The Only Design Pattern Cheat Sheet You'll Need Before Interviews

[Prateek Prabhakar]

You've probably read a lot about design patterns, most of them have long explanations, heavy UML diagrams, and abstract examples that are hard to connect with real life.
But let's be honest, when you're preparing for a tech interview, you don't need a textbook. You need a quick, crisp, memorable guide that tells you,

• What the pattern is
• Why it exists
• A real world hook to remember it
• A tiny pseudo code snippet for recall

That is exactly what this Design Pattern Cheat Sheet is.
Think of it as your one stop destination to revise all design patterns in minutes, with hooks and analogies that will stick in your head when interview pressure hits.
And if you want to go deeper, don't worry, I have already written detailed articles for each pattern. You'll find the links next to each one for a full deep dive.

Creational Patterns​

Creational design patterns are like a set of blueprints or recipes for creating objects. They separate the process of HOW objects are created from the code that actually USES them. This makes our code more flexible and easier to change later on.

1.Factory Method

// Interface
Interface Sender
    Method Send(msg)

// Concrete products
Class EmailSender implements Sender
    Send(msg) → print "Email: " + msg

Class SmsSender implements Sender
    Send(msg) → print "SMS: " + msg

// Creator
Abstract Class NotificationCreator
    Abstract Method CreateSender()

Class EmailCreator extends NotificationCreator
    CreateSender() → new EmailSender()

Class SmsCreator extends NotificationCreator
    CreateSender() → new SmsSender()

// Caller
creator = new EmailCreator()
sender  = creator.CreateSender()
sender.Send("Hey! How are you doing?")

2.Abstract Factory

// Abstract Factory
Interface UIFactory
    Method CreateButton()
    Method CreateInput()

// Concrete factories
Class DarkFactory implements UIFactory
    CreateButton() → new DarkButton()
    CreateInput()  → new DarkInput()

Class LightFactory implements UIFactory
    CreateButton() → new LightButton()
    CreateInput()  → new LightInput()

// Caller
factory = new DarkFactory()
btn = factory.CreateButton()
inp = factory.CreateInput()

3.Prototype

// Prototype interface
Interface Document
    Method Clone()

// Concrete prototype
Class Invoice implements Document
    Property header, footer
    Clone() → return shallowCopy(this)

// Caller
invoice1 = new Invoice("Logo A", "Thanks")
invoice2 = invoice1.Clone()
invoice2.footer = "Updated Footer"

4.Singleton

// Singleton
Class Config
    Private static instance
    Private Constructor()

    Static Method GetInstance()
        If instance == null
            instance = new Config()
        return instance

// Caller
c1 = Config.GetInstance()
c2 = Config.GetInstance()
// c1 == c2

5.Builder

Class Notification
    Props: Title, Message, IsEmail, IsSMS, IsPush, Priority

Class NotificationBuilder
    Constructor() -> this.n = new Notification()
    Method SetTitle(t)         -> n.Title = t; return this
    Method SetMessage(m)       -> n.Message = m; return this
    Method EnableEmail()       -> n.IsEmail = true; return this
    Method EnableSMS()         -> n.IsSMS   = true; return this
    Method EnablePush()        -> n.IsPush  = true; return this
    Method WithPriority(p)     -> n.Priority = p; return this
    Method Build()             -> return n

// Caller
notification = new NotificationBuilder()
                 .SetTitle("Big Sale!")
                 .SetMessage("50% off today")
                 .EnableEmail()
                 .EnablePush()
                 .WithPriority("High")
                 .Build()

Structural Patterns​

Structural design patterns are a set of rules for putting together classes and objects to create bigger, more useful systems. They are all about composition, which means combining objects instead of relying solely on inheritance. Think of it like building with LEGOs, you are fitting different pieces together to make a larger, more complex model.

6.Adapter

// Old interface
Class LegacyPayment
    Method MakePayment(amount)

// Adapter
Class PaymentAdapter implements NewPayment
    legacy = new LegacyPayment()
    Pay(amount) → legacy.MakePayment(amount)

// Caller
p = new PaymentAdapter()
p.Pay(100)

7.Decorator

// Base
Interface Coffee
    Method Cost()

Class SimpleCoffee implements Coffee
    Cost() → 50

// Decorator
Class MilkDecorator implements Coffee
    coffee
    Constructor(c) → coffee = c
    Cost() → coffee.Cost() + 20

// Caller
coffee = new MilkDecorator(new SimpleCoffee())
print coffee.Cost()   // 70

8.Composite

// Component
Interface FileSystemItem
    Display(indent)

// Leaf
Class File implements FileSystemItem
    Display(i) → print i + "File"

// Composite
Class Folder implements FileSystemItem
    items = []
    Add(item) → items.Add(item)
    Display(i)
        print i + "Folder"
        For item in items → item.Display(i+1)

// Caller
root = new Folder()
root.Add(new File())
root.Display(0)

9.Facade

// Subsystems
Class Lights
    Method Dim()

Class TV
    Method TurnOn()

// Facade
Class HomeTheaterFacade
    Method StartMovieNight()
        Lights.Dim()
        TV.TurnOn()

// Caller
theater = new HomeTheaterFacade()
theater.StartMovieNight()

10.Proxy

// Subject
Interface Service
    Request()

// Real subject
Class RealService implements Service
    Request() → print "Real work done"

// Proxy
Class ServiceProxy implements Service
    real = new RealService()
    Request()
        print "Proxy check..."
        real.Request()

// Caller
s = new ServiceProxy()
s.Request()

11.Flyweight

// Flyweight
Class Character
    symbol
    Constructor(s) → symbol = s

// Factory
Class CharFactory
    cache = {}
    GetChar(s)
        If not in cache → cache[s] = new Character(s)
        return cache[s]

// Caller
c1 = factory.GetChar("A")
c2 = factory.GetChar("A")
// same instance

12.Bridge

// Implementor
Interface Device
    TurnOn()

Class TV implements Device
    TurnOn() → print "TV On"

// Abstraction
Class Remote
    device
    Constructor(d) → device = d
    Press() → device.TurnOn()

// Caller
r = new Remote(new TV())
r.Press()

Behavioral Patterns​

Behavioral design patterns are about defining how objects interact and communicate with each other. They focus on delegating responsibilities and ensuring objects can work together effectively without being tightly coupled. Think of them as the traffic rules for our code. They manage the flow of requests and the assignment of tasks between different objects.

13.Strategy

// Strategy
Interface Coupon
    Apply(amount)

// Concrete strategies
Class Flat50 implements Coupon
    Apply(amount) → amount - 50

Class TenPercent implements Coupon
    Apply(amount) → amount * 0.9

// Caller
coupon = new Flat50()
print coupon.Apply(500)

14.Observer

// Subject
Class Channel
    subs = []
    AddSub(s) → subs.Add(s)
    Upload(video)
        For s in subs → s.Notify(video)

// Observer
Class User
    Notify(v) → print "New video: " + v

// Caller
ch = new Channel()
usr = new User()
ch.AddSub(usr)
ch.Upload("Design Patterns")

15.Command

// Command
Interface Command
    Execute()

Class LightOn implements Command
    Execute() → print "Light ON"

// Invoker
Class Remote
    Set(cmd) → this.cmd = cmd
    Press() → cmd.Execute()

// Caller
remote = new Remote()
remote.Set(new LightOn())
remote.Press()

16.Chain of Responsibility

// Handler
Abstract Class Handler
    next
    SetNext(h) → next = h
    Handle(req)
        If next != null → next.Handle(req)

// Concrete
Class Manager extends Handler
    Handle(req)
        If req < 1000 → print "Manager handled"
        Else super.Handle(req)

// Caller
mgr = new Manager()
dir = new Director()
mgr.SetNext(dir)
mgr.Handle(2000)

17.State

// State
Interface State
    Play()

Class HappyState implements State
    Play() → print "Party music"

Class SadState implements State
    Play() → print "Sad songs"

// Context
Class MusicPlayer
    state
    SetState(s) → state = s
    Play() → state.Play()

// Caller
mp = new MusicPlayer()
mp.SetState(new HappyState())
mp.Play()

18.Mediator

// Mediator
Class ChatRoom
    Send(msg, user) → print user + ": " + msg

// Colleague
Class User
    name, room
    Send(msg) → room.Send(msg, name)

// Caller
room = new ChatRoom()
u1 = new User("Alice", room)
u1.Send("Hello!")

19.Momento

// Memento
Class Snapshot
    state
    Constructor(s) → state = s

// Originator
Class Editor
    text
    Save() → new Snapshot(text)
    Restore(snap) → text = snap.state

// Caller
e = new Editor()
e.text = "v1"
snap = e.Save()
e.text = "v2"
e.Restore(snap)  // back to v1

20.Template Method

// Abstract
Class Game
    Play()
        Init()
        Start()
        End()

// Concrete
Class Football extends Game
    Init() → print "Setup field"
    Start() → print "Kickoff"
    End()   → print "Game over"

// Caller
g = new Football()
g.Play()

21.Visitor

// Visitor
Interface Visitor
    Visit(Book)

// Element
Class Book
    Accept(v) → v.Visit(this)

// Concrete visitor
Class TaxVisitor implements Visitor
    Visit(b) → print "Taxing book"

// Caller
book = new Book()
visitor = new TaxVisitor()
book.Accept(visitor)

22.Iterator

// Iterator
Interface Iterator
    HasNext()
    Next()

Class ProductIterator implements Iterator
    list, index=0
    HasNext() → index < list.Length
    Next() → list[index++]

// Caller
it = new ProductIterator(products)
while it.HasNext()
    print it.Next()

23.Interpreter

Interface Expression
    Method Interpret(product): bool

Class CategoryExpression implements Expression
    Constructor(category)
    Method Interpret(product) -> product.Category == category

Class PriceExpression implements Expression
    Constructor(limit)
    Method Interpret(product) -> product.Price < limit

Class AndExpression implements Expression
    Constructor(left, right)
    Method Interpret(product) -> left.Interpret(product) 
                                      AND right.Interpret(product)

// Usage
rule = new AndExpression(
          new CategoryExpression("Electronics"),
          new PriceExpression(1000))

For each product in products
    If rule.Interpret(product)
        Print "Selected: " + product.Name

Design patterns aren't about throwing jargon in interviews. They are about writing clean, extensible, and maintainable code.
This cheat sheet is meant to be your last minute revision buddy, something you can glance at before an interview and instantly recall the essence of each pattern.
Remember,

• There is no need to memorize patterns, instead it is important to understand the problem each one solves.
• Use the hooks and analogies here to explain patterns in a simple, story like way during interviews.
• When coding, reach for patterns not because they are "fancy," but because they make your design clearer and future proof.

With this in your toolkit, I am sure you will walk into interviews not just knowing the names of the patterns, but with the confidence to explain and apply them in real world scenarios.

Happy coding, and may your designs always stay simple and scalable!

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