120 lines
6.1 KiB
Markdown
120 lines
6.1 KiB
Markdown
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# Design pattern samples in Java
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[](https://gitter.im/iluwatar/java-design-patterns?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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[](https://travis-ci.org/iluwatar/java-design-patterns) [](https://coveralls.io/r/iluwatar/java-design-patterns?branch=master) <a href="https://scan.coverity.com/projects/5634">
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<img alt="Coverity Scan Build Status"
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src="https://scan.coverity.com/projects/5634/badge.svg"/>
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</a>
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<a name="top"/>
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# <a name="toc">Table of Contents</a>
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- <a href="#introduction">Introduction</a>
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- <a href="#contribute">How to contribute</a>
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- <a href="#faq">Frequently Asked Questions</a>
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- <a href="#credits">Credits</a>
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- <a href="#license">License</a>
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# <a name="introduction">Introduction</a> [↑](#top)
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Design patterns are formalized best practices that the programmer can use to
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solve common problems when designing an application or system.
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Design patterns can speed up the development process by providing tested, proven
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development paradigms.
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Reusing design patterns helps to prevent subtle issues that can cause major
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problems, and it also improves code readability for coders and architects who
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are familiar with the patterns.
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# <a name="contribute">How to contribute</a> [↑](#top)
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If you are willing to contribute to the project you will find the relevant information in our [developer wiki](https://github.com/iluwatar/java-design-patterns/wiki).
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# <a name="faq">Frequently asked questions</a> [↑](#top)
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**<a id="Q1">Q: What is the difference between State and Strategy patterns?</a>**
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While the implementation is similar they solve different problems. The State
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pattern deals with what state an object is in - it encapsulates state-dependent
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behavior.
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The Strategy pattern deals with how an object performs a certain task - it
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encapsulates an algorithm.
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**<a id="Q2">Q: What is the difference between Strategy and Template Method patterns?</a>**
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In Template Method the algorithm is chosen at compile time via inheritance.
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With Strategy pattern the algorithm is chosen at runtime via composition.
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**<a id="Q3">Q: What is the difference between Proxy and Decorator patterns?</a>**
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The difference is the intent of the patterns. While Proxy controls access to
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the object Decorator is used to add responsibilities to the object.
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**<a id="Q4">Q: What is the difference between Chain of Responsibility and Intercepting Filter patterns?</a>**
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While the implementations look similar there are differences. The Chain of
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Responsibility forms a chain of request processors and the processors are then
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executed one by one until the correct processor is found. In Intercepting
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Filter the chain is constructed from filters and the whole chain is always
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executed.
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**<a id="Q5">Q: What is the difference between Visitor and Double Dispatch patterns?</a>**
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The Visitor pattern is a means of adding a new operation to existing classes.
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Double dispatch is a means of dispatching function calls with respect to two
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polymorphic types, rather than a single polymorphic type, which is what
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languages like C++ and Java _do not_ support directly.
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**<a id="Q6">Q: What are the differences between Flyweight and Object Pool patterns?</a>**
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They differ in the way they are used.
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Pooled objects can simultaneously be used by a single "client" only. For that,
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a pooled object must be checked out from the pool, then it can be used by a
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client, and then the client must return the object back to the pool. Multiple
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instances of identical objects may exist, up to the maximal capacity of the
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pool.
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In contrast, a Flyweight object is singleton, and it can be used simultaneously
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by multiple clients.
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As for concurrent access, pooled objects can be mutable and they usually don't
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need to be thread safe, as typically, only one thread is going to use a
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specific instance at the same time. Flyweight must either be immutable (the
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best option), or implement thread safety.
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As for performance and scalability, pools can become bottlenecks, if all the
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pooled objects are in use and more clients need them, threads will become
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blocked waiting for available object from the pool. This is not the case with
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Flyweight.
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# <a name="credits">Credits</a> [↑](#top)
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* [Design Patterns: Elements of Reusable Object-Oriented Software](http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612)
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* [Effective Java (2nd Edition)](http://www.amazon.com/Effective-Java-Edition-Joshua-Bloch/dp/0321356683)
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* [Java Generics and Collections](http://www.amazon.com/Java-Generics-Collections-Maurice-Naftalin/dp/0596527756/)
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* [Let's Modify the Objects-First Approach into Design-Patterns-First](http://edu.pecinovsky.cz/papers/2006_ITiCSE_Design_Patterns_First.pdf)
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* [Pattern Languages of Program Design](http://www.amazon.com/Pattern-Languages-Program-Design-Coplien/dp/0201607344/ref=sr_1_1)
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* [Presentation Tier Patterns](http://www.javagyan.com/tutorials/corej2eepatterns/presentation-tier-patterns)
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* [Functional Programming in Java: Harnessing the Power of Java 8 Lambda Expressions](http://www.amazon.com/Functional-Programming-Java-Harnessing-Expressions/dp/1937785467/ref=sr_1_1)
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* [Patterns of Enterprise Application Architecture](http://www.amazon.com/Patterns-Enterprise-Application-Architecture-Martin/dp/0321127420)
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* [Spring Data](http://www.amazon.com/Spring-Data-Mark-Pollack/dp/1449323952/ref=sr_1_1)
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* [J2EE Design Patterns](http://www.amazon.com/J2EE-Design-Patterns-William-Crawford/dp/0596004273/ref=sr_1_2)
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* [Pattern Oriented Software Architecture Vol I-V](http://www.amazon.com/Pattern-Oriented-Software-Architecture-Volume-Patterns/dp/0471958697)
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# <a name="license">License</a> [↑](#top)
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This project is licensed under the terms of the MIT license.
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