Custom Efficient hashCode() Implementations - Alejandro Gervasio - alejandro.gervasio@gmail.com (#2301)

* Initial Commit

* Updated pom.xml

* Updated User class

* Updated pom.xml

* Updated Application class

core-java/hashcode/pom.xml

@@ -0,0 +1,39 @@
+<?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>
+    <groupId>com.baeldung.hashcode</groupId>
+    <artifactId>hashcode</artifactId>
+    <version>1.0</version>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>1.8</source>
+                    <target>1.8</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <version>4.12</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-api</artifactId>
+            <version>1.7.25</version>
+        </dependency>
+        <dependency>
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-simple</artifactId>
+            <version>1.7.25</version>
+        </dependency>
+    </dependencies>
+</project>

core-java/hashcode/src/main/java/com/baeldung/application/Application.java

@@ -0,0 +1,23 @@
+package com.baeldung.application;
+
+import com.baeldung.entities.User;
+import java.util.HashMap;
+import java.util.Map;
+
+public class Application {
+
+    public static void main(String[] args) {
+        Map<User, User> users = new HashMap<>();
+        User user1 = new User(1L, "John", "john@domain.com");
+        User user2 = new User(2L, "Jennifer", "jennifer@domain.com");
+        User user3 = new User(3L, "Mary", "mary@domain.com");
+
+        users.put(user1, user1);
+        users.put(user2, user2);
+        users.put(user3, user3);
+
+        if (users.containsKey(user1)) {
+            System.out.print("User found in the collection");
+        }
+    }
+}

core-java/hashcode/src/main/java/com/baeldung/entities/User.java

@@ -0,0 +1,38 @@
+package com.baeldung.entities;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class User {
+
+    private final Logger logger = LoggerFactory.getLogger(User.class);
+    private long id;
+    private String name;
+    private String email;
+
+    public User(long id, String name, String email) {
+        this.id = id;
+        this.name = name;
+        this.email = email;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null) return false;
+        if (this.getClass() != o.getClass()) return false;
+        User user = (User) o;
+        return id != user.id && (!name.equals(user.name) && !email.equals(user.email));
+    }
+    	
+    @Override
+    public int hashCode() {
+        int hash = 7;
+        hash = 31 * hash + (int) id;
+        hash = 31 * hash + (name == null ? 0 : name.hashCode());
+        hash = 31 * hash + (email == null ? 0 : email.hashCode());
+        logger.info("hashCode() method called - Computed hash: " + hash);
+        return hash;
+    }
+    // getters and setters here
+}

core-java/hashcode/src/test/java/com/baeldung/application/ApplicationTest.java

@@ -0,0 +1,30 @@
+package com.baeldung.application;
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import static org.junit.Assert.assertEquals;
+
+public class ApplicationTest {
+
+    private ByteArrayOutputStream outContent;
+
+    @Before
+    public void setUpPrintStreamInstance() throws Exception {
+        this.outContent = new ByteArrayOutputStream();
+        System.setOut(new PrintStream(outContent));
+    }
+
+    @After
+    public void tearDownByteArrayOutputStream() throws Exception {
+        outContent = null;
+    }
+
+    @Test
+    public void main_NoInputState_TextPrintedToConsole() throws Exception {
+        Application.main(new String[]{});
+        assertEquals("User found in the collection", outContent.toString());
+    }
+}

core-java/hashcode/src/test/java/com/baeldung/entities/UserTest.java

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+package com.baeldung.entities;
+
+import org.junit.After;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+public class UserTest {
+
+    private User user;
+    private User comparisonUser;
+
+    @Before
+    public void setUpUserInstances() {
+        this.user = new User(1L, "test", "test@domain.com");
+        this.comparisonUser = this.user;
+    }
+
+    @After
+    public void tearDownUserInstances() {
+        user = null;
+        comparisonUser = null;
+    }
+
+    @Test
+    public void equals_EqualUserInstance_TrueAssertion(){
+        Assert.assertTrue(user.equals(comparisonUser));
+    }
+
+    @Test
+    public void hashCode_UserHash_TrueAssertion() {
+        Assert.assertEquals(1792276941, user.hashCode());
+    }
+}