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Merge pull request iluwatar#573 from SrdjanPaunovic/extension-objects
iluwatar#541 Extension objects pattern
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‎extension-objects/README.md

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---
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layout: pattern
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title: Extension objects
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folder: extension-objects
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permalink: /patterns/extension-objects/
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categories: Behavioral
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tags:
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- Java
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- Difficulty-Intermediate
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---
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## Intent
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Anticipate that an object’s interface needs to be extended in the future. Additional
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interfaces are defined by extension objects.
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![Extension_objects](./etc/extension_obj.png "Extension objects")
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## Applicability
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Use the Extension Objects pattern when:
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* you need to support the addition of new or unforeseen interfaces to existing classes and you don't want to impact clients that don't need this new interface. Extension Objects lets you keep related operations together by defining them in a separate class
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* a class representing a key abstraction plays different roles for different clients. The number of roles the class can play should be open-ended. There is a need to preserve the key abstraction itself. For example, a customer object is still a customer object even if different subsystems view it differently.
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* a class should be extensible with new behavior without subclassing from it.
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## Real world examples
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* [OpenDoc](https://en.wikipedia.org/wiki/OpenDoc)
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* [Object Linking and Embedding](https://en.wikipedia.org/wiki/Object_Linking_and_Embedding)

‎extension-objects/etc/extension_obj.png

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‎extension-objects/etc/extension_obj.ucls

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.2.0" 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="concreteextensions.Soldier" project="extension-objects"
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file="/extension-objects/src/main/java/concreteextensions/Soldier.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="483" y="339"/>
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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="units.Unit" project="extension-objects"
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file="/extension-objects/src/main/java/units/Unit.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="192" y="115"/>
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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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<interface id="3" language="java" name="abstractextensions.SoldierExtension" project="extension-objects"
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file="/extension-objects/src/main/java/abstractextensions/SoldierExtension.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="510" y="229"/>
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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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</interface>
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<interface id="4" language="java" name="abstractextensions.UnitExtension" project="extension-objects"
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file="/extension-objects/src/main/java/abstractextensions/UnitExtension.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="510" y="116"/>
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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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</interface>
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<class id="5" language="java" name="units.SoldierUnit" project="extension-objects"
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file="/extension-objects/src/main/java/units/SoldierUnit.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="157" y="355"/>
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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="6" language="java" name="concreteextensions.Sergeant" project="extension-objects"
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file="/extension-objects/src/main/java/concreteextensions/Sergeant.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="650" y="375"/>
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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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<interface id="7" language="java" name="abstractextensions.SergeantExtension" project="extension-objects"
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file="/extension-objects/src/main/java/abstractextensions/SergeantExtension.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="672" y="230"/>
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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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</interface>
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<class id="8" language="java" name="units.SergeantUnit" project="extension-objects"
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file="/extension-objects/src/main/java/units/SergeantUnit.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="315" y="460"/>
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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="9" language="java" name="units.CommanderUnit" project="extension-objects"
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file="/extension-objects/src/main/java/units/CommanderUnit.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="99" width="210" x="429" y="476"/>
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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="10" language="java" name="concreteextensions.Commander" project="extension-objects"
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file="/extension-objects/src/main/java/concreteextensions/Commander.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="823" y="477"/>
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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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<interface id="11" language="java" name="abstractextensions.CommanderExtension" project="extension-objects"
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file="/extension-objects/src/main/java/abstractextensions/CommanderExtension.java" binary="false"
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corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="827" y="217"/>
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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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</interface>
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<realization id="12">
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<end type="SOURCE" refId="1"/>
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<end type="TARGET" refId="3"/>
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</realization>
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<realization id="13">
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<end type="SOURCE" refId="10"/>
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<end type="TARGET" refId="11"/>
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</realization>
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<generalization id="14">
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<end type="SOURCE" refId="9"/>
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<end type="TARGET" refId="2"/>
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</generalization>
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<association id="15">
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<end type="SOURCE" refId="1" navigable="false">
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<attribute id="16" name="unit"/>
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<multiplicity id="17" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="5" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="18">
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<end type="SOURCE" refId="10" navigable="false">
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<attribute id="19" name="unit"/>
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<multiplicity id="20" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="9" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="21">
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<end type="SOURCE" refId="2" navigable="false">
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<attribute id="22" name="unitExtension"/>
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<multiplicity id="23" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="4" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<realization id="24">
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<end type="SOURCE" refId="6"/>
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<end type="TARGET" refId="7"/>
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</realization>
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<generalization id="25">
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<end type="SOURCE" refId="3"/>
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<end type="TARGET" refId="4"/>
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</generalization>
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<generalization id="26">
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<end type="SOURCE" refId="5"/>
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<end type="TARGET" refId="2"/>
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</generalization>
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<generalization id="27">
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<end type="SOURCE" refId="7"/>
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<end type="TARGET" refId="4"/>
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</generalization>
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<generalization id="28">
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<end type="SOURCE" refId="8"/>
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<end type="TARGET" refId="2"/>
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</generalization>
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<association id="29">
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<end type="SOURCE" refId="6" navigable="false">
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<attribute id="30" name="unit"/>
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<multiplicity id="31" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="8" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<generalization id="32">
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<end type="SOURCE" refId="11"/>
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<end type="TARGET" refId="4"/>
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</generalization>
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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>

‎extension-objects/pom.xml

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<?xml version="1.0" encoding="UTF-8"?>
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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>1.16.0-SNAPSHOT</version>
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</parent>
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<modelVersion>4.0.0</modelVersion>
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<artifactId>extension-objects</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>

‎extension-objects/src/main/java/App.java

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import abstractextensions.CommanderExtension;
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import abstractextensions.SergeantExtension;
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import abstractextensions.SoldierExtension;
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import units.CommanderUnit;
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import units.SergeantUnit;
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import units.SoldierUnit;
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import units.Unit;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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/**
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* Anticipate that an object’s interface needs to be extended in the future.
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* Additional interfaces are defined by extension objects.
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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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*
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* @param args command line args
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*/
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public static void main(String[] args) {
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//Create 3 different units
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Unit soldierUnit = new SoldierUnit("SoldierUnit1");
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Unit sergeantUnit = new SergeantUnit("SergeantUnit1");
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Unit commanderUnit = new CommanderUnit("CommanderUnit1");
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//check for each unit to have an extension
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checkExtensionsForUnit(soldierUnit);
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checkExtensionsForUnit(sergeantUnit);
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checkExtensionsForUnit(commanderUnit);
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}
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private static void checkExtensionsForUnit(Unit unit) {
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final Logger logger = LoggerFactory.getLogger(App.class);
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SoldierExtension soldierExtension = (SoldierExtension) unit.getUnitExtension("SoldierExtension");
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SergeantExtension sergeantExtension = (SergeantExtension) unit.getUnitExtension("SergeantExtension");
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CommanderExtension commanderExtension = (CommanderExtension) unit.getUnitExtension("CommanderExtension");
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//if unit have extension call the method
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if (soldierExtension != null) {
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soldierExtension.soldierReady();
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} else {
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logger.info(unit.getName() + " without SoldierExtension");
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}
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if (sergeantExtension != null) {
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sergeantExtension.sergeantReady();
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} else {
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logger.info(unit.getName() + " without SergeantExtension");
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}
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if (commanderExtension != null) {
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commanderExtension.commanderReady();
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} else {
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logger.info(unit.getName() + " without CommanderExtension");
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}
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}
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}

‎extension-objects/src/main/java/abstractextensions/CommanderExtension.java

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package abstractextensions;
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/**
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* Interface with their method
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*/
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public interface CommanderExtension extends UnitExtension {
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void commanderReady();
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}

‎extension-objects/src/main/java/abstractextensions/SergeantExtension.java

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package abstractextensions;
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/**
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* Interface with their method
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*/
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public interface SergeantExtension extends UnitExtension {
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void sergeantReady();
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}

‎extension-objects/src/main/java/abstractextensions/SoldierExtension.java

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package abstractextensions;
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/**
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* Interface with their method
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*/
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public interface SoldierExtension extends UnitExtension {
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void soldierReady();
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}

‎extension-objects/src/main/java/abstractextensions/UnitExtension.java

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package abstractextensions;
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/**
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* Other Extensions will extend this interface
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*/
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public interface UnitExtension {
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}

‎extension-objects/src/main/java/concreteextensions/Commander.java

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package concreteextensions;
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import abstractextensions.CommanderExtension;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import units.CommanderUnit;
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/**
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* Class defining Commander
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*/
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public class Commander implements CommanderExtension {
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private CommanderUnit unit;
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public Commander(CommanderUnit commanderUnit) {
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this.unit = commanderUnit;
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}
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final Logger logger = LoggerFactory.getLogger(Commander.class);
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@Override
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public void commanderReady() {
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logger.info("[Commander] " + unit.getName() + " is ready!");
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}
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}

‎extension-objects/src/main/java/concreteextensions/Sergeant.java

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package concreteextensions;
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import abstractextensions.SergeantExtension;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import units.SergeantUnit;
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/**
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* Class defining Sergeant
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*/
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public class Sergeant implements SergeantExtension {
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private SergeantUnit unit;
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public Sergeant(SergeantUnit sergeantUnit) {
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this.unit = sergeantUnit;
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}
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final Logger logger = LoggerFactory.getLogger(Sergeant.class);
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@Override
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public void sergeantReady() {
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logger.info("[Sergeant] " + unit.getName() + " is ready! ");
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}
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}

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