Strategy pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the Strategy interface.

LLD

interface [1] {
    execute(): void;
}

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

AStrategy

BContext

CAlgorithm

DExecutor

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong interface name like 'Context' or 'Algorithm'.

Forgetting to define the execute method.

The Strategy pattern requires a common interface named Strategy that all concrete strategies implement.

2fill in blank

medium

Complete the code to define a concrete strategy class implementing the Strategy interface.

LLD

class ConcreteStrategyA implements [1] {
    execute(): void {
        console.log('Strategy A executed');
    }
}

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

AStrategy

BContext

CExecutor

DAlgorithm

Attempts:

3 left

💡 Hint

Common Mistakes

Implementing the wrong interface like 'Context'.

Missing the execute method implementation.

The concrete strategy class must implement the Strategy interface to provide its own algorithm.

3fill in blank

hard

Fix the error in the Context class constructor to accept a strategy.

LLD

class Context {
    strategy: [1];

    constructor(strategy: [1]) {
        this.strategy = strategy;
    }

    executeStrategy(): void {
        this.strategy.execute();
    }
}

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

AAlgorithm

BStrategy

CExecutor

DContext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Context' as the type instead of 'Strategy'.

Not defining the strategy property type.

The Context class should accept an object of type Strategy to delegate the execution to the chosen algorithm.

4fill in blank

hard

Fill both blanks to change the strategy at runtime and execute it.

LLD

const context = new Context(new ConcreteStrategyA());
context.executeStrategy();
context.[1] = new ConcreteStrategyB();
context.[2]();

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

Astrategy

BexecuteStrategy

CsetStrategy

DrunStrategy

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name instead of the property to change the strategy.

Calling a non-existent method like 'runStrategy()'.

The Context's strategy property can be reassigned to change the algorithm, and executeStrategy() runs the current strategy.

5fill in blank

hard

Fill all three blanks to implement a new concrete strategy and use it in the context.

LLD

class [1] implements Strategy {
    execute(): void {
        console.log('Strategy C executed');
    }
}

const context = new Context(new [2]());
context.[3]();

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

AConcreteStrategyC

CexecuteStrategy

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the class and constructor.

Calling a method that does not exist like 'run()'.

The new strategy class is named ConcreteStrategyC. The context is created with this strategy and calls executeStrategy() to run it.

Practice

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

easy

A. To restrict object creation to a single instance

B. To allow selecting an algorithm's behavior at runtime without changing the client code

C. To define a fixed sequence of steps for an algorithm

D. To create a single global instance of a class

Solution

1. Step 1: Understand the Strategy pattern goal

   The Strategy pattern is designed to let you swap algorithms or behaviors dynamically without changing the client code.

2. Step 2: Compare options with pattern purpose

   To allow selecting an algorithm's behavior at runtime without changing the client code correctly states this purpose. Options A and B describe Singleton pattern, and C describes Template Method pattern.

3. Final Answer:

   To allow selecting an algorithm's behavior at runtime without changing the client code -> Option B

4. Quick Check:

   Strategy pattern = runtime algorithm selection [OK]

Hint: Strategy pattern = choose behavior at runtime [OK]

Common Mistakes:

• Confusing Strategy with Singleton pattern
• Thinking Strategy fixes algorithm steps
• Assuming Strategy creates single instances

2. Which of the following is the correct way to define a Strategy interface in a typical object-oriented language?

easy

A. class Strategy { void execute(); }

B. def Strategy(): pass

C. interface Strategy { void execute(); }

D. struct Strategy { void execute(); }

Solution

1. Step 1: Identify the correct syntax for interface definition

   In many object-oriented languages like Java or C#, interface keyword is used to define a Strategy interface with method signatures.

2. Step 2: Evaluate options

   interface Strategy { void execute(); } uses interface with method execute(), which is correct. class Strategy { void execute(); } defines a class, not an interface. def Strategy(): pass is Python syntax but incomplete. struct Strategy { void execute(); } uses struct which is not typical for interfaces.

3. Final Answer:

   interface Strategy { void execute(); } -> Option C

4. Quick Check:

   Strategy interface = interface with method [OK]

Hint: Strategy interface uses 'interface' keyword in OOP [OK]

Common Mistakes:

• Using class instead of interface for Strategy
• Confusing struct with interface
• Using incomplete or wrong language syntax

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

class Context:
    def __init__(self, strategy):
        self.strategy = strategy
    def execute(self):
        return self.strategy.do_action()

class StrategyA:
    def do_action(self):
        return 'Action A'

class StrategyB:
    def do_action(self):
        return 'Action B'

context = Context(StrategyB())
print(context.execute())

medium

A. Action B

B. Action A

C. Error: StrategyB has no method do_action

D. None

Solution

1. Step 1: Trace object creation and method calls

   The Context is created with StrategyB() instance. Calling context.execute() calls StrategyB.do_action().

2. Step 2: Check StrategyB.do_action() return value

   StrategyB.do_action() returns the string 'Action B', so print(context.execute()) outputs 'Action B'.

3. Final Answer:

   Action B -> Option A

4. Quick Check:

   Context with StrategyB = 'Action B' output [OK]

Hint: Context calls strategy's method, output matches chosen strategy [OK]

Common Mistakes:

• Assuming default strategy is StrategyA
• Thinking method do_action is missing
• Confusing class and instance usage

4. Identify the error in the following Strategy pattern implementation:

class Context:
    def __init__(self, strategy):
        self.strategy = strategy
    def execute(self):
        return self.strategy.action()

class StrategyA:
    def do_action(self):
        return 'Action A'

context = Context(StrategyA())
print(context.execute())

medium

A. Context calls a method 'action' which does not exist in StrategyA

B. StrategyA class is missing the constructor

C. Context should not store strategy as an instance variable

D. No error, code runs correctly

Solution

1. Step 1: Compare method names between Context and StrategyA

   Context calls self.strategy.action(), but StrategyA defines do_action(), not action().

2. Step 2: Identify mismatch causing error

   This mismatch causes an AttributeError at runtime because action() is undefined in StrategyA.

3. Final Answer:

   Context calls a method 'action' which does not exist in StrategyA -> Option A

4. Quick Check:

   Method name mismatch = runtime error [OK]

Hint: Check method names match between context and strategy [OK]

Common Mistakes:

• Ignoring method name mismatch
• Assuming missing constructor causes error
• Thinking strategy should not be stored in context

5. You are designing a payment system that supports multiple payment methods (credit card, PayPal, cryptocurrency). How would applying the Strategy pattern improve your system design?

hard

A. It requires hardcoding all payment methods inside a single class

B. It forces all payment methods to share the same implementation details

C. It prevents runtime selection of payment methods

D. It allows adding new payment methods without changing existing code by defining each as a separate strategy

Solution

1. Step 1: Understand Strategy pattern benefits in payment methods

   Strategy pattern lets you define each payment method as a separate strategy class implementing a common interface.

2. Step 2: Analyze how this affects system design

   This design allows adding new payment methods easily without modifying existing code, and lets the system select payment method at runtime.

3. Final Answer:

   It allows adding new payment methods without changing existing code by defining each as a separate strategy -> Option D

4. Quick Check:

   Strategy pattern = easy extension and runtime choice [OK]

Hint: Strategy pattern enables easy addition and runtime choice [OK]

Common Mistakes:

• Thinking Strategy forces shared implementation
• Believing all methods must be hardcoded together
• Assuming runtime selection is not possible