SOLID principles in Software Engineering - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'S' in SOLID stand for?

It stands for <em>Single Responsibility Principle</em>. This means a class or module should have only one reason to change, focusing on a single task or responsibility.

beginner

Explain the Open/Closed Principle in simple terms.

It means software entities like classes should be open for extension but closed for modification. You can add new features without changing existing code.

intermediate

What is the Liskov Substitution Principle?

It says that objects of a parent class should be replaceable with objects of a child class without affecting the program's correctness.

intermediate

Describe the Interface Segregation Principle.

This principle advises that no client should be forced to depend on methods it does not use. It's better to have many small, specific interfaces than one large, general one.

advanced

What does the Dependency Inversion Principle promote?

It promotes that high-level modules should not depend on low-level modules. Both should depend on abstractions. Also, abstractions should not depend on details; details should depend on abstractions.

Which SOLID principle means a class should have only one reason to change?

A Single Responsibility Principle

B Open/Closed Principle

C Liskov Substitution Principle

D Dependency Inversion Principle

The Single Responsibility Principle states that a class should have only one reason to change.

What does the Open/Closed Principle encourage?

A Classes should be open for modification but closed for extension

B Classes should depend on concrete implementations

C Classes should have many responsibilities

D Classes should be open for extension but closed for modification

Open/Closed Principle means classes should be open for extension but closed for modification.

According to the Liskov Substitution Principle, a subclass should be able to replace its superclass without:

A Using interfaces

B Adding new methods

C Changing the program's correctness

D Changing variable names

Liskov Substitution Principle requires that subclasses can replace superclasses without affecting correctness.

The Interface Segregation Principle suggests:

A Use many small, specific interfaces

B Clients should depend on interfaces they do not use

C Use one large interface for all clients

D Avoid using interfaces

Interface Segregation Principle encourages many small, specific interfaces rather than one large one.

Dependency Inversion Principle states that:

A High-level modules depend on low-level modules

B Both depend on abstractions

C Low-level modules depend on high-level modules

D Abstractions depend on details

Dependency Inversion Principle says both high-level and low-level modules should depend on abstractions.

Explain each of the SOLID principles in your own words and give a simple example for one of them.

Think about how each principle helps keep code easy to maintain and extend.

Why are the SOLID principles important in software development? Describe how they help in real-life coding projects.

Consider how following these principles affects teamwork and code quality.

Practice

1. Which of the following best describes the Single Responsibility Principle (SRP) in SOLID?

easy

A. A class should have only one reason to change.

B. A class should inherit from multiple classes.

C. Functions should have multiple responsibilities.

D. Objects should be tightly coupled.

Solution

1. Step 1: Understand SRP definition

   The Single Responsibility Principle states that a class should have only one reason to change, meaning it should have one job or responsibility.

2. Step 2: Evaluate options

   A class should have only one reason to change.: A class should have only one reason to change. matches this definition exactly. Options B, C, and D contradict the principle by suggesting multiple responsibilities or tight coupling.

3. Final Answer:

   A class should have only one reason to change. -> Option A

4. Quick Check:

   SRP = One responsibility [OK]

Hint: SRP means one class, one job only [OK]

Common Mistakes:

• Thinking SRP allows multiple responsibilities
• Confusing SRP with inheritance rules
• Believing tight coupling is allowed

2. Which of the following is the correct syntax to express the Open/Closed Principle (OCP)?

easy

A. Software entities should be open for modification but closed for extension.

B. Software entities should be closed for both extension and modification.

C. Software entities should be open for both extension and modification.

D. Software entities should be open for extension but closed for modification.

Solution

1. Step 1: Recall OCP definition

   The Open/Closed Principle states that software entities should be open for extension but closed for modification, meaning you can add new features without changing existing code.

2. Step 2: Match options to definition

   Software entities should be open for extension but closed for modification.: Software entities should be open for extension but closed for modification. correctly states this principle. Software entities should be open for modification but closed for extension. reverses the terms, and options B and D contradict the principle.

3. Final Answer:

   Software entities should be open for extension but closed for modification. -> Option D

4. Quick Check:

   OCP = Open extension, closed modification [OK]

Hint: OCP means add features, don't change old code [OK]

Common Mistakes:

• Mixing up 'open for extension' and 'open for modification'
• Thinking OCP allows changing existing code freely
• Confusing OCP with SRP

3. Consider this code snippet applying the Liskov Substitution Principle (LSP):

class Bird:
    def fly(self):
        return "Flying"

class Penguin(Bird):
    def fly(self):
        return "Cannot fly"

bird = Bird()
penguin = Penguin()
print(bird.fly())
print(penguin.fly())

What is the output of this code?

medium

A. "Flying" followed by "Cannot fly"

B. "Flying" followed by "Flying"

C. Runtime error due to method override

D. "Cannot fly" followed by "Flying"

Solution

1. Step 1: Analyze Bird and Penguin classes

   Bird has a fly method returning "Flying". Penguin overrides fly to return "Cannot fly".

2. Step 2: Check print statements output

   bird.fly() returns "Flying". penguin.fly() returns "Cannot fly". So output is two lines: "Flying" then "Cannot fly".

3. Final Answer:

   "Flying" followed by "Cannot fly" -> Option A

4. Quick Check:

   bird.fly() = Flying, penguin.fly() = Cannot fly [OK]

Hint: Overridden methods can change output but not cause errors [OK]

Common Mistakes:

• Assuming penguin.fly() returns "Flying"
• Expecting runtime error from override
• Confusing method outputs order

4. Identify the error in this code violating the Interface Segregation Principle (ISP):

class WorkerInterface:
    def work(self): pass
    def eat(self): pass

class Robot(WorkerInterface):
    def work(self):
        print("Working")
    def eat(self):
        raise Exception("Robots don't eat")

medium

A. Robot class must implement eat() with a print statement.

B. WorkerInterface should have only one method.

C. Robot class should not implement eat() if it doesn't apply.

D. No error; this code follows ISP.

Solution

1. Step 1: Understand ISP

   Interface Segregation Principle says clients should not be forced to depend on methods they do not use.

2. Step 2: Analyze Robot class

   Robot implements eat() but raises an exception because it doesn't eat. This violates ISP because Robot is forced to implement an irrelevant method.

3. Final Answer:

   Robot class should not implement eat() if it doesn't apply. -> Option C

4. Quick Check:

   ISP forbids unused methods in interfaces [OK]

Hint: ISP means no forced unused methods [OK]

Common Mistakes:

• Thinking raising exception is acceptable
• Believing interface must have only one method
• Assuming no error if code runs

5. You have a payment system where new payment methods are added frequently. Which SOLID principle helps you design the system so you can add new payment types without changing existing code?

hard

A. Single Responsibility Principle

B. Open/Closed Principle

C. Liskov Substitution Principle

D. Dependency Inversion Principle

Solution

1. Step 1: Identify the problem

   The system needs to add new payment methods often without modifying existing code.

2. Step 2: Match with SOLID principles

   The Open/Closed Principle states software should be open for extension but closed for modification, exactly fitting this need.

3. Final Answer:

   Open/Closed Principle -> Option B

4. Quick Check:

   OCP = Add features without changing old code [OK]

Hint: OCP = add new without changing old [OK]

Common Mistakes:

• Confusing SRP with OCP
• Thinking LSP controls adding new types
• Assuming Dependency Inversion solves this