Memento pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the Memento class that stores the state.

LLD

class Memento:
    def __init__(self, [1]):
        self._state = state

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

Asave

Bstate

Cdata

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names like 'save' or 'data' instead of 'state'.

The Memento class stores the state passed during initialization.

2fill in blank

medium

Complete the code to restore the state from the Memento object.

LLD

class Originator:
    def restore(self, memento):
        self._state = memento.[1]

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

A_state

Bstate

Cget_state()

Dsave_state()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a method that does not exist like get_state() or save_state().

The Originator accesses the stored state directly from the Memento's _state attribute.

3fill in blank

hard

Fix the error in saving the current state into a Memento object.

LLD

class Originator:
    def save(self):
        return Memento([1])

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

Astate

Bself.save_state

Cself._state

Dself.state

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined variables like 'state' or methods like 'save_state'.

The Originator saves its current private state attribute into a new Memento.

4fill in blank

hard

Fill both blanks to implement the Caretaker storing and retrieving mementos.

LLD

class Caretaker:
    def __init__(self):
        self._mementos = []

    def add_memento(self, memento):
        self._mementos.[1](memento)

    def get_memento(self, index):
        return self._mementos[[2]]

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

Aappend

Bpop

Cindex

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop to add items or remove to get items by index.

The Caretaker appends mementos to a list and retrieves them by index.

5fill in blank

hard

Fill all three blanks to complete the Originator's state change and memento management.

LLD

class Originator:
    def __init__(self):
        self._state = None

    def set_state(self, state):
        self._state = [1]

    def save(self):
        return Memento(self._state)

    def restore(self, memento):
        self._state = memento.[2]

    def get_state(self):
        return self._state

originator = Originator()
originator.set_state('State1')
memento = originator.save()
originator.set_state('State2')
originator.restore(memento)
print(originator.[3]())

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

Astate

B_state

Cget_state

Dsave_state

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect attribute or method names that do not exist.

The Originator sets its state from the parameter, restores from the Memento's _state attribute, and uses get_state() to retrieve the current state.

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