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* Add simple implementation for strangler pattern. * Add strangler pattern in pom.xml. * change package name * Revert "change package name" This reverts commit 430bd90. * Code review for strangler Delete final of method parameters. Add final to private members. Change package name. * Revert "Code review for strangler" This reverts commit d506356. * Revert "Revert "Code review for strangler"" This reverts commit c8fd65f. * Remove unnecessary files
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‎pom.xml‎

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<module>combinator</module>
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<module>update-method</module>
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<module>leader-followers</module>
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<module>strangler</module>
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<module>arrange-act-assert</module>
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</modules>
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‎strangler/README.md‎

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---
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layout: pattern
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title: Strangler
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folder: strangler
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permalink: /patterns/strangler/
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categories: Structural
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tags:
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- Extensibility
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---
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## Intent
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Incrementally migrate a legacy system by gradually replacing specific pieces of functionality
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with new applications and services. As features from the legacy system are replaced, the new
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system eventually covers all the old system's features and may has its own new features, then
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strangling the old system and allowing you to decommission it.
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## Class diagram
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![alt text](./etc/strangler.png "Strangler")
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## Applicability
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This strangler pattern is a safe way to phase one thing out for something better, cheaper, or
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more expandable. Especially when you want to update legacy system with new techniques and need
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continuously develop new features at the same time. Note that this pattern indeed need extra effort,
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so usually use it when the system is not so simple.
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## Credits
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* [Strangler pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/strangler#context-and-problem)
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* [Legacy Application Strangulation : Case Studies](https://paulhammant.com/2013/07/14/legacy-application-strangulation-case-studies/)
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‎strangler/etc/strangler.png‎

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‎strangler/etc/strangler.puml‎

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@startuml
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package com.iluwatar.strangler {
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class App {
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+ main(args : String[]) {static}
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}
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class OldArithmetic {
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- LOGGER : Logger {static}
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- VERSION : String {static}
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- source : OldSource
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+ sum(nums : int...)
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+ mul(nums : int...)
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}
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class HalfArithmetic {
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- LOGGER : Logger {static}
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- VERSION : String {static}
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- oldSource : OldSource
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- newSource : HalfSource
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+ sum(nums : int...)
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+ mul(nums : int...)
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+ ifHasZero(nums : int...)
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}
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class NewArithmetic {
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- LOGGER : Logger {static}
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- VERSION : String {static}
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- source : NewSource
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+ sum(nums : int...)
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+ mul(nums : int...)
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+ ifHasZero(nums : int...)
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}
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class OldSource {
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- LOGGER : Logger {static}
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- VERSION : String {static}
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+ accumulateSum(nums : int...)
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+ accumulateMul(nums : int...)
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}
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class HalfSource {
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- LOGGER : Logger {static}
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- VERSION : String {static}
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+ accumulateSum(nums : int...)
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+ ifNonZero(nums : int...)
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}
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class NewSource {
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- LOGGER : Logger {static}
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- VERSION : String {static}
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+ accumulateSum(nums : int...)
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+ accumulateMul(nums : int...)
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+ ifNonZero(nums : int...)
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}
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}
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OldArithmetic o--> OldSource
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HalfArithmetic o--> OldSource
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HalfArithmetic o--> HalfSource
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NewArithmetic o--> NewSource
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@enduml

‎strangler/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<!--
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The MIT License
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Copyright © 2014-2019 Ilkka Seppälä
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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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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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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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<project 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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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<parent>
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<artifactId>java-design-patterns</artifactId>
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<groupId>com.iluwatar</groupId>
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<version>1.23.0-SNAPSHOT</version>
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</parent>
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<modelVersion>4.0.0</modelVersion>
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<artifactId>strangler</artifactId>
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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-engine</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-assembly-plugin</artifactId>
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<executions>
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<execution>
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<configuration>
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<archive>
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<manifest>
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<mainClass>com.iluwatar.strangler.App</mainClass>
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</manifest>
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</archive>
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</configuration>
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</execution>
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</executions>
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</plugin>
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</plugins>
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</build>
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</project>

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

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/*
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* The MIT License
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* Copyright © 2014-2019 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 com.iluwatar.strangler;
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/**
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*
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* <p>The Strangler pattern is a software design pattern that incrementally migrate a legacy
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* system by gradually replacing specific pieces of functionality with new applications and
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* services. As features from the legacy system are replaced, the new system eventually
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* replaces all of the old system's features, strangling the old system and allowing you
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* to decommission it.</p>
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*
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* <p>This pattern is not only about updating but also enhancement.</p>
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*
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* <p>In this example, {@link OldArithmetic} indicates old system and its implementation depends
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* on its source ({@link OldSource}). Now we tend to update system with new techniques and
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* new features. In reality, the system may too complex, so usually need gradual migration.
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* {@link HalfArithmetic} indicates system in the process of migration, its implementation
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* depends on old one ({@link OldSource}) and under development one ({@link HalfSource}). The
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* {@link HalfSource} covers part of {@link OldSource} and add new functionality. You can release
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* this version system with new features, which also supports old version system functionalities.
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* After whole migration, the new system ({@link NewArithmetic}) only depends on new source
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* ({@link NewSource}).</p>
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*
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*/
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public class App {
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/**
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* Program entry point.
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* @param args command line args
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*/
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public static void main(final String[] args) {
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final var nums = new int[]{1, 2, 3, 4, 5};
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//Before migration
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final var oldSystem = new OldArithmetic(new OldSource());
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oldSystem.sum(nums);
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oldSystem.mul(nums);
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//In process of migration
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final var halfSystem = new HalfArithmetic(new HalfSource(), new OldSource());
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halfSystem.sum(nums);
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halfSystem.mul(nums);
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halfSystem.ifHasZero(nums);
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//After migration
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final var newSystem = new NewArithmetic(new NewSource());
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newSystem.sum(nums);
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newSystem.mul(nums);
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newSystem.ifHasZero(nums);
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}
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}

‎strangler/src/main/java/com/iluwatar/strangler/HalfArithmetic.java‎

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/*
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* The MIT License
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* Copyright © 2014-2019 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 com.iluwatar.strangler;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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/**
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* System under migration. Depends on old version source ({@link OldSource}) and
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* developing one ({@link HalfSource}).
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*/
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public class HalfArithmetic {
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private static final Logger LOGGER = LoggerFactory.getLogger(HalfArithmetic.class);
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private static final String VERSION = "1.5";
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private final HalfSource newSource;
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private final OldSource oldSource;
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public HalfArithmetic(HalfSource newSource, OldSource oldSource) {
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this.newSource = newSource;
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this.oldSource = oldSource;
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}
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/**
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* Accumulate sum.
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* @param nums numbers need to add together
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* @return accumulate sum
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*/
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public int sum(int... nums) {
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LOGGER.info("Arithmetic sum {}", VERSION);
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return newSource.accumulateSum(nums);
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}
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/**
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* Accumulate multiplication.
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* @param nums numbers need to multiply together
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* @return accumulate multiplication
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*/
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public int mul(int... nums) {
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LOGGER.info("Arithmetic mul {}", VERSION);
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return oldSource.accumulateMul(nums);
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}
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/**
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* Chech if has any zero.
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* @param nums numbers need to check
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* @return if has any zero, return true, else, return false
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*/
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public boolean ifHasZero(int... nums) {
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LOGGER.info("Arithmetic check zero {}", VERSION);
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return !newSource.ifNonZero(nums);
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}
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}

‎strangler/src/main/java/com/iluwatar/strangler/HalfSource.java‎

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/*
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* The MIT License
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* Copyright © 2014-2019 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 com.iluwatar.strangler;
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import java.util.Arrays;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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/**
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* Source under development. Replace part of old source and has added some new features.
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*/
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public class HalfSource {
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private static final Logger LOGGER = LoggerFactory.getLogger(HalfSource.class);
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private static final String VERSION = "1.5";
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/**
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* Implement accumulate sum with new technique.
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* Replace old one in {@link OldSource}
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*/
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public int accumulateSum(int... nums) {
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LOGGER.info("Source module {}", VERSION);
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return Arrays.stream(nums).reduce(0, Integer::sum);
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}
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/**
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* Check if all number is not zero.
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* New feature.
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*/
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public boolean ifNonZero(int... nums) {
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LOGGER.info("Source module {}", VERSION);
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return Arrays.stream(nums).allMatch(num -> num != 0);
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}
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}

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