Decorator pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the base component interface.

LLD

interface [1] {
    void operation();
}

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

AComponent

BClient

CConcreteComponent

DDecorator

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Decorator' as the base interface name instead of 'Component'.

Naming the interface after the concrete implementation.

The base interface is named Component which defines the operation method to be decorated.

2fill in blank

medium

Complete the code to declare the decorator class that implements the component interface.

LLD

class [1] implements Component {
    protected Component component;
    public [1](Component component) {
        this.component = component;
    }
    public void operation() {
        component.operation();
    }
}

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

ADecorator

BConcreteDecorator

CConcreteComponent

DClient

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ConcreteDecorator' as the decorator class name here.

Not implementing the component interface.

The decorator class is named Decorator and it implements the Component interface, forwarding calls to the wrapped component.

3fill in blank

hard

Fix the error in the concrete decorator's operation method to add extra behavior.

LLD

class ConcreteDecorator extends Decorator {
    public ConcreteDecorator(Component component) {
        super(component);
    }
    public void operation() {
        [1];
        super.operation();
    }
}

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

Asuper.operation()

Bcomponent.operation()

CextraBehavior()

Doperation()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling component.operation() directly instead of using super.operation().

Calling operation() recursively causing infinite loop.

The concrete decorator adds extra behavior by calling extraBehavior() before delegating to the base operation.

4fill in blank

hard

Fill both blanks to implement a decorator that adds logging before and after the operation.

LLD

class LoggingDecorator extends Decorator {
    public LoggingDecorator(Component component) {
        super(component);
    }
    public void operation() {
        System.out.println([1]);
        super.operation();
        System.out.println([2]);
    }
}

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

A"Start operation"

B"End operation"

C"Operation started"

D"Operation finished"

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the start and end messages.

Using the same message for both prints.

The logging decorator prints "Start operation" before and "End operation" after the base operation.

5fill in blank

hard

Fill all three blanks to create a client code that wraps a concrete component with two decorators.

LLD

Component component = new ConcreteComponent();
Component decorated = new [1](component);
Component doubleDecorated = new [2](decorated);
doubleDecorated.[3]();

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

ALoggingDecorator

BConcreteDecorator

Coperation

DDecorator

Attempts:

3 left

💡 Hint

Common Mistakes

Using the base Decorator class directly instead of concrete decorators.

Calling a method other than operation.

The client wraps the ConcreteComponent first with ConcreteDecorator, then with LoggingDecorator, and calls operation() on the outermost decorator.

Practice

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

easy

A. To create multiple instances of a class

B. To add new features to objects without changing their original code

C. To replace an object with a completely new one

D. To store objects in a database

Solution

1. Step 1: Understand the Decorator pattern goal

   The Decorator pattern is designed to add new behavior or features to objects dynamically without modifying their original code.

2. Step 2: Compare options with the pattern goal

   Options B, C, and D do not describe adding features without changing code. Only To add new features to objects without changing their original code matches the pattern's purpose.

3. Final Answer:

   To add new features to objects without changing their original code -> Option B

4. Quick Check:

   Decorator pattern = add features without code change [OK]

Hint: Decorator adds features by wrapping, not by changing code [OK]

Common Mistakes:

• Confusing Decorator with Factory pattern
• Thinking it replaces objects instead of wrapping
• Assuming it stores data instead of adding behavior

2. Which of the following is the correct way to implement a decorator in a class-based design?

easy

A. Use a global variable to store extra features

B. Modify the original class to include new methods directly

C. Delete the original class and write a new one with extra features

D. Create a new class that wraps the original class and adds new methods

Solution

1. Step 1: Recall Decorator implementation style

   Decorator pattern uses a wrapper class that holds an instance of the original class and adds new behavior.

2. Step 2: Evaluate each option

   Create a new class that wraps the original class and adds new methods correctly describes wrapping with a new class. Options A, B, and C do not follow the decorator pattern principles.

3. Final Answer:

   Create a new class that wraps the original class and adds new methods -> Option D

4. Quick Check:

   Decorator = wrapper class adding methods [OK]

Hint: Decorator wraps original class; never modifies it directly [OK]

Common Mistakes:

• Changing original class instead of wrapping
• Replacing class instead of extending behavior
• Using global variables for decoration

3. Consider this pseudocode using the Decorator pattern:

class Coffee:
    def cost(self):
        return 5

class MilkDecorator:
    def __init__(self, coffee):
        self.coffee = coffee
    def cost(self):
        return self.coffee.cost() + 2

order = MilkDecorator(Coffee())
print(order.cost())

What will be the output of print(order.cost())?

medium

A. 7

B. 5

C. 2

D. Error

Solution

1. Step 1: Understand the base cost

   The Coffee class returns 5 when cost() is called.

2. Step 2: Calculate decorated cost

   The MilkDecorator adds 2 to the coffee cost by calling self.coffee.cost() and adding 2.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   5 + 2 = 7 [OK]

Hint: Decorator adds to base cost by calling wrapped object's method [OK]

Common Mistakes:

• Forgetting to call wrapped object's method
• Returning only decorator's added cost
• Confusing base and added costs

4. In the following code snippet, what is the main issue that prevents the decorator from working correctly?

class LoggerDecorator:
    def __init__(self, component):
        self.component = component
    def execute(self):
        print('Logging start')
        self.execute()
        print('Logging end')

medium

A. The decorator calls itself recursively causing infinite loop

B. The decorator does not call the wrapped component's execute method

C. The constructor is missing a parameter

D. The print statements are incorrect

Solution

1. Step 1: Analyze method calls inside execute()

   The execute() method calls self.execute(), which calls itself recursively without a base case.

2. Step 2: Identify correct call to wrapped component

   The decorator should call self.component.execute() to delegate the call, not call itself.

3. Final Answer:

   The decorator calls itself recursively causing infinite loop -> Option A

4. Quick Check:

   Recursive call without base case = infinite loop [OK]

Hint: Decorator must call wrapped object's method, not itself [OK]

Common Mistakes:

• Calling self.execute() instead of wrapped component
• Missing delegation to wrapped object
• Ignoring recursion issues

5. You are designing a notification system where messages can be sent via Email, SMS, and Push notifications. You want to add features like encryption and logging dynamically without changing the core notification classes. How would you apply the Decorator pattern here?

hard

A. Use inheritance to create subclasses for every combination of features

B. Add encryption and logging code directly inside each notification class

C. Create decorator classes for encryption and logging that wrap notification objects

D. Store all notifications in a database and process features later

Solution

1. Step 1: Identify need for dynamic feature addition

   Encryption and logging should be added without modifying core notification classes, so dynamic wrapping is needed.

2. Step 2: Apply Decorator pattern for flexible feature addition

   Create decorator classes that wrap notification objects and add encryption or logging behavior dynamically.

3. Final Answer:

   Create decorator classes for encryption and logging that wrap notification objects -> Option C

4. Quick Check:

   Decorator pattern = dynamic wrapping for features [OK]

Hint: Wrap notifications with decorators for features, avoid code changes [OK]

Common Mistakes:

• Modifying core classes directly
• Using inheritance explosion for feature combos
• Ignoring dynamic behavior needs