5 Must-Know Design Patterns Every Developer Should Master

At some point in our careers we’ve all been tangled in if-else nests and duct-taped functions just trying to make things work. But what if I told you there’s a better way? A way used by seasoned developers to write cleaner, smarter, and more maintainable code?

We’ve all heard the term “design patterns” thrown around — usually by that one senior dev who speaks fluent Lorem ipsum.
Maybe you’ve even noticed how certain built-in functions or framework methods just make sense — like they were crafted with some secret blueprint. Well, spoiler alert: they were. That blueprint is what we call a design pattern — a time-tested solution to a common software design problem. In this article, I'll break down the Top 5 Design Patterns you’ve probably seen in the wild (even if you didn’t know their names), and show you how to use them like a pro.

1. Strategy Pattern

Intent: Define a family of algorithms, encapsulate each one, and make them interchangeable.

Use When:

• You have multiple ways of doing something (e.g., sorting, payment, compression).

• You want to switch algorithms at runtime.

Structure:

• Strategy Interface

• Concrete Strategies

• Context (uses Strategy)

Sample Code

// Strategy
interface PaymentStrategy {
    void pay(int amount);
}

// Concrete Strategy
class CreditCardPayment implements PaymentStrategy {
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using credit card");
    }
}

class PayPalPayment implements PaymentStrategy {
    public void pay(int amount) {
        System.out.println("Paid " + amount + " using PayPal");
    }
}

// Context
class ShoppingCart {
    private PaymentStrategy strategy;

    public void setPaymentStrategy(PaymentStrategy strategy) {
        this.strategy = strategy;
    }

    public void checkout(int amount) {
        strategy.pay(amount);
    }
}

2. Observer Pattern

Intent: Define a one-to-many dependency so when one object (subject) changes state, all its dependents (observers) are notified.

Use When:

• An object changes state and others need to know.

• For event-driven systems (e.g., UI, stock price updates).

Structure:

• Subject

• Observer Interface

• Concrete Observers

Sample Code

interface Observer {
    void update(String message);
}

class EmailSubscriber implements Observer {
    public void update(String message) {
        System.out.println("Email received: " + message);
    }
}

class Publisher {
    private List<Observer> observers = new ArrayList<>();

    public void subscribe(Observer obs) {
        observers.add(obs);
    }

    public void notifyObservers(String message) {
        for (Observer obs : observers) {
            obs.update(message);
        }
    }
}

3. Decorator Pattern

Intent: Attach additional responsibilities to an object dynamically. It’s like wrapping an object with extra behavior.

Use When:

• You want to add features without changing the base class.

• Avoid subclass explosion.

Structure:

• Component Interface

• Concrete Component

• Decorator (implements component and wraps another)

Sample Code

interface Coffee {
    String getDescription();
    int cost();
}

class BasicCoffee implements Coffee {
    public String getDescription() { return "Basic Coffee"; }
    public int cost() { return 50; }
}

class MilkDecorator implements Coffee {
    private Coffee coffee;
    public MilkDecorator(Coffee coffee) { this.coffee = coffee; }

    public String getDescription() {
        return coffee.getDescription() + ", Milk";
    }
    public int cost() {
        return coffee.cost() + 10;
    }
}

4. Factory Pattern

Intent: Create objects without exposing the instantiation logic to the client. Use a method to create the object.

Use When:

• You want to delegate object creation to a separate method/class.

• Need to avoid tight coupling.

Structure:

• Interface or Superclass

• Factory method to create instances

Sample Code

interface Animal {
    void speak();
}

class Dog implements Animal {
    public void speak() {
        System.out.println("Woof!");
    }
}

class AnimalFactory {
    public static Animal getAnimal(String type) {
        if (type.equals("dog")) return new Dog();
        throw new IllegalArgumentException("Unknown type");
    }
}

5. Singleton Pattern

Intent: Ensure a class has only one instance and provide a global access point to it.

Use When:

• You need only one instance (e.g., DB connection, config).

• To coordinate actions across the system.

Structure:

• Private constructor

• Static instance variable

• Public static getter method

Sample Code

class ConfigManager {
    private static ConfigManager instance;

    private ConfigManager() {}

    public static ConfigManager getInstance() {
        if (instance == null) {
            instance = new ConfigManager();
        }
        return instance;
    }
}

Thread Safe Version:

public class ThreadSafeSingleton {
    private static final ThreadSafeSingleton instance = new ThreadSafeSingleton();

    private ThreadSafeSingleton() {}

    public static ThreadSafeSingleton getInstance() {
        return instance;
    }
}

Here’s a quick table to help you map the purpose of each pattern to its core concept.

So whether you’re building a game, an API, or the next unicorn startup from your garage, these patterns will save you time, headaches, and therapy bills.