Merge pull request #5733 from cror/equals-hashcode

BAEL-2388: examples for equals and hashCode

core-java-lang/pom.xml

@@ -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>

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

@@ -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;
+    }
+
+}

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

@@ -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;
+    }
+
+}

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

@@ -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;
+    }
+}

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

@@ -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;
+    }
+
+}

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

@@ -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;
+    }
+}

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

@@ -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));
+    }
+
+}

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

@@ -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();
+    }
+
+}