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Added Null Object pattern.
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-4
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9 files changed

+225
-4
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‎null-object/etc/test.png‎

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‎null-object/etc/test.ucls‎

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<?xml version="1.0" encoding="UTF-8"?>
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<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true"
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realizations="true" associations="true" dependencies="false" nesting-relationships="true">
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<interface id="1" language="java" name="com.iluwatar.Node" project="null-object"
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file="/null-object/src/main/java/com/iluwatar/Node.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="535" y="139"/>
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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="2" language="java" name="com.iluwatar.NodeImpl" project="null-object"
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file="/null-object/src/main/java/com/iluwatar/NodeImpl.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="857" y="348"/>
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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.NullNode" project="null-object"
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file="/null-object/src/main/java/com/iluwatar/NullNode.java" binary="false" corner="BOTTOM_RIGHT">
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<position height="-1" width="-1" x="535" y="408"/>
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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="2" navigable="false">
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<attribute id="5" name="left"/>
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<multiplicity id="6" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="1" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<association id="7">
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<end type="SOURCE" refId="2" navigable="false">
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<attribute id="8" name="right"/>
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<multiplicity id="9" minimum="0" maximum="1"/>
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</end>
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<end type="TARGET" refId="1" navigable="true"/>
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<display labels="true" multiplicity="true"/>
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</association>
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<realization id="10">
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<end type="SOURCE" refId="2"/>
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<end type="TARGET" refId="1"/>
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</realization>
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<realization id="11">
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<end type="SOURCE" refId="3"/>
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<end type="TARGET" refId="1"/>
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</realization>
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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>

‎null-object/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>1.0-SNAPSHOT</version>
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</parent>
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<artifactId>null-object</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>

‎null-object/src/main/java/com/iluwatar/App.java‎

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package com.iluwatar;
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/**
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*
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* Null Object pattern replaces null values with neutral objects.
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* Many times this simplifies algorithms since no extra null checks
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* are needed.
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*
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* In this example we build a binary tree where the nodes are either
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* normal or Null Objects. No null values are used in the tree making
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* the traversal easy.
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*
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*/
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public class App
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{
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public static void main( String[] args ) {
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Node root = new NodeImpl("1",
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new NodeImpl("11",
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new NodeImpl("111",
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new NullNode(),
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new NullNode()),
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new NullNode()),
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new NodeImpl("12",
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new NullNode(),
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new NodeImpl("122",
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new NullNode(),
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new NullNode())));
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root.walk();
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}
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}

‎null-object/src/main/java/com/iluwatar/Node.java‎

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package com.iluwatar;
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/**
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*
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* Interface for binary tree node.
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*
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*/
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public interface Node {
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String getName();
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int getTreeSize();
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Node getLeft();
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Node getRight();
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void walk();
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}

‎null-object/src/main/java/com/iluwatar/NodeImpl.java‎

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package com.iluwatar;
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/**
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*
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* Implementation for binary tree's normal nodes.
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*
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*/
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public class NodeImpl implements Node {
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private final String name;
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private final Node left;
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private final Node right;
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public NodeImpl(String name, Node left, Node right) {
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this.name = name;
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this.left = left;
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this.right = right;
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}
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@Override
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public int getTreeSize() {
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return 1 + left.getTreeSize() + right.getTreeSize();
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}
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@Override
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public Node getLeft() {
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return left;
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}
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@Override
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public Node getRight() {
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return right;
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}
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@Override
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public String getName() {
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return name;
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}
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@Override
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public void walk() {
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System.out.println(name);
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if (left.getTreeSize() > 0) {
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left.walk();
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}
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if (right.getTreeSize() > 0) {
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right.walk();
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}
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}
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}

‎null-object/src/main/java/com/iluwatar/NullNode.java‎

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package com.iluwatar;
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/**
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*
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* Null Object implementation for binary tree node.
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*
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*/
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public class NullNode implements Node {
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@Override
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public int getTreeSize() {
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return 0;
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}
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@Override
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public Node getLeft() {
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return null;
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}
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@Override
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public Node getRight() {
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return null;
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}
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@Override
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public String getName() {
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return null;
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}
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@Override
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public void walk() {
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}
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}

‎null-object/src/test/java/com/iluwatar/AppTest.java‎

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package com.iluwatar;
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import org.junit.Test;
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public class AppTest {
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@Test
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public void test() {
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String[] args = {};
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App.main(args);
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}
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}

‎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" 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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<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<groupId>com.iluwatar</groupId>
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<module>double-checked-locking</module>
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<module>servant</module>
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<module>service-locator</module>
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</modules>
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<module>null-object</module>
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</modules>
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<dependencyManagement>
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<dependencies>
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</plugins>
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</build>
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</project>
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</project>

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