Interactive runtime simulators, real-world metaphors, UML structures, and production code in Java, TypeScript, and Python for all Creational, Structural, and Behavioral patterns.
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Defines an interface for creating an object, but lets subclasses decide which class to instantiate.
A logistics company dispatch center: the core logistics system asks for a 'Transport', while sub-departments dispatch Trucks, Ships, or Planes depending on terrain.
Provides an interface for creating families of related or dependent objects without specifying their concrete classes.
A modern furniture store with Victorian, Modern, and Art Deco lines. Ordering a 'Sofa + Chair + CoffeeTable' from ModernFactory guarantees all 3 pieces match in style.
Separates the construction of a complex object from its representation, allowing the same construction process to create different representations.
Customizing a pizza or building a custom gaming PC. Rather than passing 18 parameters to a giant constructor (`new PC(cpu, gpu, ram, rgb, liquidCooling...)`), you assemble parts step-by-step.
Ensures a class has only one instance and provides a global point of access to it.
The President of a country: there can only be one official president at any given time, accessible to all government departments.
Allows objects with incompatible interfaces to collaborate by converting the interface of one object into another.
A US-to-UK power plug adapter. The UK wall socket and the US laptop plug are incompatible, but the adapter bridges them without modifying the wall or the laptop.
Attaches additional responsibilities to an object dynamically, providing a flexible alternative to subclassing.
Putting on clothes in cold weather. You put on a base shirt, then wrap yourself in a warm sweater, then wrap a raincoat over that. Each layer adds a behavior without altering your core body.
Provides a simplified, unified interface to a complex subsystem of classes, libraries, or frameworks.
A hotel concierge. You tell the concierge: 'Book me dinner and a theater ticket'. The concierge coordinates with restaurants, taxi drivers, and ticket vendors behind the scenes.
Provides a surrogate or placeholder for another object to control access to it.
A debit card is a proxy for your bank account cash. You swipe the card instead of carrying paper bills; the card authenticates you, verifies balance, and delegates to the real account.
Defines a family of algorithms, encapsulates each one, and makes them interchangeable at runtime.
Traveling to the airport. Depending on your budget and time, you can choose a strategy: taking the Subway, calling an Uber, or riding a Bicycle. The destination is the same, but the algorithm differs.
Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified automatically.
A newspaper subscription or YouTube channel. When the YouTuber uploads a new video, all subscribed viewers receive an instant notification on their phones.
Encapsulates a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
A waiter in a restaurant taking your order on a slip of paper. The slip encapsulates your order request. It can be queued, passed to the chef, tracked, or cancelled before cooking.
Passes requests along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it to the next handler in the chain.
A customer support phone tree. The first automated agent handles FAQs. If unhandled, it transfers to Tier-1 support. If unresolved, it escalates to Tier-2 engineering.
Allows an object to alter its behavior when its internal state changes. The object will appear to change its class.
A smartphone. Pressing the power button behaves differently depending on current state: if phone is Locked -> Screen wakes up. If phone is Unlocked -> Screen turns off. If phone is Ringing -> Silences ringer.
Defines the skeleton of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.
Building a prefabricated house. The master architectural blueprint defines the sequence: 1. Lay Foundation -> 2. Build Walls -> 3. Add Roof. Subclasses choose whether walls are wooden, brick, or glass.
Specifies the kinds of objects to create using a prototypical instance, and creates new objects by copying this prototype.
Cell division (mitosis). A biological cell duplicates its DNA and splits into an identical copy, without having to rebuild the cell from raw atoms from scratch.
Composes objects into tree structures to represent part-whole hierarchies, letting clients treat individual objects and compositions uniformly.
A computer file system. A Folder can contain Files AND other Folders. Calling `getSize()` on a Folder computes the total size recursively, treating leaf files and container folders uniformly.
Fits more objects into the available amount of RAM by sharing common parts of state between multiple objects instead of keeping all of the data in each object.
Rendering 1,000,000 trees in a video game forest. Rather than loading the 3D mesh and texture 1,000,000 times, you share 1 TreeType in RAM and only store unique (X, Y) coordinates for each tree.
Lets you traverse elements of a collection without exposing its underlying representation (list, stack, tree, graph).
A tour guide in a museum. The guide walks you through exhibits in a curated sequence (chronological, theme-based) without you needing to know the museum's private floorplan.
Restricts direct communications between objects and forces them to collaborate only via a mediator object.
An Air Traffic Control (ATC) tower. Airplanes do not talk directly to each other to negotiate runway landings; every pilot talks exclusively to the ATC tower, which coordinates safe spacing.
Lets you save and restore the previous state of an object without revealing the details of its implementation.
A game save checkpoint or git commit. You take a snapshot of the current state and stash it. If you die or make a mistake, you can reload the exact snapshot without exposing private variables.
Lets you separate algorithms from the objects on which they operate by using Double Dispatch.
A tax auditor visiting different properties (Residential home, Commercial factory, Agricultural farm). Each property accepts the auditor (`property.accept(auditor)`), and the auditor executes property-specific tax calculations without changing the property's class.
Detects failures and encapsulates the logic of preventing a failure from constantly recurring during maintenance, temporary external or system difficulties.
An electrical circuit breaker in your house. If a wire shorts out, the breaker trips to prevent electrical fires and damage to appliances.