Template Method pattern in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Template Method pattern?

The Template Method pattern defines the skeleton of an algorithm in a method, deferring some steps to subclasses. It lets subclasses redefine certain steps without changing the algorithm's structure.

beginner

Which part of the Template Method pattern is fixed and which part is customizable?

The fixed part is the overall algorithm structure defined in the template method. The customizable parts are the steps or methods that subclasses override to provide specific behavior.

intermediate

Why use the Template Method pattern instead of just writing separate algorithms?

It promotes code reuse by sharing common steps in a base class and allows flexibility by letting subclasses customize specific steps. This avoids code duplication and keeps the algorithm structure consistent.

intermediate

In the Template Method pattern, what is a 'hook' method?

A hook is an optional method in the base class that subclasses can override to add extra behavior. It usually has an empty or default implementation and does not affect the algorithm if not overridden.

beginner

Give a real-life example of the Template Method pattern.

Making a cup of tea or coffee: the steps like boiling water and pouring are fixed, but adding tea leaves or coffee powder is customizable. The overall process stays the same, but details vary.

What does the Template Method pattern primarily help with?

A Replacing inheritance with composition

B Creating multiple unrelated algorithms

C Defining an algorithm's structure while allowing some steps to vary

D Managing object creation

The Template Method pattern fixes the algorithm's structure but lets subclasses customize certain steps.

In the Template Method pattern, where is the template method usually defined?

A In the base class

B In each subclass

C In a separate utility class

D In the client code

The template method is defined in the base class to outline the algorithm's structure.

What is the role of subclasses in the Template Method pattern?

A Rewrite the entire algorithm

B Prevent changes to the algorithm

C Call the template method directly

D Override specific steps of the algorithm

Subclasses override certain steps to customize behavior while keeping the overall algorithm intact.

Which of the following is NOT a benefit of the Template Method pattern?

A Complete algorithm flexibility

B Algorithm consistency

C Code reuse

D Ease of maintenance

The pattern fixes the algorithm structure, so it does not allow complete flexibility.

What is a 'hook' in the Template Method pattern?

A A method that calls the template method

B An optional method subclasses can override

C A required method subclasses must implement

D A method that prevents subclassing

Hooks are optional methods with default behavior that subclasses may override to add extra steps.

Explain the Template Method pattern and how it helps in designing algorithms.

Think about a recipe where the main steps stay the same but ingredients can change.

Describe a real-world example that illustrates the Template Method pattern.

Consider a daily routine with some fixed and some flexible parts.

Practice

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

easy

A. To replace all methods with completely new implementations

B. To define a fixed sequence of steps with some customizable parts

C. To allow direct modification of the entire process by subclasses

D. To create multiple unrelated classes with no shared behavior

Solution

1. Step 1: Understand the Template Method pattern goal

   The pattern fixes the overall process flow but allows subclasses to customize certain steps.

2. Step 2: Analyze each option

   To define a fixed sequence of steps with some customizable parts correctly describes this fixed sequence with customizable parts. Options A, B, and C do not match the pattern's intent.

3. Final Answer:

   To define a fixed sequence of steps with some customizable parts -> Option B

4. Quick Check:

   Template Method = fixed process + flexible steps [OK]

Hint: Remember: fixed order, flexible details [OK]

Common Mistakes:

• Thinking subclasses rewrite the whole process
• Confusing Template Method with Strategy pattern
• Believing the pattern removes all common code
• Assuming no steps are customizable

2. Which of the following is the correct way to declare a template method in a class?

easy

A. Override all methods in subclasses without calling base methods

B. Define multiple unrelated methods without calling each other

C. Define a method that calls other methods in a fixed order

D. Use a method that randomly calls other methods

Solution

1. Step 1: Identify the structure of a template method

   A template method is a method that defines the sequence of steps by calling other methods in order.

2. Step 2: Evaluate each option

   Define a method that calls other methods in a fixed order matches this definition. Options B, C, and D do not follow the fixed sequence concept.

3. Final Answer:

   Define a method that calls other methods in a fixed order -> Option C

4. Quick Check:

   Template method = fixed calls order [OK]

Hint: Template method calls steps in order [OK]

Common Mistakes:

• Not calling methods in a fixed sequence
• Overriding template method instead of steps
• Ignoring the fixed process structure
• Using random or unordered calls

3. Consider this simplified template method code in Python:

class Game:
    def play(self):
        self.start()
        self.play_turn()
        self.end()

    def start(self):
        print('Game started')

    def play_turn(self):
        print('Playing turn')

    def end(self):
        print('Game ended')

class Chess(Game):
    def play_turn(self):
        print('Chess turn played')

chess = Chess()
chess.play()

What will be the output when chess.play() is called?

medium

A. Game ended
Chess turn played
Game started

B. Chess turn played
Game started
Game ended

C. Game started
Playing turn
Game ended

D. Game started
Chess turn played
Game ended

Solution

1. Step 1: Trace the play() method calls

   The play() method calls start(), play_turn(), and end() in order.

2. Step 2: Identify overridden methods

   Chess overrides play_turn(), so Chess's version prints 'Chess turn played'. start() and end() use base class prints.

3. Final Answer:

   Game started Chess turn played Game ended -> Option D

4. Quick Check:

   Template calls base start/end + overridden play_turn [OK]

Hint: Overridden steps print their own messages [OK]

Common Mistakes:

• Ignoring method overriding
• Assuming base play_turn() runs
• Mixing order of prints
• Thinking play() is overridden

4. In the following code, what is the main problem that breaks the Template Method pattern?

class Report:
    def generate(self):
        self.load_data()
        self.process_data()
        self.save_report()

    def load_data(self):
        print('Loading data')

    def process_data(self):
        print('Processing data')

    def save_report(self):
        print('Saving report')

class CustomReport(Report):
    def generate(self):
        print('Custom generate start')
        self.load_data()
        self.process_data()
        self.save_report()
        print('Custom generate end')

medium

A. The subclass overrides the template method instead of steps

B. The base class does not define any methods

C. The subclass does not call any base methods

D. The base class methods are private

Solution

1. Step 1: Identify the template method and overrides

   The base class defines generate() as the template method. The subclass overrides generate() itself.

2. Step 2: Understand Template Method pattern rules

   In this pattern, subclasses should override steps, not the template method, to keep the fixed process intact.

3. Final Answer:

   The subclass overrides the template method instead of steps -> Option A

4. Quick Check:

   Template method must not be overridden [OK]

Hint: Override steps, not the template method [OK]

Common Mistakes:

• Overriding the whole template method
• Ignoring base class method definitions
• Assuming private methods cause issues
• Thinking subclass must call base generate() explicitly

5. You are designing a document processing system where each document type requires a specific sequence of steps: open, parse, validate, save, and close. You want to ensure the sequence is fixed but allow each document type to customize parsing and validation. How should you apply the Template Method pattern here?

hard

A. Create a base class with a template method calling open, parse, validate, save, close; subclasses override parse and validate

B. Let each subclass implement its own full process without a base class

C. Use separate unrelated classes for each step without a fixed sequence

D. Make all steps abstract and force subclasses to implement the entire process

Solution

1. Step 1: Identify fixed and customizable parts

   The sequence (open, parse, validate, save, close) is fixed; parse and validate vary by document type.

2. Step 2: Apply Template Method pattern correctly

   Create a base class with a template method that calls all steps in order. Subclasses override parse and validate to customize behavior.

3. Final Answer:

   Create a base class with a template method calling open, parse, validate, save, close; subclasses override parse and validate -> Option A

4. Quick Check:

   Fixed sequence + customizable steps = Template Method [OK]

Hint: Fix sequence in base; override variable steps in subclasses [OK]

Common Mistakes:

• Not fixing the sequence in one place
• Forcing subclasses to rewrite entire process
• Ignoring the need for a template method
• Separating steps without order control