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Merge pull request #5733 from cror/equals-hashcode

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BAEL-2388: examples for equals and hashCode
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Eric Martin 2018-11-27 18:58:29 -06:00 committed by GitHub
commit 26a6cd6b94
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8 changed files with 270 additions and 0 deletions
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7
core-java-lang/pom.xml
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@ -66,6 +66,12 @@
<artifactId>mail</artifactId>
<version>${javax.mail.version}</version>
</dependency>
<dependency>
<groupId>nl.jqno.equalsverifier</groupId>
<artifactId>equalsverifier</artifactId>
<version>${equalsverifier.version}</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
@ -424,6 +430,7 @@
<maven-shade-plugin.version>3.1.1</maven-shade-plugin.version>
<spring-boot-maven-plugin.version>2.0.3.RELEASE</spring-boot-maven-plugin.version>
<exec-maven-plugin.version>1.6.0</exec-maven-plugin.version>
<equalsverifier.version>3.0.3</equalsverifier.version>
</properties>
</project>

36
core-java-lang/src/main/java/com/baeldung/equalshashcode/Money.java Normal file
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@ -0,0 +1,36 @@
package com.baeldung.equalshashcode;
class Money {
int amount;
String currencyCode;
Money(int amount, String currencyCode) {
this.amount = amount;
this.currencyCode = currencyCode;
}
@Override
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Money))
return false;
Money other = (Money)o;
boolean currencyCodeEquals = (this.currencyCode == null && other.currencyCode == null)
|| (this.currencyCode != null && this.currencyCode.equals(other.currencyCode));
return this.amount == other.amount
&& currencyCodeEquals;
}
@Override
public int hashCode() {
int result = 17;
result = 31 * result + amount;
if (currencyCode != null) {
result = 31 * result + currencyCode.hashCode();
}
return result;
}
}

39
core-java-lang/src/main/java/com/baeldung/equalshashcode/Team.java Normal file
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@ -0,0 +1,39 @@
package com.baeldung.equalshashcode;
class Team {
final String city;
final String department;
Team(String city, String department) {
this.city = city;
this.department = department;
}
@Override
public final boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Team))
return false;
Team otherTeam = (Team)o;
boolean cityEquals = (this.city == null && otherTeam.city == null)
|| this.city != null && this.city.equals(otherTeam.city);
boolean departmentEquals = (this.department == null && otherTeam.department == null)
|| this.department != null && this.department.equals(otherTeam.department);
return cityEquals && departmentEquals;
}
@Override
public final int hashCode() {
int result = 17;
if (city != null) {
result = 31 * result + city.hashCode();
}
if (department != null) {
result = 31 * result + department.hashCode();
}
return result;
}
}

38
core-java-lang/src/main/java/com/baeldung/equalshashcode/Voucher.java Normal file
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@ -0,0 +1,38 @@
package com.baeldung.equalshashcode;
class Voucher {
private Money value;
private String store;
Voucher(int amount, String currencyCode, String store) {
this.value = new Money(amount, currencyCode);
this.store = store;
}
@Override
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Voucher))
return false;
Voucher other = (Voucher)o;
boolean valueEquals = (this.value == null && other.value == null)
|| (this.value != null && this.value.equals(other.value));
boolean storeEquals = (this.store == null && other.store == null)
|| (this.store != null && this.store.equals(other.store));
return valueEquals && storeEquals;
}
@Override
public int hashCode() {
int result = 17;
if (this.value != null) {
result = 31 * result + value.hashCode();
}
if (this.store != null) {
result = 31 * result + store.hashCode();
}
return result;
}
}

30
core-java-lang/src/main/java/com/baeldung/equalshashcode/WrongTeam.java Normal file
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@ -0,0 +1,30 @@
package com.baeldung.equalshashcode;
/* (non-Javadoc)
* This class overrides equals, but it doesn't override hashCode.
*
* To see which problems this leads to:
* TeamUnitTest.givenMapKeyWithoutHashCode_whenSearched_thenReturnsWrongValue
*/
class WrongTeam {
String city;
String department;
WrongTeam(String city, String department) {
this.city = city;
this.department = department;
}
@Override
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof WrongTeam))
return false;
WrongTeam otherTeam = (WrongTeam)o;
return this.city == otherTeam.city
&& this.department == otherTeam.department;
}
}

47
core-java-lang/src/main/java/com/baeldung/equalshashcode/WrongVoucher.java Normal file
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@ -0,0 +1,47 @@
package com.baeldung.equalshashcode;
/* (non-Javadoc)
* This class extends the Money class that has overridden the equals method and once again overrides the equals method.
*
* To see which problems this leads to:
* MoneyUnitTest.givenMoneyAndVoucherInstances_whenEquals_thenReturnValuesArentSymmetric
*/
class WrongVoucher extends Money {
private String store;
WrongVoucher(int amount, String currencyCode, String store) {
super(amount, currencyCode);
this.store = store;
}
@Override
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof WrongVoucher))
return false;
WrongVoucher other = (WrongVoucher)o;
boolean currencyCodeEquals = (this.currencyCode == null && other.currencyCode == null)
|| (this.currencyCode != null && this.currencyCode.equals(other.currencyCode));
boolean storeEquals = (this.store == null && other.store == null)
|| (this.store != null && this.store.equals(other.store));
return this.amount == other.amount
&& currencyCodeEquals
&& storeEquals;
}
@Override
public int hashCode() {
int result = 17;
result = 31 * result + amount;
if (this.currencyCode != null) {
result = 31 * result + currencyCode.hashCode();
}
if (this.store != null) {
result = 31 * result + store.hashCode();
}
return result;
}
}

27
core-java-lang/src/test/java/com/baeldung/equalshashcode/MoneyUnitTest.java Normal file
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@ -0,0 +1,27 @@
package com.baeldung.equalshashcode;
import static org.junit.Assert.assertTrue;
import static org.junit.Assert.assertFalse;
import org.junit.Test;
public class MoneyUnitTest {
@Test
public void givenMoneyInstancesWithSameAmountAndCurrency_whenEquals_thenReturnsTrue() {
Money income = new Money(55, "USD");
Money expenses = new Money(55, "USD");
assertTrue(income.equals(expenses));
}
@Test
public void givenMoneyAndVoucherInstances_whenEquals_thenReturnValuesArentSymmetric() {
Money cash = new Money(42, "USD");
WrongVoucher voucher = new WrongVoucher(42, "USD", "Amazon");
assertFalse(voucher.equals(cash));
assertTrue(cash.equals(voucher));
}
}

46
core-java-lang/src/test/java/com/baeldung/equalshashcode/TeamUnitTest.java Normal file
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@ -0,0 +1,46 @@
package com.baeldung.equalshashcode;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;
import java.util.HashMap;
import java.util.Map;
import org.junit.Test;
import nl.jqno.equalsverifier.EqualsVerifier;
public class TeamUnitTest {
@Test
public void givenMapKeyWithHashCode_whenSearched_thenReturnsCorrectValue() {
Map<Team,String> leaders = new HashMap<>();
leaders.put(new Team("New York", "development"), "Anne");
leaders.put(new Team("Boston", "development"), "Brian");
leaders.put(new Team("Boston", "marketing"), "Charlie");
Team myTeam = new Team("New York", "development");
String myTeamleader = leaders.get(myTeam);
assertEquals("Anne", myTeamleader);
}
@Test
public void givenMapKeyWithoutHashCode_whenSearched_thenReturnsWrongValue() {
Map<WrongTeam,String> leaders = new HashMap<>();
leaders.put(new WrongTeam("New York", "development"), "Anne");
leaders.put(new WrongTeam("Boston", "development"), "Brian");
leaders.put(new WrongTeam("Boston", "marketing"), "Charlie");
WrongTeam myTeam = new WrongTeam("New York", "development");
String myTeamleader = leaders.get(myTeam);
assertFalse("Anne".equals(myTeamleader));
}
@Test
public void equalsContract() {
EqualsVerifier.forClass(Team.class).verify();
}
}