diff --git a/core-java-lang/pom.xml b/core-java-lang/pom.xml
index ace39de274..2f307859f1 100644
--- a/core-java-lang/pom.xml
+++ b/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>
diff --git a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/Money.java b/core-java-lang/src/main/java/com/baeldung/equalshashcode/Money.java
similarity index 60%
rename from java-collections-maps/src/main/java/com/baeldung/map/hashCode/Money.java
rename to core-java-lang/src/main/java/com/baeldung/equalshashcode/Money.java
index 0ba494d944..60c043545d 100644
--- a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/Money.java
+++ b/core-java-lang/src/main/java/com/baeldung/equalshashcode/Money.java
@@ -1,4 +1,4 @@
-package com.baeldung.map.hashcode;
+package com.baeldung.equalshashcode;
 
 class Money {
 
@@ -17,15 +17,19 @@ class Money {
         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
-          && this.currencyCode == other.currencyCode;
+          && currencyCodeEquals;
     }
 
     @Override
     public int hashCode() {
         int result = 17;
         result = 31 * result + amount;
-        result = 31 * result + currencyCode.hashCode();
+        if (currencyCode != null) {
+            result = 31 * result + currencyCode.hashCode();
+        }
         return result;
     }
 
diff --git a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/Team.java b/core-java-lang/src/main/java/com/baeldung/equalshashcode/Team.java
similarity index 96%
rename from java-collections-maps/src/main/java/com/baeldung/map/hashCode/Team.java
rename to core-java-lang/src/main/java/com/baeldung/equalshashcode/Team.java
index 6c1473b8e9..c2dee1de6b 100644
--- a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/Team.java
+++ b/core-java-lang/src/main/java/com/baeldung/equalshashcode/Team.java
@@ -1,4 +1,4 @@
-package com.baeldung.map.hashcode;
+package com.baeldung.equalshashcode;
 
 class Team {
 
diff --git a/core-java-lang/src/main/java/com/baeldung/equalshashcode/Voucher.java b/core-java-lang/src/main/java/com/baeldung/equalshashcode/Voucher.java
new file mode 100644
index 0000000000..19f46e0358
--- /dev/null
+++ b/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;
+    }
+}
\ No newline at end of file
diff --git a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/WrongTeam.java b/core-java-lang/src/main/java/com/baeldung/equalshashcode/WrongTeam.java
similarity index 67%
rename from java-collections-maps/src/main/java/com/baeldung/map/hashCode/WrongTeam.java
rename to core-java-lang/src/main/java/com/baeldung/equalshashcode/WrongTeam.java
index 57fb218ce0..c4477aa790 100644
--- a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/WrongTeam.java
+++ b/core-java-lang/src/main/java/com/baeldung/equalshashcode/WrongTeam.java
@@ -1,5 +1,11 @@
-package com.baeldung.map.hashcode;
+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;
diff --git a/core-java-lang/src/main/java/com/baeldung/equalshashcode/WrongVoucher.java b/core-java-lang/src/main/java/com/baeldung/equalshashcode/WrongVoucher.java
new file mode 100644
index 0000000000..97935bf8de
--- /dev/null
+++ b/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;
+    }
+}
\ No newline at end of file
diff --git a/java-collections-maps/src/test/java/com/baeldung/map/hashCode/MoneyUnitTest.java b/core-java-lang/src/test/java/com/baeldung/equalshashcode/MoneyUnitTest.java
similarity index 94%
rename from java-collections-maps/src/test/java/com/baeldung/map/hashCode/MoneyUnitTest.java
rename to core-java-lang/src/test/java/com/baeldung/equalshashcode/MoneyUnitTest.java
index f4cba4d114..60584fdb53 100644
--- a/java-collections-maps/src/test/java/com/baeldung/map/hashCode/MoneyUnitTest.java
+++ b/core-java-lang/src/test/java/com/baeldung/equalshashcode/MoneyUnitTest.java
@@ -1,4 +1,4 @@
-package com.baeldung.map.hashcode;
+package com.baeldung.equalshashcode;
 
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.assertFalse;
diff --git a/java-collections-maps/src/test/java/com/baeldung/map/hashCode/TeamUnitTest.java b/core-java-lang/src/test/java/com/baeldung/equalshashcode/TeamUnitTest.java
similarity index 97%
rename from java-collections-maps/src/test/java/com/baeldung/map/hashCode/TeamUnitTest.java
rename to core-java-lang/src/test/java/com/baeldung/equalshashcode/TeamUnitTest.java
index 89a7462438..a2de408796 100644
--- a/java-collections-maps/src/test/java/com/baeldung/map/hashCode/TeamUnitTest.java
+++ b/core-java-lang/src/test/java/com/baeldung/equalshashcode/TeamUnitTest.java
@@ -1,4 +1,4 @@
-package com.baeldung.map.hashcode;
+package com.baeldung.equalshashcode;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
diff --git a/java-collections-maps/pom.xml b/java-collections-maps/pom.xml
index 6af8f7a5c6..0803866c51 100644
--- a/java-collections-maps/pom.xml
+++ b/java-collections-maps/pom.xml
@@ -41,12 +41,6 @@
             <artifactId>streamex</artifactId>
             <version>0.6.5</version>
         </dependency>
-        <dependency>
-            <groupId>nl.jqno.equalsverifier</groupId>
-            <artifactId>equalsverifier</artifactId>
-            <version>3.0.3</version>
-            <scope>test</scope>
-        </dependency>
     </dependencies>
 
     <properties>
diff --git a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/Voucher.java b/java-collections-maps/src/main/java/com/baeldung/map/hashCode/Voucher.java
deleted file mode 100644
index b4536b38ea..0000000000
--- a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/Voucher.java
+++ /dev/null
@@ -1,31 +0,0 @@
-package com.baeldung.map.hashcode;
-
-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;
-        return this.value == other.value
-          && this.store == other.store;
-    }
-
-    @Override
-    public int hashCode() {
-        int result = 17;
-        result = 31 * result + value.hashCode();
-        result = 31 * result + store.hashCode();
-        return result;
-    }
-}
\ No newline at end of file
diff --git a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/WrongVoucher.java b/java-collections-maps/src/main/java/com/baeldung/map/hashCode/WrongVoucher.java
deleted file mode 100644
index 03122c7cc8..0000000000
--- a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/WrongVoucher.java
+++ /dev/null
@@ -1,33 +0,0 @@
-package com.baeldung.map.hashcode;
-
-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;
-        return this.amount == other.amount
-          && this.currencyCode == other.currencyCode
-          && this.store == other.store;
-    }
-
-    @Override
-    public int hashCode() {
-        int result = 17;
-        result = 31 * result + amount;
-        result = 31 * result + currencyCode.hashCode();
-        result = 31 * result + store.hashCode();
-        return result;
-    }
-}
\ No newline at end of file
diff --git a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/equalsverifier_notes.txt b/java-collections-maps/src/main/java/com/baeldung/map/hashCode/equalsverifier_notes.txt
deleted file mode 100644
index adc14f393a..0000000000
--- a/java-collections-maps/src/main/java/com/baeldung/map/hashCode/equalsverifier_notes.txt
+++ /dev/null
@@ -1 +0,0 @@
- Non-nullity: hashCode throws NullPointerException on field city.