Design Patterns in Java: The Complete Guide

🔵 Low-Level Design · Hub · Fresher-friendly → 🟠 senior

Design patterns are proven, reusable solutions to problems that come up again and again in object-oriented design. They're not code you copy — they're templates for structuring classes, and knowing them gives you a shared vocabulary ("let's use a Factory here") that instantly communicates a design. This hub explains what patterns are, the three families they fall into, and links to a deep-dive on each one. Think of it as your map to the whole topic.

Why patterns matter (especially in interviews)

🟠 In a low-level design round, reaching for the right pattern shows maturity. When asked "how would you support multiple payment methods without a giant if/else?", answering "a Strategy pattern — each method is a class behind a common interface" is far stronger than ad-hoc code. Patterns also make you faster: you're not inventing structure from scratch, you're applying a known-good one. And they're a shared language every senior engineer speaks.

The three families

The classic 23 "Gang of Four" patterns split into three groups by what problem they solve:

Creational — how objects get made

These control object creation so it's flexible and decoupled from the concrete classes:

• Singleton — exactly one shared instance (e.g. a config or logger).
• Factory Method — a method decides which subclass to instantiate.
• Abstract Factory — create families of related objects.
• Builder — construct a complex object step by step.
• Prototype — create new objects by cloning an existing one.

Structural — how objects are put together

These compose classes and objects into larger structures while keeping them flexible:

• Adapter — make an incompatible interface fit.
• Decorator — add behaviour to an object at runtime, without subclassing.
• Facade — a simple front over a complex subsystem.
• Proxy — a stand-in that controls access to another object.
• Composite — treat individual objects and groups uniformly (trees).

Behavioral — how objects communicate

These manage algorithms and the flow of responsibility between objects:

• Strategy — swap interchangeable algorithms behind one interface.
• Observer — notify many objects when one changes (pub/sub).
• State — change behaviour as an object's internal state changes.
• Command — wrap a request as an object (undo, queues).
• Template Method — define an algorithm's skeleton, let subclasses fill steps.
• Chain of Responsibility — pass a request along a chain of handlers.

How to actually use patterns

🟠 A caution every senior gives: don't force patterns. Applying a pattern where a simple solution would do (over-engineering) is a red flag, not a green one. Patterns are tools for real recurring problems — reach for one when you feel the specific pain it solves (a growing type-switch → Strategy/Factory; needing runtime add-ons → Decorator). The goal is always cleaner, more flexible code, never pattern-name-dropping. Understanding the problem each solves matters far more than memorising its structure.

Where to start

If you're new, learn these five first — they're the most common in interviews and real code: Singleton, Factory, Strategy, Observer, and Decorator. Each linked article explains the pattern from scratch with a Java example, a UML sketch, and exactly when to use it.

What to read next

• SOLID Principles — the foundation every pattern applies
• Strategy Pattern — the most useful one to learn first
• Design a Parking Lot — patterns applied in a full LLD problem
• ← The complete System Design guide (hub)

← UML class diagrams · Singleton pattern →