BAEL-1848 final and immutable objects in Java (#4515)

* Strange git issue with README.MD, wouldn't revert the file

* final and immutable objects in Java

* Move tests to src/test/

* BAEL-1848 renamed test class

core-java/src/main/java/com/baeldung/immutableobjects/Currency.java

@@ -0,0 +1,18 @@
+package com.baeldung.immutableobjects;
+
+public final class Currency {
+
+    private final String value;
+    
+    private Currency(String currencyValue) {
+        value = currencyValue;
+    }
+    
+    public String getValue() {
+        return value;
+    }
+    
+    public static Currency of(String value) {
+        return new Currency(value);
+    }
+}

core-java/src/main/java/com/baeldung/immutableobjects/Money.java

@@ -0,0 +1,20 @@
+package com.baeldung.immutableobjects;
+
+// 4. Immutability in Java
+public final class Money {
+    private final double amount;
+    private final Currency currency;
+
+    public Money(double amount, Currency currency) {
+        this.amount = amount;
+        this.currency = currency;
+    }
+
+    public Currency getCurrency() {
+        return currency;
+    }
+
+    public double getAmount() {
+        return amount;
+    }
+}

core-java/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());
+    }
+}