Payment strategy pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the PaymentStrategy interface.

LLD

interface [1] {
    void pay(int amount);
}

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

APayment

BPaymentStrategy

CPayStrategy

DPaymentMethod

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class name instead of an interface name.

Choosing a name that does not reflect the strategy pattern.

The interface name should be PaymentStrategy to follow the pattern naming.

2fill in blank

medium

Complete the code to implement the pay method in CreditCardPayment class.

LLD

class CreditCardPayment implements PaymentStrategy {
    public void [1](int amount) {
        System.out.println("Paid " + amount + " using Credit Card.");
    }
}

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

Apay

Bprocess

CmakePayment

Dexecute

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name than the interface.

Forgetting to implement the interface method.

The method must be named pay to implement the interface method.

3fill in blank

hard

Fix the error in the PaymentContext constructor to accept a PaymentStrategy.

LLD

class PaymentContext {
    private PaymentStrategy strategy;

    public PaymentContext([1] strategy) {
        this.strategy = strategy;
    }
}

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

AStrategy

BPaymentMethod

CPaymentStrategy

DPayment

Attempts:

3 left

💡 Hint

Common Mistakes

Using a concrete class type instead of the interface.

Mismatching parameter type and field type.

The constructor parameter type must be PaymentStrategy to accept any payment strategy.

4fill in blank

hard

Fill both blanks to complete the pay method call in PaymentContext.

LLD

class PaymentContext {
    private PaymentStrategy strategy;

    public void pay(int amount) {
        strategy.[1]([2]);
    }
}

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

Apay

Bamount

Cprocess

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method name like 'process'.

Passing wrong argument like 'value'.

The method pay is called with the amount parameter.

5fill in blank

hard

Fill all three blanks to create a PaymentContext with PayPalPayment and pay 100.

LLD

PaymentStrategy strategy = new [1]();
PaymentContext context = new PaymentContext([2]);
context.[3](100);

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

APayPalPayment

Bstrategy

Cpay

DCreditCardPayment

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong payment class like CreditCardPayment.

Passing wrong variable to context.

Calling wrong method name.

Create a PayPalPayment instance, pass it as strategy to context, then call pay with amount 100.

Practice

1. What is the main benefit of using the Payment Strategy Pattern in a payment system?

easy

A. It allows switching between different payment methods without changing the main code.

B. It forces all payment methods to use the same currency.

C. It stores all payment data in a single database table.

D. It encrypts payment information automatically.

Solution

1. Step 1: Understand the purpose of the Payment Strategy Pattern

   The pattern is designed to let the system switch payment methods easily without modifying the main logic.

2. Step 2: Analyze the options

   Only It allows switching between different payment methods without changing the main code. describes this benefit correctly. Other options describe unrelated features.

3. Final Answer:

   It allows switching between different payment methods without changing the main code. -> Option A

4. Quick Check:

   Payment Strategy Pattern = Switch payment methods easily [OK]

Hint: Focus on flexibility to switch payment methods without code changes [OK]

Common Mistakes:

• Confusing strategy pattern with data storage or encryption
• Thinking it enforces currency or database rules
• Assuming it handles security automatically

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

easy

A. interface PaymentStrategy { void pay(double amount); }

B. class PaymentStrategy { void pay(amount); }

C. function PaymentStrategy(amount) { return pay; }

D. var PaymentStrategy = pay => amount;

Solution

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

   In object-oriented languages, interfaces declare method signatures without implementation. interface PaymentStrategy { void pay(double amount); } uses 'interface' and a method signature correctly.

2. Step 2: Check other options

   class PaymentStrategy { void pay(amount); } is a class, not an interface. Options C and D use function syntax, not interface definitions.

3. Final Answer:

   interface PaymentStrategy { void pay(double amount); } -> Option A

4. Quick Check:

   Interface syntax = interface PaymentStrategy { void pay(double amount); } [OK]

Hint: Look for 'interface' keyword and method signature format [OK]

Common Mistakes:

• Using class instead of interface for strategy definition
• Confusing function syntax with interface
• Missing method parameter types

3. Given the following code snippet implementing the Payment Strategy Pattern, what will be the output?

class PaymentStrategy {
  pay(amount) { throw 'Not implemented'; }
}

class CreditCardPayment extends PaymentStrategy {
  pay(amount) { return `Paid ${amount} with Credit Card`; }
}

class PayPalPayment extends PaymentStrategy {
  pay(amount) { return `Paid ${amount} with PayPal`; }
}

class PaymentContext {
  constructor(strategy) { this.strategy = strategy; }
  executePayment(amount) { return this.strategy.pay(amount); }
}

const context = new PaymentContext(new PayPalPayment());
console.log(context.executePayment(100));

medium

A. Paid 100 with Credit Card

B. Not implemented

C. Paid 100 with PayPal

D. Error: strategy.pay is not a function

Solution

1. Step 1: Trace the object creation and method calls

   The PaymentContext is created with a PayPalPayment strategy. Calling executePayment(100) calls PayPalPayment's pay method.

2. Step 2: Understand the pay method output

   PayPalPayment's pay returns 'Paid 100 with PayPal'. This string is printed.

3. Final Answer:

   Paid 100 with PayPal -> Option C

4. Quick Check:

   Context uses PayPalPayment = Output with PayPal [OK]

Hint: Check which strategy instance is passed to context [OK]

Common Mistakes:

• Assuming default or CreditCardPayment is used
• Expecting an error from base class
• Confusing method override behavior

4. Identify the error in the following Payment Strategy Pattern implementation:

class PaymentStrategy {
  pay(amount) { console.log('Paying ' + amount); }
}

class BitcoinPayment extends PaymentStrategy {
  pay() { console.log('Paying with Bitcoin'); }
}

const payment = new BitcoinPayment();
payment.pay(50);

medium

A. PaymentStrategy should not have a pay method implementation.

B. BitcoinPayment's pay method does not accept the amount parameter.

C. BitcoinPayment should not extend PaymentStrategy.

D. Calling pay with 50 causes a syntax error.

Solution

1. Step 1: Compare method signatures in base and subclass

   PaymentStrategy's pay expects an amount parameter, but BitcoinPayment's pay method does not accept any parameters.

2. Step 2: Understand the impact of signature mismatch

   Calling payment.pay(50) passes an argument, but BitcoinPayment's pay ignores it, causing unexpected behavior or errors.

3. Final Answer:

   BitcoinPayment's pay method does not accept the amount parameter. -> Option B

4. Quick Check:

   Method signature mismatch = BitcoinPayment's pay method does not accept the amount parameter. [OK]

Hint: Check if subclass methods match base method parameters [OK]

Common Mistakes:

• Thinking base class should not implement pay
• Assuming inheritance is wrong
• Confusing runtime error with syntax error

5. You are designing a payment system that must support credit cards, PayPal, and a new cryptocurrency payment method. Using the Payment Strategy Pattern, which design approach best supports adding the new method with minimal changes?

hard

A. Use a global variable to switch payment methods inside the main payment function.

B. Modify the existing CreditCardPayment class to handle cryptocurrency payments.

C. Add cryptocurrency payment logic inside the PaymentContext class directly.

D. Create a new class implementing the PaymentStrategy interface for cryptocurrency and pass it to the payment context.

Solution

1. Step 1: Understand the open/closed principle in design

   The system should be open for extension but closed for modification. Adding a new payment method should not require changing existing classes.

2. Step 2: Evaluate each option

   Create a new class implementing the PaymentStrategy interface for cryptocurrency and pass it to the payment context. creates a new class implementing the interface, fitting the pattern and minimizing changes. Options B and C modify existing classes, violating the principle. Use a global variable to switch payment methods inside the main payment function. uses a global variable, which is poor design.

3. Final Answer:

   Create a new class implementing the PaymentStrategy interface for cryptocurrency and pass it to the payment context. -> Option D

4. Quick Check:

   New class for new method = Create a new class implementing the PaymentStrategy interface for cryptocurrency and pass it to the payment context. [OK]

Hint: Add new payment as new class, avoid changing existing code [OK]

Common Mistakes:

• Modifying existing payment classes
• Adding logic inside context class
• Using global variables for strategy switching