Proxy pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the Proxy class implementing the Subject interface.

LLD

class Proxy implements [1] {
    private RealSubject realSubject;

    public void request() {
        if (realSubject == null) {
            realSubject = new RealSubject();
        }
        realSubject.request();
    }
}

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

AProxy

BSubject

CRealSubject

DClient

Attempts:

3 left

💡 Hint

Common Mistakes

Using RealSubject instead of Subject as the implemented interface.

Not implementing any interface.

Implementing Proxy interface which does not exist.

The Proxy class must implement the Subject interface to act as a substitute for RealSubject.

2fill in blank

medium

Complete the code to forward the request from Proxy to RealSubject.

LLD

public void request() {
    if (realSubject == null) {
        realSubject = new RealSubject();
    }
    realSubject.[1]();
}

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

Arequest

Bexecute

Chandle

Dprocess

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names not defined in the Subject interface.

Calling a method that does not exist in RealSubject.

The Proxy forwards the call to the RealSubject's request() method.

3fill in blank

hard

Fix the error in the Proxy constructor to initialize the RealSubject lazily.

LLD

public Proxy() {
    [1] = null;
}

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

Asubject

BRealSubject

CrealSubject

Dproxy

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name RealSubject instead of the variable name.

Initializing realSubject to a new RealSubject() in the constructor.

The instance variable realSubject should be set to null to delay creation.

4fill in blank

hard

Fill both blanks to add access control in Proxy before forwarding the request.

LLD

public void request() {
    if ([1]()) {
        realSubject.[2]();
    } else {
        System.out.println("Access denied.");
    }
}

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

AcheckAccess

Brequest

Cvalidate

Dexecute

Attempts:

3 left

💡 Hint

Common Mistakes

Calling execute() instead of request() on realSubject.

Using validate() without defining it.

The Proxy checks access with checkAccess() before calling realSubject.request().

5fill in blank

hard

Fill all three blanks to implement caching in Proxy for the result of RealSubject's request.

LLD

private String cache;

public String request() {
    if (cache == null) {
        cache = realSubject.[1]();
    }
    return [2];
}

public void clearCache() {
    [3] = null;
}

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

Aresult

Bcache

Drequest

Attempts:

3 left

💡 Hint

Common Mistakes

Returning realSubject.request() every time without caching.

Clearing the wrong variable in clearCache().

The Proxy caches the result of realSubject.request() in cache and returns it. clearCache() resets cache to null.

Practice

1.

What is the main purpose of the Proxy pattern in system design?

easy

A. To replace the real object permanently with a new one

B. To control access to an object by using a placeholder

C. To directly modify the real object's data

D. To create multiple instances of an object

Solution

1. Step 1: Understand the Proxy pattern role

   The Proxy pattern acts as a stand-in or placeholder to control access to another object.

2. Step 2: Compare options with the pattern definition

   Only To control access to an object by using a placeholder correctly describes controlling access using a placeholder, which matches the Proxy pattern.

3. Final Answer:

   To control access to an object by using a placeholder -> Option B

4. Quick Check:

   Proxy pattern = control access placeholder [OK]

Hint: Proxy controls access via placeholder, not object creation [OK]

Common Mistakes:

• Confusing Proxy with Singleton pattern
• Thinking Proxy modifies the real object directly
• Assuming Proxy replaces the real object permanently

2.

Which of the following is the correct way to declare a proxy class that implements the same interface as the real object?

interface Image {
    void display();
}

// Which proxy class declaration is correct?

easy

A. interface ImageProxy implements Image { void display(); }

B. class ImageProxy extends Image { void display() { /* control access */ } }

C. class ImageProxy { void display() { /* control access */ } }

D. class ImageProxy implements Image { void display() { /* control access */ } }

Solution

1. Step 1: Recall interface implementation syntax

   In many languages, a class implements an interface using the 'implements' keyword, not 'extends'.

2. Step 2: Check each option's correctness

   class ImageProxy implements Image { void display() { /* control access */ } } correctly declares a class implementing the interface. class ImageProxy extends Image { void display() { /* control access */ } } wrongly uses 'extends' for an interface. class ImageProxy { void display() { /* control access */ } } misses interface implementation. interface ImageProxy implements Image { void display(); } wrongly declares an interface implementing another interface.

3. Final Answer:

   class ImageProxy implements Image { void display() { /* control access */ } } -> Option D

4. Quick Check:

   Proxy class implements interface = class ImageProxy implements Image { void display() { /* control access */ } } [OK]

Hint: Proxy class must implement interface, not extend it [OK]

Common Mistakes:

• Using 'extends' instead of 'implements' for interfaces
• Not implementing the interface at all
• Declaring proxy as an interface instead of a class

3.

Given the following code snippet, what will be the output?

class RealImage {
    void display() { System.out.println("Displaying real image"); }
}

class ProxyImage {
    RealImage realImage;
    void display() {
        if (realImage == null) {
            realImage = new RealImage();
        }
        realImage.display();
    }
}

ProxyImage proxy = new ProxyImage();
proxy.display();
proxy.display();

medium

A. Displaying real image
Displaying real image

B. Displaying real image

C. No output

D. NullPointerException

Solution

1. Step 1: Analyze the first call to display()

   Since realImage is null initially, it is created and then display() prints "Displaying real image".

2. Step 2: Analyze the second call to display()

   realImage is no longer null, so display() is called directly again, printing the same message.

3. Final Answer:

   Displaying real image Displaying real image -> Option A

4. Quick Check:

   Lazy loading calls display twice [OK]

Hint: Proxy lazy loads real object, so display prints twice [OK]

Common Mistakes:

• Assuming display prints only once
• Expecting NullPointerException due to uninitialized realImage
• Thinking proxy suppresses output

4.

Identify the error in the following proxy implementation and select the fix:

class Proxy {
    RealObject real;
    void request() {
        real.request();
    }
}

medium

A. Remove the proxy and call RealObject directly

B. Change method name from request() to execute()

C. Initialize 'real' before calling request()

D. Make 'real' a static variable

Solution

1. Step 1: Identify the problem in the code

   The proxy calls real.request() but 'real' is never initialized, causing a NullPointerException.

2. Step 2: Determine the correct fix

   Initializing 'real' before calling request() ensures the real object exists and avoids errors.

3. Final Answer:

   Initialize 'real' before calling request() -> Option C

4. Quick Check:

   Uninitialized real causes error; initialize it [OK]

Hint: Always initialize real object before proxy calls it [OK]

Common Mistakes:

• Ignoring null initialization error
• Changing method names unnecessarily
• Removing proxy defeats pattern purpose

5.

You are designing a system where loading a large image file is expensive. You want to use the Proxy pattern to improve performance by loading the image only when needed. Which of the following design choices best applies the Proxy pattern for this scenario?

hard

A. Create a proxy class that holds the image filename and loads the real image only when display() is called

B. Load the real image immediately when the proxy is created to avoid delays

C. Use a proxy that duplicates the image data to speed up access

D. Replace the real image with a smaller thumbnail permanently

Solution

1. Step 1: Understand the problem context

   Loading large images is expensive, so delaying loading until necessary improves performance.

2. Step 2: Match design choices to Proxy pattern benefits

   Create a proxy class that holds the image filename and loads the real image only when display() is called uses lazy loading by deferring real image loading until display() is called, which fits Proxy pattern use.

3. Final Answer:

   Create a proxy class that holds the image filename and loads the real image only when display() is called -> Option A

4. Quick Check:

   Lazy loading with proxy = Create a proxy class that holds the image filename and loads the real image only when display() is called [OK]

Hint: Proxy delays expensive load until needed (lazy loading) [OK]

Common Mistakes:

• Loading real object immediately defeats proxy benefit
• Duplicating data wastes resources
• Replacing with thumbnail changes object behavior