diff --git a/core-java-modules/core-java-collections-maps-4/README.md b/core-java-modules/core-java-collections-maps-4/README.md
new file mode 100644
index 0000000000..4449d3859b
--- /dev/null
+++ b/core-java-modules/core-java-collections-maps-4/README.md
@@ -0,0 +1,6 @@
+## Java Collections Cookbooks and Examples
+
+This module contains articles about Map data structures in Java.
+
+### Relevant Articles: 
+- [Using a Custom Class as a Key in a Java HashMap](https://www.baeldung.com/custom-key-hashmap)
diff --git a/core-java-modules/core-java-collections-maps-4/pom.xml b/core-java-modules/core-java-collections-maps-4/pom.xml
new file mode 100644
index 0000000000..1835e3ceac
--- /dev/null
+++ b/core-java-modules/core-java-collections-maps-4/pom.xml
@@ -0,0 +1,34 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-collections-maps-4</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-collections-maps-4</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../pom.xml</relativePath>
+    </parent>
+
+    <build>
+        <pluginManagement>
+            <plugins>
+                <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
+                    <artifactId>maven-surefire-plugin</artifactId>
+                    <version>3.0.0-M5</version>
+                    <configuration>
+                        <!-- those tests are not made for CI purposes, but for manual testing -->
+                        <groups>!performance</groups>
+                    </configuration>
+                </plugin>
+            </plugins>
+        </pluginManagement>
+    </build>
+
+</project>
diff --git a/core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/maps/CoordinateKey.java b/core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/maps/CoordinateKey.java
new file mode 100644
index 0000000000..34ae3d7952
--- /dev/null
+++ b/core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/maps/CoordinateKey.java
@@ -0,0 +1,39 @@
+package com.baeldung.maps;
+
+import java.util.Objects;
+
+public class CoordinateKey {
+
+    private final int x;
+    private final int y;
+    private final int hashCode;
+
+    public CoordinateKey(int x, int y) {
+        this.x = x;
+        this.y = y;
+        this.hashCode = Objects.hash(x, y);
+    }
+
+    public int getX() {
+        return x;
+    }
+
+    public int getY() {
+        return y;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+        CoordinateKey that = (CoordinateKey) o;
+        return x == that.x && y == that.y;
+    }
+
+    @Override
+    public int hashCode() {
+        return this.hashCode;
+    }
+}
diff --git a/core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/maps/CoordinateMutableKey.java b/core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/maps/CoordinateMutableKey.java
new file mode 100644
index 0000000000..08b329de3d
--- /dev/null
+++ b/core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/maps/CoordinateMutableKey.java
@@ -0,0 +1,45 @@
+package com.baeldung.maps;
+
+import java.util.Objects;
+
+public class CoordinateMutableKey {
+
+    private int x;
+    private int y;
+
+    public CoordinateMutableKey(int x, int y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    public int getX() {
+        return x;
+    }
+
+    public void setX(int x) {
+        this.x = x;
+    }
+
+    public int getY() {
+        return y;
+    }
+
+    public void setY(int y) {
+        this.y = y;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (o == null || getClass() != o.getClass())
+            return false;
+        CoordinateMutableKey that = (CoordinateMutableKey) o;
+        return x == that.x && y == that.y;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(x, y);
+    }
+}
diff --git a/core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/maps/CoordinateSlowKey.java b/core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/maps/CoordinateSlowKey.java
new file mode 100644
index 0000000000..9fe54bd9ea
--- /dev/null
+++ b/core-java-modules/core-java-collections-maps-4/src/main/java/com/baeldung/maps/CoordinateSlowKey.java
@@ -0,0 +1,13 @@
+package com.baeldung.maps;
+
+public class CoordinateSlowKey extends CoordinateKey {
+
+    public CoordinateSlowKey(int x, int y) {
+        super(x, y);
+    }
+
+    @Override
+    public int hashCode() {
+        return 1;
+    }
+}
diff --git a/core-java-modules/core-java-collections-maps-4/src/test/java/com/baeldung/maps/CoordinateKeyUnitTest.java b/core-java-modules/core-java-collections-maps-4/src/test/java/com/baeldung/maps/CoordinateKeyUnitTest.java
new file mode 100644
index 0000000000..8ee68c4a3d
--- /dev/null
+++ b/core-java-modules/core-java-collections-maps-4/src/test/java/com/baeldung/maps/CoordinateKeyUnitTest.java
@@ -0,0 +1,80 @@
+package com.baeldung.maps;
+
+import org.junit.jupiter.api.Tag;
+import org.junit.jupiter.api.Test;
+
+import java.awt.*;
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+public class CoordinateKeyUnitTest {
+
+    private Map<CoordinateKey, Color> pixels = new HashMap<>();
+
+    @Test
+    void testOptimalKey() {
+        // setup
+        CoordinateKey coord = new CoordinateKey(1, 2);
+        pixels.put(coord, Color.CYAN);
+        // read out color correctly
+        assertEquals(Color.CYAN, pixels.get(coord));
+    }
+
+    @Test
+    void testSlowKey() {
+        // setup
+        CoordinateKey coord = new CoordinateSlowKey(1, 2);
+        pixels.put(coord, Color.CYAN);
+        // read out color correctly
+        assertEquals(Color.CYAN, pixels.get(coord));
+    }
+
+    // Performance Test Parameters - change here
+    private static final int MAX_X = 100;
+    private static final int MAX_Y = 100;
+    private static final int COUNT_OF_QUERIES = 1000;
+    private static final int QUERY_X = 1;
+    private static final int QUERY_Y = 1;
+
+    @Tag("performance")
+    @Test
+    void testKeyPerformance() {
+        // generate some sample keys and values
+        for (int x = 0; x < MAX_X; x++) {
+            for (int y = 0; y < MAX_Y; y++) {
+                pixels.put(new CoordinateKey(x, y), new Color(x % 255, y % 255, (x + y) % 255));
+            }
+        }
+        // read out multiple times and measure time
+        CoordinateKey coord = new CoordinateKey(QUERY_X, QUERY_Y);
+        long t1 = System.currentTimeMillis();
+        for (int i = 0; i < COUNT_OF_QUERIES; i++) {
+            assertNotNull(pixels.get(coord));
+        }
+        long t2 = System.currentTimeMillis();
+        System.out.printf("Optimal key performance: %d ms%n", t2 - t1);
+    }
+
+    @Tag("performance")
+    @Test
+    void testSlowKeyPerformance() {
+        // generate some sample keys and values
+        for (int x = 0; x < MAX_X; x++) {
+            for (int y = 0; y < MAX_Y; y++) {
+                pixels.put(new CoordinateSlowKey(x, y), new Color(x % 255, y % 255, (x + y) % 255));
+            }
+        }
+        // read out multiple times and measure time
+        CoordinateKey coord = new CoordinateSlowKey(QUERY_X, QUERY_Y);
+        long t1 = System.currentTimeMillis();
+        for (int i = 0; i < COUNT_OF_QUERIES; i++) {
+            assertNotNull(pixels.get(coord));
+        }
+        long t2 = System.currentTimeMillis();
+        System.out.printf("Slow key performance: %d ms%n", t2 - t1);
+    }
+
+}
diff --git a/core-java-modules/core-java-collections-maps-4/src/test/java/com/baeldung/maps/CoordinateMutableKeyUnitTest.java b/core-java-modules/core-java-collections-maps-4/src/test/java/com/baeldung/maps/CoordinateMutableKeyUnitTest.java
new file mode 100644
index 0000000000..54a61be230
--- /dev/null
+++ b/core-java-modules/core-java-collections-maps-4/src/test/java/com/baeldung/maps/CoordinateMutableKeyUnitTest.java
@@ -0,0 +1,27 @@
+package com.baeldung.maps;
+
+import org.junit.jupiter.api.Test;
+
+import java.awt.*;
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+public class CoordinateMutableKeyUnitTest {
+
+    @Test
+    void testKeyMutable() {
+        // setup
+        Map<CoordinateMutableKey, Color> pixels = new HashMap<>();
+        CoordinateMutableKey coord = new CoordinateMutableKey(1, 2);
+        pixels.put(coord, Color.CYAN);
+        // read out color correctly
+        assertEquals(Color.CYAN, pixels.get(coord));
+        // change key's hashcode should result in null value
+        coord.setX(10);
+        assertNull(pixels.get(coord));
+    }
+
+}
diff --git a/core-java-modules/pom.xml b/core-java-modules/pom.xml
index b272d2aa13..5291c8c3ca 100644
--- a/core-java-modules/pom.xml
+++ b/core-java-modules/pom.xml
@@ -38,6 +38,7 @@
         <module>core-java-collections-maps</module>
         <module>core-java-collections-maps-2</module>
         <module>core-java-collections-maps-3</module>
+        <module>core-java-collections-maps-4</module>
         <module>core-java-concurrency-2</module>
         <module>core-java-concurrency-advanced</module>
         <module>core-java-concurrency-advanced-2</module>
@@ -138,4 +139,4 @@
         <junit-jupiter.version>5.6.2</junit-jupiter.version>
     </properties>
 
-</project>
\ No newline at end of file
+</project>