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Merge pull request iluwatar#717 from waisuan/master
Dirty Flag pattern iluwatar#560
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‎dirty-flag/README.md‎

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---
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layout: pattern
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title: Dirty Flag
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folder: dirty-flag
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permalink: /patterns/dirty-flag/
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categories: Other
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tags:
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- Java
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- Difficulty-Easy
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- Performance
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---
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## Intent
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To avoid expensive re-acquisition of resources. The resources retain their identity, are kept in some
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fast-access storage, and are re-used to avoid having to acquire them again.
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![alt text](./etc/dirty-flag.png "Dirty Flag")
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## Applicability
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Use the Dirty Flag pattern when
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* Repetitious acquisition, initialization, and release of the same resource causes unnecessary performance overhead.
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## Credits
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* [Design Patterns: Dirty Flag](https://www.takeupcode.com/podcast/89-design-patterns-dirty-flag/)

‎dirty-flag/etc/dirty-flag.png‎

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‎dirty-flag/etc/dirty-flag.ucls‎

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.2.2" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
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realizations="true" associations="true" dependencies="false" nesting-relationships="true" router="FAN">
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<class id="1" language="java" name="com.iluwatar.dirtyflag.App" project="dirty-flag"
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file="/dirty-flag/src/main/java/com/iluwatar/dirtyflag/App.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="266" y="188"/>
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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.dirtyflag.DataFetcher" project="dirty-flag"
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file="/dirty-flag/src/main/java/com/iluwatar/dirtyflag/DataFetcher.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="153" width="125" x="66" y="291"/>
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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.dirtyflag.World" project="dirty-flag"
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file="/dirty-flag/src/main/java/com/iluwatar/dirtyflag/World.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="379" y="366"/>
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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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<association id="4">
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<end type="SOURCE" refId="3" navigable="false">
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<attribute id="5" name="df"/>
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<multiplicity id="6" minimum="0" maximum="1"/>
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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>

‎dirty-flag/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>1.20.0-SNAPSHOT</version>
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</parent>
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<groupId>com.iluwatar</groupId>
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<artifactId>dirty-flag</artifactId>
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<version>1.20.0-SNAPSHOT</version>
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<name>dirty-flag</name>
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<url>http://maven.apache.org</url>
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<properties>
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<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
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</properties>
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<dependencies>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-api</artifactId>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-engine</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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</project>

‎dirty-flag/src/main/java/com/iluwatar/dirtyflag/App.java‎

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/**
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* The MIT License
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* Copyright (c) 2014-2016 Ilkka Seppälä
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* <p>
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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* <p>
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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* <p>
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package com.iluwatar.dirtyflag;
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import java.util.List;
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import java.util.concurrent.Executors;
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import java.util.concurrent.ScheduledExecutorService;
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import java.util.concurrent.TimeUnit;
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/**
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*
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* This application demonstrates the <b>Dirty Flag</b> pattern. The dirty flag behavioral pattern allows you to avoid
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* expensive operations that would just need to be done again anyway. This is a simple pattern that really just explains
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* how to add a bool value to your class that you can set anytime a property changes. This will let your class know that
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* any results it may have previously calculated will need to be calculated again when they’re requested. Once the
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* results are re-calculated, then the bool value can be cleared.
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*
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* There are some points that need to be considered before diving into using this pattern:- there are some things you’ll
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* need to consider:- (1) Do you need it? This design pattern works well when the results to be calculated are difficult
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* or resource intensive to compute. You want to save them. You also don’t want to be calculating them several times in
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* a row when only the last one counts. (2) When do you set the dirty flag? Make sure that you set the dirty flag within
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* the class itself whenever an important property changes. This property should affect the result of the calculated
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* result and by changing the property, that makes the last result invalid. (3) When do you clear the dirty flag? It
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* might seem obvious that the dirty flag should be cleared whenever the result is calculated with up-to-date
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* information but there are other times when you might want to clear the flag.
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*
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* In this example, the {@link DataFetcher} holds the <i>dirty flag</i>. It fetches and re-fetches from <i>world.txt</i>
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* when needed. {@link World} mainly serves the data to the front-end.
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*/
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public class App {
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/**
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* Program execution point
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*/
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public void run() {
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final ScheduledExecutorService executorService = Executors.newSingleThreadScheduledExecutor();
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executorService.scheduleAtFixedRate(new Runnable() {
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@Override
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public void run() {
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World world = new World();
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List<String> countries = world.fetch();
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System.out.println("Our world currently has the following countries:-");
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for (String country : countries) {
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System.out.println("\t" + country);
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}
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}
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}, 0, 15, TimeUnit.SECONDS); // Run at every 15 seconds.
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}
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/**
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* Program entry point
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*
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* @param args
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* command line args
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*/
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public static void main(String[] args) {
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App app = new App();
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app.run();
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}
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}

‎dirty-flag/src/main/java/com/iluwatar/dirtyflag/DataFetcher.java‎

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package com.iluwatar.dirtyflag;
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import java.io.BufferedReader;
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import java.io.File;
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import java.io.FileReader;
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import java.io.IOException;
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import java.util.ArrayList;
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import java.util.List;
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/**
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* A mock database manager -- Fetches data from a raw file.
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*
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* @author swaisuan
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*
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*/
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public class DataFetcher {
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private final String filename = "world.txt";
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private long lastFetched;
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public DataFetcher() {
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this.lastFetched = -1;
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}
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private boolean isDirty(long fileLastModified) {
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if (lastFetched != fileLastModified) {
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lastFetched = fileLastModified;
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return true;
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}
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return false;
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}
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/**
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* Fetches data/content from raw file.
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*
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* @return List of strings
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*/
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public List<String> fetch() {
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ClassLoader classLoader = getClass().getClassLoader();
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File file = new File(classLoader.getResource(filename).getFile());
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if (isDirty(file.lastModified())) {
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System.out.println(filename + " is dirty! Re-fetching file content...");
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List<String> data = new ArrayList<String>();
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try (BufferedReader br = new BufferedReader(new FileReader(file))) {
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String line;
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while ((line = br.readLine()) != null) {
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data.add(line);
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}
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} catch (IOException e) {
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e.printStackTrace();
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}
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return data;
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}
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return new ArrayList<String>();
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}
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}

‎dirty-flag/src/main/java/com/iluwatar/dirtyflag/World.java‎

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package com.iluwatar.dirtyflag;
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import java.util.ArrayList;
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import java.util.List;
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/**
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*
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* A middle-layer app that calls/passes along data from the back-end.
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*
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* @author swaisuan
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*
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*/
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public class World {
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private List<String> countries;
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private DataFetcher df;
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public World() {
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this.countries = new ArrayList<String>();
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this.df = new DataFetcher();
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}
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/**
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*
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* Calls {@link DataFetcher} to fetch data from back-end.
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*
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* @return List of strings
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*/
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public List<String> fetch() {
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List<String> data = df.fetch();
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countries = data.isEmpty() ? countries : data;
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return countries;
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}
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}

‎dirty-flag/src/main/resources/world.txt‎

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UNITED_KINGDOM
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MALAYSIA
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UNITED_STATES

‎dirty-flag/src/test/java/org/dirty/flag/AppTest.java‎

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/**
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* The MIT License
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* Copyright (c) 2014-2016 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package org.dirty.flag;
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import java.io.IOException;
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import org.junit.jupiter.api.Test;
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import com.iluwatar.dirtyflag.App;
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/**
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* Tests that Dirty-Flag example runs without errors.
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*/
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public class AppTest {
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@Test
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public void test() throws IOException {
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String[] args = {};
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App.main(args);
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}
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}

‎dirty-flag/src/test/java/org/dirty/flag/DirtyFlagTest.java‎

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/**
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* The MIT License
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* Copyright (c) 2014-2016 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package org.dirty.flag;
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import static org.junit.jupiter.api.Assertions.assertTrue;
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import java.util.List;
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import org.junit.jupiter.api.Test;
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import com.iluwatar.dirtyflag.DataFetcher;
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/**
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*
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* Application test
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*
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*/
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public class DirtyFlagTest {
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@Test
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public void testIsDirty() {
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DataFetcher df = new DataFetcher();
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List<String> countries = df.fetch();
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assertTrue(!countries.isEmpty());
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}
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@Test
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public void testIsNotDirty() {
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DataFetcher df = new DataFetcher();
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df.fetch();
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List<String> countries = df.fetch();
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assertTrue(countries.isEmpty());
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}
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}

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