Proxy pattern in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the Proxy Pattern Purpose

Which of the following best describes the primary purpose of the Proxy pattern in system design?

A To provide a placeholder for another object to control access to it

B To create multiple instances of an object to improve performance

C To directly modify the behavior of an object without changing its code

D To combine multiple objects into a single interface for easier use

💡 Hint

Think about how a proxy acts as a gatekeeper or representative.

The Proxy pattern provides a surrogate or placeholder for another object to control access to it, often adding additional behavior like lazy loading or access control.

❓ Architecture

intermediate

Proxy Pattern Components Identification

In a Proxy pattern architecture, which component is responsible for implementing the same interface as the real object and controlling access to it?

A Real Subject

B Proxy

C Client

D Adapter

💡 Hint

This component stands between the client and the real object.

The Proxy implements the same interface as the real object and controls access to it, acting as a middleman.

❓ scaling

advanced

Scaling a System Using Proxy Pattern

You have a system where creating the real object is expensive. Using the Proxy pattern, which approach best helps to scale the system efficiently?

A Use a Remote Proxy to replicate the real object on multiple servers

B Use a Protection Proxy to limit the number of clients accessing the real object

C Use a Virtual Proxy to delay the creation of the real object until it is needed

D Use a Cache Proxy to store multiple copies of the real object in memory

💡 Hint

Think about delaying expensive operations until absolutely necessary.

A Virtual Proxy delays the creation of the real object until it is actually needed, saving resources and improving scalability.

❓ tradeoff

advanced

Tradeoffs of Using Proxy Pattern

What is a common tradeoff when using the Proxy pattern in system design?

A Difficulty in testing because proxies cannot be replaced with mocks

B Reduced security because the proxy exposes internal details of the real object

C Loss of flexibility since the proxy cannot implement the same interface as the real object

D Increased complexity and potential performance overhead due to extra indirection

💡 Hint

Consider what extra step the proxy adds to method calls.

The Proxy pattern adds an extra layer of indirection which can increase complexity and cause some performance overhead.

❓ component

expert

Request Flow in Proxy Pattern

Given a Proxy pattern implementation, what is the correct sequence of components involved when a client requests a service?

A 1,2,3,4

B 1,3,2,4

C 2,1,3,4

D 1,2,4,3

💡 Hint

Think about the natural flow from client to proxy to real object and back.

The client first sends the request, the proxy receives and optionally checks it, then forwards it to the real subject, which processes and returns the response.

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