Gang of Four · Interview Prep Guide

Design Patterns
Mastery Guide

All 23 GoF design patterns explained with intent, structure diagrams, Java code, real-world examples, pros/cons, and related pattern comparisons — everything you need for technical interviews.

23Patterns
3Categories
JavaCode Examples

Creational (5)

Deal with object creation mechanisms, aiming to create objects in a manner suitable to the situation.

Structural (7)

Deal with object composition, creating relationships between objects to form larger structures.

Behavioral (11)

Deal with communication between objects, how objects interact and distribute responsibility.

Each Pattern Covers

Intent · Problem · Solution · Diagram · Java Code · When to Use · Real World · Pros/Cons · Related Patterns · Recap

Creational
01
Singleton
Ensure a class has only one instance and provide a global access point
● Beginner
02
Factory Method
Define an interface for creating an object, but let subclasses decide which class to instantiate
● Intermediate
03
Abstract Factory
Provide an interface for creating families of related or dependent objects
● Intermediate
04
Builder
Separate construction of a complex object from its representation
● Intermediate
05
Prototype
Specify the kinds of objects to create using a prototypical instance, and create by copying
● Beginner
Structural
06
Adapter
Convert the interface of a class into another interface clients expect
● Beginner
07
Bridge
Decouple an abstraction from its implementation so the two can vary independently
● Advanced
08
Composite
Compose objects into tree structures to represent part-whole hierarchies
● Intermediate
09
Decorator
Attach additional responsibilities to an object dynamically
● Intermediate
10
Facade
Provide a simplified interface to a complex subsystem
● Beginner
11
Flyweight
Use sharing to efficiently support large numbers of fine-grained objects
● Advanced
12
Proxy
Provide a surrogate or placeholder for another object to control access
● Intermediate
Behavioral
13
Chain of Responsibility
Pass a request along a chain of handlers until one handles it
● Intermediate
14
Command
Encapsulate a request as an object, parameterize clients, and support undo
● Intermediate
15
Interpreter
Define a grammar and provide an interpreter to deal with that grammar
● Advanced
16
Iterator
Provide a way to access elements of an aggregate object sequentially
● Beginner
17
Mediator
Define an object that encapsulates how a set of objects interact
● Intermediate
18
Memento
Capture and externalize an object's internal state for later restoration
● Intermediate
19
Observer
Define a one-to-many dependency so when one object changes state, all dependents are notified
● Beginner
20
State
Allow an object to alter its behavior when its internal state changes
● Intermediate
21
Strategy
Define a family of algorithms, encapsulate each one, and make them interchangeable
● Beginner
22
Template Method
Define the skeleton of an algorithm in a base class, deferring some steps to subclasses
● Beginner
23
Visitor
Represent an operation to be performed on elements of an object structure
● Advanced