Singleton
Ensure a class has only one instance, and provide a global point of access to it.
Interview Prep
Design Patterns
25 Gang of Four patterns - Creational, Structural, and Behavioral. Each with intent, code examples, and interview angles.
6 patterns
Creational
How objects are created - control instantiation, hide complexity, avoid direct new calls.
Factory Method
Define an interface for creating an object, but let subclasses decide which class to instantiate.
Abstract Factory
Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
Builder
Separate the construction of a complex object from its representation so the same construction process can create different representations.
Prototype
Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
Object Pool
Reuse a set of initialized objects kept ready for use rather than creating and destroying on demand.
7 patterns
Structural
How objects are composed - build flexible structures from simple parts.
Adapter
Convert the interface of a class into another interface clients expect. Adapter lets classes work together that could not otherwise because of incompatible interfaces.
Bridge
Decouple an abstraction from its implementation so that the two can vary independently.
Composite
Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
Decorator
Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
Facade
Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
Flyweight
Use sharing to support large numbers of fine-grained objects efficiently.
Proxy
Provide a surrogate or placeholder for another object to control access to it.
12 patterns
Behavioral
How objects communicate - define algorithms, delegate responsibilities, manage state.
Chain of Responsibility
Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
Command
Encapsulate a request as an object, thereby allowing parameterization of clients with different requests, queuing or logging of requests, and support for undoable operations.
Interpreter
Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
Iterator
Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
Mediator
Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly.
Memento
Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.
Observer
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
State
Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
Strategy
Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
Template Method
Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.
Visitor
Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.
Null Object
Provide a default object with do-nothing (no-op) behavior instead of using null, to avoid null checks and NullPointerExceptions.