Also known as

Kit

Intent

Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

Explanation

Real-world example

To create a kingdom we need objects with a common theme. The elven kingdom needs an elven king, elven castle, and elven army whereas the orcish kingdom needs an orcish king, orcish castle, and orcish army. There is a dependency between the objects in the kingdom.

Also known as

Wrapper

Intent

Convert the interface of a class into another interface the clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.

Explanation

Real-world example

Consider that you have some pictures on your memory card and you need to transfer them to your computer. To transfer them, you need some kind of adapter that is compatible with your computer ports so that you can attach a memory card to your computer. In this case card reader is an adapter. Another example would be the famous power adapter; a three-legged plug can't be connected to a two-pronged outlet, it needs to use a power adapter that makes it compatible with the two-pronged outlets. Yet another example would be a translator translating words spoken by one person to another

Also known as

Handle/Body

Intent

Decouple an abstraction from its implementation so that the two can vary independently.

Explanation

Real-world example

Consider you have a weapon with different enchantments, and you are supposed to allow mixing different weapons with different enchantments. What would you do? Create multiple copies of each of the weapons for each of the enchantments or would you just create separate enchantment and set it for the weapon as needed? Bridge pattern allows you to do the second.

Intent

Separate the construction of a complex object from its representation so that the same construction process can create different representations.

Explanation

Real-world example

Imagine a character generator for a role-playing game. The easiest option is to let the computer create the character for you. If you want to manually select the character details like profession, gender, hair color, etc. the character generation becomes a step-by-step process that completes when all the selections are ready.

Intent

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.

Explanation

Real-world example

The Orc King gives loud orders to his army. The closest one to react is the commander, then an officer, and then a soldier. The commander, officer, and soldier form a chain of responsibility.

Also known as

Action, Transaction

Intent

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Explanation

Real-world example

There is a wizard casting spells on a goblin. The spells are executed on the goblin one by one. The first spell shrinks the goblin and the second makes him invisible. Then the wizard reverses the spells one by one. Each spell here is a command object that can be undone.

Intent

Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Explanation

Real-world example

Every sentence is composed of words which are in turn composed of characters. Each of these objects are printable and they can have something printed before or after them like sentence always ends with full stop and word always has space before it.

Also known as

Wrapper

Intent

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

Explanation

Real-world example

There is an angry troll living in the nearby hills. Usually, it goes bare-handed but sometimes it has a weapon. To arm the troll it's not necessary to create a new troll but to decorate it dynamically with a suitable weapon.

Also known as

• Simple Factory
• Static Factory Method

Intent

Providing a static method encapsulated in a class called the factory, to hide the implementation logic and make client code focus on usage rather than initializing new objects.

Explanation

Also known as

Virtual Constructor

Intent

Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

Explanation

Real-world example

Blacksmith manufactures weapons. Elves require Elvish weapons and orcs require Orcish weapons. Depending on the customer at hand the right type of blacksmith is summoned.