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Nicolai Parlog
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[50] Create demo and tests for defensive copies
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‎README.md‎

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# Effective Java
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This project contains examples and benchmarks for my [YouTube series on _Effective Java, Third Edition_](https://www.youtube.com/playlist?list=PL_-IO8LOLuNqUzvXfRCWRRJBswKEbLhgN).
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This project contains examples, tests, and benchmarks for my [YouTube series on _Effective Java, Third Edition_](https://www.youtube.com/playlist?list=PL_-IO8LOLuNqUzvXfRCWRRJBswKEbLhgN).
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The tests can be run with `mvn test` or any IDE with JUnit 5 integration.
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To run the benchmarks:
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```
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* _Prefer dependency injection to hardwiring resources_
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* Item 6: _Avoid creating unnecessary objects_ -
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[benchmark](src/main/java/org/codefx/demo/effective_java/_06_unnecessary_objects/StringMatches.java)
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## Methods
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* Item 50: _Make defensive copies when needed_ -
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[example](src/main/java/org/codefx/demo/effective_java/_50_defensive_copies) and
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[tests](src/test/java/org/codefx/demo/effective_java/_50_defensive_copies/MegacorpInvarianceTests.java)

‎pom.xml‎

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<properties>
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<maven.compiler.source>10</maven.compiler.source>
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<maven.compiler.target>10</maven.compiler.target>
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<junit-jupiter-version>5.4.1</junit-jupiter-version>
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<jmh-version>1.21</jmh-version>
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<uberjar-name>benchmarks</uberjar-name>
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</properties>
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<version>25.0-jre</version>
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</dependency>
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<!-- tests -->
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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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<version>${junit-jupiter-version}</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-engine</artifactId>
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<version>${junit-jupiter-version}</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.assertj</groupId>
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<artifactId>assertj-core</artifactId>
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<version>3.10.0</version>
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<scope>test</scope>
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</dependency>
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<!-- jmh -->
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<dependency>
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<groupId>org.openjdk.jmh</groupId>

‎src/main/java/org/codefx/demo/effective_java/_50_defensive_copies/Megacorp.java‎

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package org.codefx.demo.effective_java._50_defensive_copies;
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import java.util.ArrayList;
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import java.util.List;
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import java.util.Objects;
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public class Megacorp {
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private final String name;
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private int totalRevenue;
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private final List<Subsidiary> subsidiaries;
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public Megacorp(String name, List<Subsidiary> subsidiaries) {
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this.name = name;
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this.totalRevenue = subsidiaries.stream()
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.mapToInt(Subsidiary::revenue)
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.sum();
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this.subsidiaries = new ArrayList<>(subsidiaries);
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if (this.subsidiaries.isEmpty())
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throw new IllegalArgumentException(name + " needs at least one subsidiary.");
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}
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public String name() {
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return name;
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}
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public int totalRevenue() {
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return totalRevenue;
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}
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public List<Subsidiary> subsidiaries() {
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return List.copyOf(subsidiaries);
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}
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public void acquire(Subsidiary subsidiary) {
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subsidiaries.add(subsidiary);
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totalRevenue += subsidiary.revenue();
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}
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@Override
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public boolean equals(Object o) {
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if (this == o)
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return true;
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if (o == null || getClass() != o.getClass())
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return false;
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Megacorp megacorp = (Megacorp) o;
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return Objects.equals(name, megacorp.name);
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}
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@Override
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public int hashCode() {
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return Objects.hash(name);
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}
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@Override
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public String toString() {
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return name + "(" + totalRevenue + " M€)";
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}
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}

‎src/main/java/org/codefx/demo/effective_java/_50_defensive_copies/Subsidiary.java‎

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package org.codefx.demo.effective_java._50_defensive_copies;
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import java.util.Objects;
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public class Subsidiary {
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private final String name;
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private final int revenue;
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public Subsidiary(String name, int revenue) {
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this.name = name;
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this.revenue = revenue;
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}
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public String name() {
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return name;
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}
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public int revenue() {
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return revenue;
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}
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@Override
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public boolean equals(Object o) {
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if (this == o)
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return true;
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if (o == null || getClass() != o.getClass())
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return false;
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Subsidiary that = (Subsidiary) o;
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return Objects.equals(name, that.name);
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}
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@Override
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public int hashCode() {
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return Objects.hash(name);
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}
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@Override
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public String toString() {
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return name + "(" + revenue + " M€)";
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}
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}

‎src/test/java/org/codefx/demo/effective_java/_50_defensive_copies/MegacorpInvarianceTests.java‎

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package org.codefx.demo.effective_java._50_defensive_copies;
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import org.junit.jupiter.api.Nested;
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import org.junit.jupiter.api.Test;
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import java.util.ArrayList;
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import java.util.ConcurrentModificationException;
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import java.util.List;
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import java.util.concurrent.CountDownLatch;
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import java.util.concurrent.ExecutorService;
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import java.util.concurrent.Executors;
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import java.util.concurrent.atomic.LongAdder;
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import static org.assertj.core.api.Assertions.assertThat;
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import static org.assertj.core.api.Assertions.assertThatThrownBy;
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class MegacorpInvarianceTests {
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private final List<Subsidiary> mutableSubsidiaries;
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private final Subsidiary orbitalDynamix = new Subsidiary("Orbital Dynamix", 693_264);
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MegacorpInvarianceTests() {
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mutableSubsidiaries = new ArrayList<>();
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mutableSubsidiaries.add(new Subsidiary("Arianespace", 1_023_684));
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mutableSubsidiaries.add(new Subsidiary("Krupp Group", 847_793));
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mutableSubsidiaries.add(new Subsidiary("Ruhr Nuclear", 295_203));
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}
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private static void assertReportedAndActualTotalRevenueEqual(Megacorp corp) {
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int actualTotalRevenue = corp.subsidiaries().stream()
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.mapToInt(Subsidiary::revenue)
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.sum();
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assertThat(corp.totalRevenue()).isEqualTo(actualTotalRevenue);
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}
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@Nested
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class EstablishingInvariants {
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@Test
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void nonEmptySubsidiaries() {
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assertThatThrownBy(() -> new Megacorp("Saeder-Krupp", new ArrayList<>()))
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.isInstanceOf(IllegalArgumentException.class);
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}
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@Test
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void revenueAfterConstruction() {
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var saederKrupp = new Megacorp("Saeder-Krupp", mutableSubsidiaries);
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assertReportedAndActualTotalRevenueEqual(saederKrupp);
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}
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@Test
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void revenueAfterAcquisition() {
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var saederKrupp = new Megacorp("Saeder-Krupp", mutableSubsidiaries);
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saederKrupp.acquire(orbitalDynamix);
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assertReportedAndActualTotalRevenueEqual(saederKrupp);
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}
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}
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@Nested
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class AttackingInvariants {
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@Test
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void makesDefensiveCopy() {
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var saederKrupp = new Megacorp("Saeder-Krupp", mutableSubsidiaries);
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mutableSubsidiaries.add(orbitalDynamix);
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assertReportedAndActualTotalRevenueEqual(saederKrupp);
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}
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@Test
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void copyBeforeTest() throws InterruptedException {
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var withoutSubsidiaries = new LongAdder();
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var tasks = Executors.newFixedThreadPool(2);
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for (int i = 0; i < 100; i++) {
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var subsidiaries = new ArrayList<Subsidiary>(mutableSubsidiaries);
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var taskLatch = new CountDownLatch(2);
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tasks.submit(() -> createMegacorpFromSubsidiaries(withoutSubsidiaries, subsidiaries, taskLatch));
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tasks.submit(() -> clearSubsidiaries(subsidiaries, taskLatch));
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taskLatch.await();
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}
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assertThat(withoutSubsidiaries.sum()).isZero();
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}
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private void clearSubsidiaries(ArrayList<Subsidiary> subsidiaries, CountDownLatch latch) {
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subsidiaries.clear();
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latch.countDown();
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}
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private void createMegacorpFromSubsidiaries(
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LongAdder withoutSubsidiaries, ArrayList<Subsidiary> subsidiaries, CountDownLatch latch) {
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try {
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var saederKrupp = new Megacorp("Saeder-Krupp", subsidiaries);
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if (saederKrupp.subsidiaries().isEmpty())
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// this should never happen!
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// ~> invariant broken!
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withoutSubsidiaries.increment();
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} catch (IllegalArgumentException ex) {
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// indicates that the construction failed because
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// the list of subsidiaries is empty
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// ~> invariant upheld
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} catch (RuntimeException ex) {
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// indicates that the stream pipeline in `Megacorp::new` failed
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// because of concurrent modification of the underlying collection
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// ~> invariant upheld
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}
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latch.countDown();
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}
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@Test
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void returnsDefensiveCopy() {
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try {
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var saederKrupp = new Megacorp("Saeder-Krupp", mutableSubsidiaries);
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saederKrupp.subsidiaries().add(orbitalDynamix);
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assertReportedAndActualTotalRevenueEqual(saederKrupp);
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} catch (RuntimeException ex) {
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// indicates that the returned collection was immutable
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// ~> invariant upheld
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}
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}
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}
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}

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