oop - Examples of GoF Design Patterns in Java's core libraries

I am learning GoF Java Design Patterns and I want to see some real life examples of them. What are some good examples of these Design Patterns in Java's core libraries?

Answer

You can find an overview of a lot of design patterns in Wikipedia. It also mentions which patterns are mentioned by GoF. I'll sum them up here and try to assign as many pattern implementations as possible, found in both the Java SE and Java EE APIs.

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Abstract factory ^{_{(recognizeable by creational methods returning the factory itself which in turn can be used to create another abstract/interface type)}}

Builder ^{_{(recognizeable by creational methods returning the instance itself)}}

Factory method ^{_{(recognizeable by creational methods returning an implementation of an abstract/interface type)}}

Prototype ^{_{(recognizeable by creational methods returning a different instance of itself with the same properties)}}

Singleton ^{_{(recognizeable by creational methods returning the same instance (usually of itself) everytime)}}

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Adapter ^{_{(recognizeable by creational methods taking an instance of different abstract/interface type and returning an implementation of own/another abstract/interface type which decorates/overrides the given instance)}}

Bridge ^{_{(recognizeable by creational methods taking an instance of different abstract/interface type and returning an implementation of own abstract/interface type which delegates/uses the given instance)}}

• None comes to mind yet. A fictive example would be new LinkedHashMap(LinkedHashSet, List) which returns an unmodifiable linked map which doesn't clone the items, but uses them. The java.util.Collections#newSetFromMap() and singletonXXX() methods however comes close.

Composite ^{_{(recognizeable by behavioral methods taking an instance of same abstract/interface type into a tree structure)}}

Decorator ^{_{(recognizeable by creational methods taking an instance of same abstract/interface type which adds additional behaviour)}}

Facade ^{_{(recognizeable by behavioral methods which internally uses instances of different independent abstract/interface types)}}

Flyweight ^{_{(recognizeable by creational methods returning a cached instance, a bit the "multiton" idea)}}

Proxy ^{_{(recognizeable by creational methods which returns an implementation of given abstract/interface type which in turn delegates/uses a different implementation of given abstract/interface type)}}

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Chain of responsibility ^{_{(recognizeable by behavioral methods which (indirectly) invokes the same method in another implementation of same abstract/interface type in a queue)}}

Command ^{_{(recognizeable by behavioral methods in an abstract/interface type which invokes a method in an implementation of a different abstract/interface type which has been encapsulated by the command implementation during its creation)}}

Interpreter ^{_{(recognizeable by behavioral methods returning a structurally different instance/type of the given instance/type; note that parsing/formatting is not part of the pattern, determining the pattern and how to apply it is)}}

Iterator ^{_{(recognizeable by behavioral methods sequentially returning instances of a different type from a queue)}}

Mediator ^{_{(recognizeable by behavioral methods taking an instance of different abstract/interface type (usually using the command pattern) which delegates/uses the given instance)}}

Memento ^{_{(recognizeable by behavioral methods which internally changes the state of the whole instance)}}

Observer (or Publish/Subscribe) ^{_{(recognizeable by behavioral methods which invokes a method on an instance of another abstract/interface type, depending on own state)}}

State ^{_{(recognizeable by behavioral methods which changes its behaviour depending on the instance's state which can be controlled externally)}}

Strategy ^{_{(recognizeable by behavioral methods in an abstract/interface type which invokes a method in an implementation of a different abstract/interface type which has been passed-in as method argument into the strategy implementation)}}

Template method ^{_{(recognizeable by behavioral methods which already have a "default" behaviour defined by an abstract type)}}

Visitor ^{_{(recognizeable by two different abstract/interface types which has methods defined which takes each the other abstract/interface type; the one actually calls the method of the other and the other executes the desired strategy on it)}}