[BAEL-10839] - Added missed class and moved articles related to hashcode and equals

core-java-lang-oop/README.md

@@ -20,4 +20,6 @@
 - [Guide to the this Java Keyword](http://www.baeldung.com/java-this)
 - [Immutable Objects in Java](http://www.baeldung.com/java-immutable-object)
 - [Inheritance and Composition (Is-a vs Has-a relationship) in Java](http://www.baeldung.com/java-inheritance-composition)
-- [A Guide to Constructors in Java](https://www.baeldung.com/java-constructors)
+- [A Guide to Constructors in Java](https://www.baeldung.com/java-constructors)
+- [Generate equals() and hashCode() with Eclipse](http://www.baeldung.com/java-eclipse-equals-and-hashcode)
+- [Java equals() and hashCode() Contracts](https://www.baeldung.com/java-equals-hashcode-contracts)

core-java-lang-oop/src/test/java/com/baeldung/immutableobjects/ImmutableObjectsUnitTest.java

@@ -0,0 +1,36 @@
+package com.baeldung.immutableobjects;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.Test;
+
+public class ImmutableObjectsUnitTest {
+
+    @Test
+    public void whenCallingStringReplace_thenStringDoesNotMutate() {
+        // 2. What's an Immutable Object?
+        final String name = "baeldung";
+        final String newName = name.replace("dung", "----");
+
+        assertEquals("baeldung", name);
+        assertEquals("bael----", newName);
+    }
+
+    public void whenReassignFinalValue_thenCompilerError() {
+        // 3. The final Keyword in Java (1)
+        final String name = "baeldung";
+        // name = "bael...";
+    }
+
+    @Test
+    public void whenAddingElementToList_thenSizeChange() {
+        // 3. The final Keyword in Java (2)
+        final List<String> strings = new ArrayList<>();
+        assertEquals(0, strings.size());
+        strings.add("baeldung");
+        assertEquals(1, strings.size());
+    }
+}