[BAEL-6859] Inner Classes vs. Subclasses in Java (#14608)

* [BAEL-6859] inner and sub classes

* [BAEL-6859] unit test cases added

---------

Co-authored-by: Bhaskar <bhaskar.dastidar@freshworks.com>

core-java-modules/core-java-lang-oop-inheritance/src/main/java/com/baeldung/subclassinnerclass/EmailNotifier.java

@@ -0,0 +1,17 @@
+package com.baeldung.subclassinnerclass;
+
+import java.util.HashMap;
+
+public class EmailNotifier extends Notifier {
+    @Override
+    void notify(Message e) {
+        // Provide email specific implementation here
+    }
+
+    // Inner class for email connection
+    static class EmailConnector {
+        private String emailHost;
+        private int emailPort;
+        // Getter Setters
+    }
+}

core-java-modules/core-java-lang-oop-inheritance/src/main/java/com/baeldung/subclassinnerclass/Message.java

@@ -0,0 +1,6 @@
+package com.baeldung.subclassinnerclass;
+
+public class Message {
+    private int message;
+    // getter setter and other atteibutes
+}

core-java-modules/core-java-lang-oop-inheritance/src/main/java/com/baeldung/subclassinnerclass/NotificationService.java

@@ -0,0 +1,17 @@
+package com.baeldung.subclassinnerclass;
+
+public class NotificationService {
+    void notifyMessages() {
+        // Sending a Text Message
+        Message textMessage = new Message();
+        Notifier textNotifier = new TextMessageNotifier();
+
+        textNotifier.notify(textMessage);
+
+        // Sending an Email Message
+        Message emailMessage = new Message();
+        Notifier emailNotifier = new EmailNotifier();
+
+        emailNotifier.notify(emailMessage);
+    }
+}

core-java-modules/core-java-lang-oop-inheritance/src/main/java/com/baeldung/subclassinnerclass/Notifier.java

@@ -0,0 +1,5 @@
+package com.baeldung.subclassinnerclass;
+
+public abstract class Notifier {
+    abstract void notify(Message e);
+}

core-java-modules/core-java-lang-oop-inheritance/src/main/java/com/baeldung/subclassinnerclass/TextMessageNotifier.java

@@ -0,0 +1,14 @@
+package com.baeldung.subclassinnerclass;
+
+public class TextMessageNotifier extends Notifier {
+    @Override
+    void notify(Message e) {
+        // Provide text message specific implementation here
+    }
+
+    // Inner class for text message connection
+    private static class SMSConnector {
+        private String smsHost;
+        // Getter Setters
+    }
+}

core-java-modules/core-java-lang-oop-inheritance/src/test/java/com/baeldung/subclassinnerclass/InnerClassUnitTest.java

@@ -0,0 +1,16 @@
+package com.baeldung.subclassinnerclass;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+import org.junit.Test;
+
+public class InnerClassUnitTest {
+    @Test
+    public void givenInnerStaticClassWhenInstantiatedThenOuterClassIsInstantiated() {
+        Notifier emailNotifier = new EmailNotifier();
+        EmailNotifier.EmailConnector emailConnector = new EmailNotifier.EmailConnector();
+
+        assertThat(emailNotifier).hasSameClassAs(new EmailNotifier());
+        assertThat(emailConnector).isInstanceOf(EmailNotifier.EmailConnector.class);
+    }
+}

core-java-modules/core-java-lang-oop-inheritance/src/test/java/com/baeldung/subclassinnerclass/SubClassUnitTest.java

@@ -0,0 +1,18 @@
+package com.baeldung.subclassinnerclass;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+import org.junit.Test;
+
+public class SubClassUnitTest {
+    @Test
+    public void givenSubclassWhenInstantiatedThenSubclassObjectIsPossible() {
+        Notifier emailNotifier = new EmailNotifier();
+        assertThat(emailNotifier).hasSameClassAs(new EmailNotifier());
+        assertThat(emailNotifier).isExactlyInstanceOf(EmailNotifier.class);
+
+        Notifier textMessageNotifier = new TextMessageNotifier();
+        assertThat(textMessageNotifier).isInstanceOf(Notifier.class);
+        assertThat(textMessageNotifier).isExactlyInstanceOf(TextMessageNotifier.class);
+    }
+}