface to another compatible interface

C. To manage concurrent access to shared resources

D. To save and restore an object's state without exposing its internal details

Solution

1. Step 1: Understand the role of Memento pattern

   The Memento pattern is designed to capture and externalize an object's internal state so that it can be restored later without exposing the object's implementation details.

2. Step 2: Compare with other design patterns

   Other options describe different patterns: A is about object creation (Factory), C is about synchronization (Mutex), D is about interface compatibility (Adapter).

3. Final Answer:

   To save and restore an object's state without exposing its internal details -> Option D

4. Quick Check:

   Memento = Save & Restore State [OK]

Hint: Memento = save state secretly, no details shown [OK]

Common Mistakes:

• Confusing Memento with Factory or Adapter patterns
• Thinking it manages concurrency
• Assuming it changes object interfaces

2. Which of the following correctly represents the key components of the Memento pattern?

easy

A. Subject, Observer, ConcreteObserver

B. Originator, Memento, Caretaker

C. Client, Proxy, RealSubject

D. Component, Decorator, ConcreteComponent

Solution

1. Step 1: Identify components of Memento pattern

   The Memento pattern consists of three main parts: Originator (the object whose state is saved), Memento (the object storing the state), and Caretaker (manages mementos).

2. Step 2: Eliminate other patterns

   Options B, C, and D correspond to Observer, Proxy, and Decorator patterns respectively, which are unrelated to Memento.

3. Final Answer:

   Originator, Memento, Caretaker -> Option B

4. Quick Check:

   Components = Originator + Memento + Caretaker [OK]

Hint: Remember 3 parts: Originator, Memento, Caretaker [OK]

Common Mistakes:

• Mixing Memento components with Observer or Proxy
• Forgetting the Caretaker role
• Confusing Memento with Decorator pattern

3. Consider this simplified Python code using the Memento pattern:

class Memento:
    def __init__(self, state):
        self._state = state

class Originator:
    def __init__(self):
        self._state = ""
    def set_state(self, state):
        self._state = state
    def save(self):
        return Memento(self._state)
    def restore(self, memento):
        self._state = memento._state

originator = Originator()
originator.set_state("State1")
memento = originator.save()
originator.set_state("State2")
originator.restore(memento)
print(originator._state)

What will be printed?

medium

A. None

B. State2

C. State1

D. Error

Solution

1. Step 1: Trace state changes in Originator

   Initially, Originator's state is set to "State1". Then a Memento is saved capturing "State1". Next, state changes to "State2".

2. Step 2: Restore state from Memento

   Calling restore with the saved Memento sets the state back to "State1". The print statement outputs the restored state.

3. Final Answer:

   State1 -> Option C

4. Quick Check:

   Restore resets state to saved value [OK]

Hint: Restore sets state back to saved snapshot [OK]

Common Mistakes:

• Assuming print shows latest state before restore
• Confusing save and restore methods
• Expecting error due to private variable access

4. In the following code snippet, what is the main issue that breaks the Memento pattern?

class Originator:
    def __init__(self):
        self._state = ""
    def set_state(self, state):
        self._state = state
    def save(self):
        return self._state  # returns state directly
    def restore(self, memento):
        self._state = memento

originator = Originator()
originator.set_state("State1")
memento = originator.save()
originator.set_state("State2")
originator.restore(memento)
print(originator._state)

medium

A. The save method returns state directly, exposing internal details

B. The restore method does not update the state

C. The Originator class lacks a Memento class

D. The set_state method is missing

Solution

1. Step 1: Analyze the save method

   The save method returns the internal state directly instead of encapsulating it in a Memento object, exposing internal details.

2. Step 2: Understand Memento pattern principle

   The pattern requires hiding the internal state inside a Memento object to prevent external access. Returning raw state breaks encapsulation.

3. Final Answer:

   The save method returns state directly, exposing internal details -> Option A

4. Quick Check:

   Save must hide state in Memento [OK]

Hint: Save must return Memento, not raw state [OK]

Common Mistakes:

• Thinking restore method is faulty
• Believing Memento class is mandatory in code
• Ignoring encapsulation principle

5. You are designing a text editor with undo functionality using the Memento pattern. Which approach best balances memory usage and undo capability?

hard

A. Store a Memento only after significant changes or at checkpoints

B. Store a Memento after every single character change

C. Store all changes as raw text snapshots without Memento objects

D. Do not store any state; rely on user to retype

Solution

1. Step 1: Consider memory and undo tradeoff

   Storing a Memento after every character change (Store a Memento after every single character change) uses excessive memory and is inefficient.

2. Step 2: Evaluate checkpoint strategy

   Storing Mementos after significant changes or checkpoints (Store a Memento only after significant changes or at checkpoints) reduces memory use while allowing meaningful undo steps.

3. Step 3: Assess other options

   Store all changes as raw text snapshots without Memento objects wastes memory by storing raw snapshots without encapsulation; Do not store any state; rely on user to retype removes undo capability.

4. Final Answer:

   Store a Memento only after significant changes or at checkpoints -> Option A

5. Quick Check:

   Checkpoint Mementos balance memory and undo [OK]

Hint: Save states at checkpoints, not every keystroke [OK]

Common Mistakes:

• Saving state too frequently causing memory bloat
• Ignoring encapsulation by storing raw snapshots
• Not implementing undo at all