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eric-martineric-martin
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Merge pull request eugenp#5733 from cror/equals-hashcode
BAEL-2388: examples for equals and hashCode
2 parents 3d2f6ef + a67c10e commit 26a6cd6

8 files changed

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‎core-java-lang/pom.xml‎

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<artifactId>mail</artifactId>
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<version>${javax.mail.version}</version>
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</dependency>
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<dependency>
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<groupId>nl.jqno.equalsverifier</groupId>
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<artifactId>equalsverifier</artifactId>
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<version>${equalsverifier.version}</version>
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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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<maven-shade-plugin.version>3.1.1</maven-shade-plugin.version>
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<spring-boot-maven-plugin.version>2.0.3.RELEASE</spring-boot-maven-plugin.version>
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<exec-maven-plugin.version>1.6.0</exec-maven-plugin.version>
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<equalsverifier.version>3.0.3</equalsverifier.version>
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</properties>
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</project>

‎core-java-lang/src/main/java/com/baeldung/equalshashcode/Money.java‎

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package com.baeldung.equalshashcode;
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class Money {
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int amount;
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String currencyCode;
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Money(int amount, String currencyCode) {
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this.amount = amount;
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this.currencyCode = currencyCode;
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}
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@Override
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public boolean equals(Object o) {
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if (o == this)
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return true;
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if (!(o instanceof Money))
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return false;
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Money other = (Money)o;
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boolean currencyCodeEquals = (this.currencyCode == null && other.currencyCode == null)
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|| (this.currencyCode != null && this.currencyCode.equals(other.currencyCode));
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return this.amount == other.amount
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&& currencyCodeEquals;
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}
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@Override
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public int hashCode() {
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int result = 17;
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result = 31 * result + amount;
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if (currencyCode != null) {
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result = 31 * result + currencyCode.hashCode();
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}
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return result;
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}
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}

‎core-java-lang/src/main/java/com/baeldung/equalshashcode/Team.java‎

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package com.baeldung.equalshashcode;
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class Team {
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final String city;
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final String department;
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Team(String city, String department) {
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this.city = city;
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this.department = department;
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}
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@Override
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public final boolean equals(Object o) {
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if (o == this)
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return true;
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if (!(o instanceof Team))
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return false;
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Team otherTeam = (Team)o;
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boolean cityEquals = (this.city == null && otherTeam.city == null)
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|| this.city != null && this.city.equals(otherTeam.city);
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boolean departmentEquals = (this.department == null && otherTeam.department == null)
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|| this.department != null && this.department.equals(otherTeam.department);
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return cityEquals && departmentEquals;
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}
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@Override
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public final int hashCode() {
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int result = 17;
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if (city != null) {
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result = 31 * result + city.hashCode();
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}
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if (department != null) {
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result = 31 * result + department.hashCode();
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}
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return result;
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}
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}

‎core-java-lang/src/main/java/com/baeldung/equalshashcode/Voucher.java‎

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package com.baeldung.equalshashcode;
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class Voucher {
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private Money value;
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private String store;
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Voucher(int amount, String currencyCode, String store) {
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this.value = new Money(amount, currencyCode);
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this.store = store;
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}
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@Override
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public boolean equals(Object o) {
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if (o == this)
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return true;
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if (!(o instanceof Voucher))
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return false;
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Voucher other = (Voucher)o;
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boolean valueEquals = (this.value == null && other.value == null)
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|| (this.value != null && this.value.equals(other.value));
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boolean storeEquals = (this.store == null && other.store == null)
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|| (this.store != null && this.store.equals(other.store));
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return valueEquals && storeEquals;
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}
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@Override
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public int hashCode() {
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int result = 17;
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if (this.value != null) {
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result = 31 * result + value.hashCode();
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}
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if (this.store != null) {
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result = 31 * result + store.hashCode();
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}
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return result;
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}
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}

‎core-java-lang/src/main/java/com/baeldung/equalshashcode/WrongTeam.java‎

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package com.baeldung.equalshashcode;
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/* (non-Javadoc)
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* This class overrides equals, but it doesn't override hashCode.
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*
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* To see which problems this leads to:
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* TeamUnitTest.givenMapKeyWithoutHashCode_whenSearched_thenReturnsWrongValue
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*/
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class WrongTeam {
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String city;
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String department;
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WrongTeam(String city, String department) {
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this.city = city;
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this.department = department;
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}
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@Override
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public boolean equals(Object o) {
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if (o == this)
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return true;
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if (!(o instanceof WrongTeam))
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return false;
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WrongTeam otherTeam = (WrongTeam)o;
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return this.city == otherTeam.city
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&& this.department == otherTeam.department;
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}
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}

‎core-java-lang/src/main/java/com/baeldung/equalshashcode/WrongVoucher.java‎

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package com.baeldung.equalshashcode;
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/* (non-Javadoc)
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* This class extends the Money class that has overridden the equals method and once again overrides the equals method.
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*
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* To see which problems this leads to:
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* MoneyUnitTest.givenMoneyAndVoucherInstances_whenEquals_thenReturnValuesArentSymmetric
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*/
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class WrongVoucher extends Money {
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private String store;
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WrongVoucher(int amount, String currencyCode, String store) {
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super(amount, currencyCode);
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this.store = store;
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}
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@Override
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public boolean equals(Object o) {
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if (o == this)
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return true;
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if (!(o instanceof WrongVoucher))
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return false;
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WrongVoucher other = (WrongVoucher)o;
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boolean currencyCodeEquals = (this.currencyCode == null && other.currencyCode == null)
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|| (this.currencyCode != null && this.currencyCode.equals(other.currencyCode));
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boolean storeEquals = (this.store == null && other.store == null)
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|| (this.store != null && this.store.equals(other.store));
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return this.amount == other.amount
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&& currencyCodeEquals
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&& storeEquals;
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}
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@Override
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public int hashCode() {
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int result = 17;
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result = 31 * result + amount;
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if (this.currencyCode != null) {
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result = 31 * result + currencyCode.hashCode();
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}
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if (this.store != null) {
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result = 31 * result + store.hashCode();
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}
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return result;
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}
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}

‎core-java-lang/src/test/java/com/baeldung/equalshashcode/MoneyUnitTest.java‎

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package com.baeldung.equalshashcode;
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import static org.junit.Assert.assertTrue;
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import static org.junit.Assert.assertFalse;
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import org.junit.Test;
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public class MoneyUnitTest {
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@Test
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public void givenMoneyInstancesWithSameAmountAndCurrency_whenEquals_thenReturnsTrue() {
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Money income = new Money(55, "USD");
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Money expenses = new Money(55, "USD");
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assertTrue(income.equals(expenses));
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}
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@Test
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public void givenMoneyAndVoucherInstances_whenEquals_thenReturnValuesArentSymmetric() {
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Money cash = new Money(42, "USD");
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WrongVoucher voucher = new WrongVoucher(42, "USD", "Amazon");
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assertFalse(voucher.equals(cash));
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assertTrue(cash.equals(voucher));
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}
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}

‎core-java-lang/src/test/java/com/baeldung/equalshashcode/TeamUnitTest.java‎

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package com.baeldung.equalshashcode;
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import static org.junit.Assert.assertEquals;
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import static org.junit.Assert.assertFalse;
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import java.util.HashMap;
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import java.util.Map;
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import org.junit.Test;
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import nl.jqno.equalsverifier.EqualsVerifier;
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public class TeamUnitTest {
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@Test
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public void givenMapKeyWithHashCode_whenSearched_thenReturnsCorrectValue() {
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Map<Team,String> leaders = new HashMap<>();
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leaders.put(new Team("New York", "development"), "Anne");
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leaders.put(new Team("Boston", "development"), "Brian");
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leaders.put(new Team("Boston", "marketing"), "Charlie");
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Team myTeam = new Team("New York", "development");
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String myTeamleader = leaders.get(myTeam);
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assertEquals("Anne", myTeamleader);
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}
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@Test
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public void givenMapKeyWithoutHashCode_whenSearched_thenReturnsWrongValue() {
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Map<WrongTeam,String> leaders = new HashMap<>();
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leaders.put(new WrongTeam("New York", "development"), "Anne");
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leaders.put(new WrongTeam("Boston", "development"), "Brian");
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leaders.put(new WrongTeam("Boston", "marketing"), "Charlie");
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WrongTeam myTeam = new WrongTeam("New York", "development");
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String myTeamleader = leaders.get(myTeam);
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assertFalse("Anne".equals(myTeamleader));
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}
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@Test
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public void equalsContract() {
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EqualsVerifier.forClass(Team.class).verify();
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}
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}

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