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Monostate pattern iluwatar#85 
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‎monostate/etc/MonoState.ucls‎

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.1.8" icons="true" always-add-relationships="false" generalizations="true" realizations="true"
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associations="true" dependencies="false" nesting-relationships="true">
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<class id="1" language="java" name="com.iluwatar.monostate.LoadBalancer" project="monostate"
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file="/monostate/src/main/java/com/iluwatar/monostate/LoadBalancer.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="187" width="158" x="100" y="66"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="2" language="java" name="com.iluwatar.monostate.Server" project="monostate"
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file="/monostate/src/main/java/com/iluwatar/monostate/Server.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="176" y="372"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<class id="3" language="java" name="com.iluwatar.monostate.Request" project="monostate"
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file="/monostate/src/main/java/com/iluwatar/monostate/Request.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="329" y="352"/>
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<display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</display>
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</class>
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<dependency id="4">
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<end type="SOURCE" refId="1"/>
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<end type="TARGET" refId="3"/>
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</dependency>
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<dependency id="5">
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<end type="SOURCE" refId="2"/>
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<end type="TARGET" refId="3"/>
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</dependency>
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<association id="6">
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<end type="SOURCE" refId="1" navigable="false">
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<attribute id="7" name="servers"/>
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<multiplicity id="8" minimum="0" maximum="2147483647"/>
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</end>
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<end type="TARGET" refId="2" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true"
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sort-features="false" accessors="true" visibility="true">
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<attributes public="true" package="true" protected="true" private="true" static="true"/>
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<operations public="true" package="true" protected="true" private="true" static="true"/>
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</classifier-display>
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<association-display labels="true" multiplicity="true"/>
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</class-diagram>

‎monostate/etc/monostate.png‎

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‎monostate/index.md‎

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---
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layout: pattern
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title: MonoState
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folder: monostate
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permalink: /patterns/monostate/
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categories: Creational
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tags: Java
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---
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**Intent:** Enforces a behaviour like sharing the same state amongst all instances.
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![alt text](./etc/monostate.png "MonoState")
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**Applicability:** Use the Monostate pattern when
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* The same state must be shared across all instances of a class.
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* Typically this pattern might be used everywhere a SingleTon might be used. Singleton usage however is not transparent, Monostate usage is.
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* Monostate has one major advantage over singleton. The subclasses might decorate the shared state as they wish and hence can provide dynamically different behaviour than the base class.
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**Typical Use Case:**
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* the logging class
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* managing a connection to a database
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* file manager
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**Real world examples:**
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Yet to see this.

‎monostate/pom.xml‎

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<?xml version="1.0"?>
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<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.iluwatar</groupId>
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<artifactId>java-design-patterns</artifactId>
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<version>1.6.0</version>
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</parent>
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<artifactId>monostate</artifactId>
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<dependencies>
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<dependency>
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<groupId>junit</groupId>
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<artifactId>junit</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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</project>

‎monostate/src/main/java/com/iluwatar/monostate/App.java‎

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package com.iluwatar.monostate;
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public class App {
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public static void main(String[] args) {
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LoadBalancer loadBalancer1 = new LoadBalancer();
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LoadBalancer loadBalancer2 = new LoadBalancer();
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loadBalancer1.serverequest(new Request("Hello"));
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loadBalancer2.serverequest(new Request("Hello World"));
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}
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}

‎monostate/src/main/java/com/iluwatar/monostate/LoadBalancer.java‎

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package com.iluwatar.monostate;
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import java.util.ArrayList;
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import java.util.List;
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public class LoadBalancer {
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private static List<Server> servers = new ArrayList<>();
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private static int id = 0;
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private static int lastServedId = 0;
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static {
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servers.add(new Server("localhost", 8081, ++id));
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servers.add(new Server("localhost", 8080, ++id));
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servers.add(new Server("localhost", 8082, ++id));
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servers.add(new Server("localhost", 8083, ++id));
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servers.add(new Server("localhost", 8084, ++id));
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}
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public final void addServer(Server server) {
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synchronized (servers) {
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servers.add(server);
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}
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}
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public final int getNoOfServers() {
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return servers.size();
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}
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public void serverequest(Request request) {
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if (lastServedId >= servers.size()) {
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lastServedId = 0;
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}
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Server server = servers.get(lastServedId++);
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server.serve(request);
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}
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}

‎monostate/src/main/java/com/iluwatar/monostate/Request.java‎

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package com.iluwatar.monostate;
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public class Request {
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public final String value;
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public Request(String value) {
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super();
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this.value = value;
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}
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}

‎monostate/src/main/java/com/iluwatar/monostate/Server.java‎

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package com.iluwatar.monostate;
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public class Server {
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public final String host;
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public final int port;
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public final int id;
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public Server(String host, int port, int id) {
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this.host = host;
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this.port = port;
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this.id = id;
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}
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public String getHost() {
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return host;
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}
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public int getPort() {
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return port;
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}
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public final void serve(Request request) {
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System.out.println("Server ID "+id + " processed request with value "+request.value);
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}
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}

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