Add first test

immutables/src/main/java/com/baeldung/immutable/Address.java

@@ -1,5 +1,9 @@
 package com.baeldung.immutable;
 
-public class Address {
+import org.immutables.value.Value;
 
+@Value.Immutable
+public interface Address {
+    String getStreetName();
+    Integer getNumber();
 }

immutables/src/main/java/com/baeldung/immutable/Person.java

@@ -3,6 +3,7 @@ package com.baeldung.immutable;
 import org.immutables.value.Value;
 
 @Value.Immutable
-public class Person {
-    private String name;
+public abstract class Person {
+    abstract String getName();
+    abstract Integer getAge();
 }

immutables/src/test/java/com/baeldung/immutable/ImmutablePersonTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.immutable;
+
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class ImmutablePersonTest {
+
+    @Test
+    public void whenModifying_shouldCreateNewInstance() throws Exception {
+        final com.baeldung.immutable.ImmutablePerson john = com.baeldung.immutable.ImmutablePerson.builder()
+          .age(42)
+          .name("John")
+          .build();
+
+        final com.baeldung.immutable.ImmutablePerson john43 = john.withAge(43);
+
+        assertThat(john)
+          .isNotSameAs(john43);
+        assertThat(john.getAge())
+          .isEqualTo(42);
+    }
+}