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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?

Shog9
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unj2
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7 Answers7

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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.

Creational patterns

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)

Structural patterns

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<K>, List<V>) 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)

Behavioral patterns

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)

Community
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BalusC
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    impressive.. :) +1. `javax.lang.model.element` defines visitors ;) I'm not quite sure whether `doXXX` and `doFilter` are "strategies". – Bozho Apr 26 '10 at 13:14
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    @Bozho: Thank you for the visitor example! (that whole API is actually new to me btw). In the `doXXX()` and `doFilter()` you basically write the strategy algorithm alone. The server loads and executes it. – BalusC Apr 26 '10 at 13:24
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    hm, yes.. my impression is that the strategies have to be objects conforming to a strategy interface, and thus interchangeable at runtime – Bozho Apr 26 '10 at 13:31
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    This related blog post just appeared http://www.briandupreez.net/2010/11/design-patterns-in-jdk.html – Bozho Nov 23 '10 at 19:09
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    The mentioned builders e.g. StrinbgBuilder are all not an example for the Builder-Pattern. It is a very common mistake however to consider them as builders (so you are not really to blame ^_^) – Angel O'Sphere May 25 '11 at 13:41
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    @BalusC: are you sure that 'All non-abstract methods of java.io.InputStream, java.io.OutputStream, java.io.Reader and java.io.Writer.' are template methods? For example, InputStream#available(). – Oleksandr Bondarenko Mar 09 '12 at 09:29
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    The Adapter examples are too shallow. I was hoping to see clearly how one adapter interface supports different adaptee interfaces, but it's not obvious from the examples. – Fuhrmanator Nov 10 '12 at 14:20
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    @BalusC, I have a question to ask you. Did you read the **WHOLE** source code of Java and JSF? – Tapas Bose Jan 09 '13 at 21:39
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    @Tapas: I did not read everything, only parts which I needed to, or were just curious as to how "they" did it. – BalusC Jan 09 '13 at 21:41
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    @BalusC In the [Singleton design pattern](http://en.wikipedia.org/wiki/Singleton_pattern), `getInstance()` is the name of the method that returns the singleton, in all books I have read so far (incl the Wikipedia page I am referring). I find it very confusing that the the JDK actually doesn't return a singleton on methods named `getInstance()` – rds Feb 26 '13 at 13:09
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    Most of the examples under "Factory Method"are examples of "static factory" which is not a GoF pattern. Not correct. – ring bearer May 04 '15 at 11:16
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    https://docs.oracle.com/javase/tutorial/jaxp/sax/parsing.html Standard SAX parsing APIs would be an example of the Visitor pattern – nixau Jun 24 '15 at 13:39
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    And what would be `java.awt.GridBagConstraints`? State? Also I wonder if [`MultiValueMap`](https://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/map/MultiValueMap.html#multiValueMap%28java.util.Map,%20java.lang.Class%29) is proper candidate for bridge (however it is not Java core). Thanks. – dma_k Aug 05 '15 at 17:23
  • Adapter example: java.util.Collections # enumeration(Collection c) : It allows you to enumerate over ArrayList – Deen John Jun 25 '16 at 15:12
  • Introduced in 1.8, [Calendar.Builder](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8u40-b25/java/util/Calendar.java#Calendar.Builder) is also a good example of the builder pattern. –  Jul 15 '16 at 20:15
  • @BalusC `InputStreamReader` is documented to be a bridge from byte streams to char streams in official javadoc of Oracle. Is there a reason it is under Adapter? – Narendra Pathai Aug 20 '16 at 08:58
  • @BalusC How can serialization be called Memento? By itself serialization is dumb and does not do anything special. Yes serialization can be used as one of the techniques to implement memento. Any insights would be great. Thanks. – Narendra Pathai Aug 20 '16 at 13:55
  • Memento vs Serialization http://stackoverflow.com/questions/14076772/difference-between-memento-pattern-and-serialization – Narendra Pathai Aug 20 '16 at 14:20
  • Template example: Template lays down the steps of an algorithm or process and usually involves multiple abstract methods whose implementation must be filled in by concrete classes. But I doubt whether InputStream and OutputStream non abstract methods qualify as template methods – Narendra Pathai Aug 20 '16 at 14:57
  • Isn't `javax.xml.parsers.DocumentBuilderFactory` part of a **Builder** pattern rather than **Abstract Factory**? – Malvon Sep 09 '16 at 19:42
  • @Malvon: a class can implement multiple design patterns. – BalusC Sep 09 '16 at 20:54
  • @BalusC Understood. How about JCA framework and/or Java encryption? Do we have any Creational pattern? – Malvon Sep 12 '16 at 18:26
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    Read the most voted answer, and found that the Abstract Factory and Factory Method mentioned here is not the design patterns defined in GOF book. That is to say, this answer in WRONG when talking about the Abstract Factory and Factory Method. – ZhaoGang Nov 10 '16 at 02:10
  • java.util.Arrays#asList() is not an adapter as of state of JDK8, it takes Objects and creates ArrayList from them, so it transforms not adapts. – MaxNevermind Jan 06 '17 at 17:37
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    the StringBuilder is an example of the "java builder" design pattern and not the GOF Builder pattern. They're different patterns and often confused – Andrew Norman Mar 17 '17 at 21:28
  • @BalusC "note that parsing/formatting is not part of the pattern, determining the pattern and how to apply it is". Can you please elaborate this with respect to java.util.Pattern and java.text.Format? Format class must do i)pattern matching and ii)formatting for you to say that statement? If so, what is the clear distinction/boundary between these 2 steps? In other words, what is the RESULT of just "pattern matching"(interpreting) step which is used by the "formatting" step? – user104309 Mar 26 '17 at 13:46
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    You can find the visual model of the GoF Design Patterns at http://www.visual-paradigm.org/category/gof-design/ – Rain Wong Aug 22 '17 at 08:20
  • Am I right saying that [`org.xml.sax.ContentHandler`](https://docs.oracle.com/javase/7/docs/api/org/xml/sax/ContentHandler.html) is also a visitor that visits XML elements? And [`javax.xml.stream.XMLEventReader`](https://docs.oracle.com/javase/7/docs/api/javax/xml/stream/XMLEventReader.html) is iterator... – dma_k Feb 08 '18 at 15:36
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    I count *three*: Behavioral, Creational, Structural. Why is it called Gang of *Four*? *edit:* Named after the authors: **Gamma, Erich; Helm, Richard; Johnson, Ralph; Vlissides, John (1995). Design Patterns: Elements of Reusable Object-Oriented Software.** – Vinnie James Jan 06 '19 at 20:20
  • Are there API examples of the Adapter Pattern with respect to JDBC? The Oracle coined "DAO" or "Data Access Object Pattern" is a subordinate type of adapter pattern: ["Essentially, the DAO acts as an adapter between the component and the data source." - OTN](https://www.oracle.com/java/technologies/dataaccessobject.html). I want to know if there are any specific DAO examples that qualify as implementations GoF adapter pattern. – John Sep 10 '20 at 17:00
  • Awesome :). Just adding the missing, which is very common. Java Exceptions are using the Chain-of-Responsibility pattern – user2739602 Oct 30 '20 at 18:55
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  1. Observer pattern throughout whole swing (Observable, Observer)
  2. MVC also in swing
  3. Adapter pattern: InputStreamReader and OutputStreamWriter NOTE: ContainerAdapter, ComponentAdapter, FocusAdapter, KeyAdapter, MouseAdapter are not adapters; they are actually Null Objects. Poor naming choice by Sun.
  4. Decorator pattern (BufferedInputStream can decorate other streams such as FilterInputStream)
  5. AbstractFactory Pattern for the AWT Toolkit and the Swing pluggable look-and-feel classes
  6. java.lang.Runtime#getRuntime() is Singleton
  7. ButtonGroup for Mediator pattern
  8. Action, AbstractAction may be used for different visual representations to execute same code -> Command pattern
  9. Interned Strings or CellRender in JTable for Flyweight Pattern (Also think about various pools - Thread pools, connection pools, EJB object pools - Flyweight is really about management of shared resources)
  10. The Java 1.0 event model is an example of Chain of Responsibility, as are Servlet Filters.
  11. Iterator pattern in Collections Framework
  12. Nested containers in AWT/Swing use the Composite pattern
  13. Layout Managers in AWT/Swing are an example of Strategy

and many more I guess

Cerbrus
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jitter
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    Thanks for the tip on MouseAdapter. I found this exaplanation: http://stackoverflow.com/questions/9244185/why-is-mouseadapter-an-adapter – Lincoln May 20 '15 at 14:24
  • Note that Swing is [only loosely based](https://www.oracle.com/technetwork/java/architecture-142923.html) on MVC. It has collapsed View and Controller into one class. – Matthias Braun Dec 23 '19 at 08:05
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  1. Flyweight is used with some values of Byte, Short, Integer, Long and String.
  2. Facade is used in many place but the most obvious is Scripting interfaces.
  3. Singleton - java.lang.Runtime comes to mind.
  4. Abstract Factory - Also Scripting and JDBC API.
  5. Command - TextComponent's Undo/Redo.
  6. Interpreter - RegEx (java.util.regex.) and SQL (java.sql.) API.
  7. Prototype - Not 100% sure if this count, but I thinkg clone() method can be used for this purpose.
NawaMan
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    Concerning **Flyweight** pattern: it could be different Layout Managers from `java.awt` and `java.swing` packages. Indeed, they share almost identical intrinsic attributes and extrinsic attributes are different UI components that they lay out in UI form. – Vitaly Oct 23 '15 at 16:48
  • @NawaMan You said 5. Comand TextComponent's Undo/Redo. I think it is memento not command. Or most probably both. – Stimpson Cat Sep 03 '18 at 11:37
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RMI is based on Proxy.

Should be possible to cite one for most of the 23 patterns in GoF:

  1. Abstract Factory: java.sql interfaces all get their concrete implementations from JDBC JAR when driver is registered.
  2. Builder: java.lang.StringBuilder.
  3. Factory Method: XML factories, among others.
  4. Prototype: Maybe clone(), but I'm not sure I'm buying that.
  5. Singleton: java.lang.System
  6. Adapter: Adapter classes in java.awt.event, e.g., WindowAdapter.
  7. Bridge: Collection classes in java.util. List implemented by ArrayList.
  8. Composite: java.awt. java.awt.Component + java.awt.Container
  9. Decorator: All over the java.io package.
  10. Facade: ExternalContext behaves as a facade for performing cookie, session scope and similar operations.
  11. Flyweight: Integer, Character, etc.
  12. Proxy: java.rmi package
  13. Chain of Responsibility: Servlet filters
  14. Command: Swing menu items
  15. Interpreter: No directly in JDK, but JavaCC certainly uses this.
  16. Iterator: java.util.Iterator interface; can't be clearer than that.
  17. Mediator: JMS?
  18. Memento:
  19. Observer: java.util.Observer/Observable (badly done, though)
  20. State:
  21. Strategy:
  22. Template:
  23. Visitor:

I can't think of examples in Java for 10 out of the 23, but I'll see if I can do better tomorrow. That's what edit is for.

Shashank Kadne
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duffymo
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The Abstract Factory pattern is used in various places. E.g., DatagramSocketImplFactory, PreferencesFactory. There are many more---search the Javadoc for interfaces which have the word "Factory" in their name.

Also there are quite a few instances of the Factory pattern, too.

tharindu_DG
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uckelman
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Even though I'm sort of a broken clock with this one, Java XML API uses Factory a lot. I mean just look at this:

Document doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(source);
String title = XPathFactory.newInstance().newXPath().evaluate("//title", doc);

...and so on and so forth.

Additionally various Buffers (StringBuffer, ByteBuffer, StringBuilder) use Builder.

Esko
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java.util.Collection#Iterator is a good example of a Factory Method. Depending on the concrete subclass of Collection you use, it will create an Iterator implementation. Because both the Factory superclass (Collection) and the Iterator created are interfaces, it is sometimes confused with AbstractFactory. Most of the examples for AbstractFactory in the the accepted answer (BalusC) are examples of Factory, a simplified version of Factory Method, which is not part of the original GoF patterns. In Facory the Factory class hierarchy is collapsed and the factory uses other means to choose the product to be returned.

  • Abstract Factory

An abstract factory has multiple factory methods, each creating a different product. The products produced by one factory are intended to be used together (your printer and cartridges better be from the same (abstract) factory). As mentioned in answers above the families of AWT GUI components, differing from platform to platform, are an example of this (although its implementation differs from the structure described in Gof).

Catweazle
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