Bal 4152 covariant return type (#9561)

* BAEL-4152: added covariant return type material

* BAEL-4152: covariant return type article related code is moved to another package

core-java-modules/core-java-lang-oop-methods/src/main/java/com/baeldung/covariance/IntegerProducer.java

@@ -0,0 +1,8 @@
+package com.baeldung.covariance;
+
+public class IntegerProducer extends Producer {
+    @Override
+    public Integer produce(String input) {
+        return Integer.parseInt(input);
+    }
+}

core-java-modules/core-java-lang-oop-methods/src/main/java/com/baeldung/covariance/Producer.java

@@ -0,0 +1,8 @@
+package com.baeldung.covariance;
+
+public class Producer {
+    public Object produce(String input) {
+        Object result = input.toLowerCase();
+        return result;
+    }
+}

core-java-modules/core-java-lang-oop-methods/src/test/java/com/baeldung/covariance/CovariantProducersUnitTest.java

@@ -0,0 +1,49 @@
+package com.baeldung.covariance;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class CovariantProducersUnitTest {
+
+    @Test
+    public void whenInputIsArbitrary_thenProducerProducesString() {
+        String arbitraryInput = "just a random text";
+        Producer producer = new Producer();
+
+        Object objectOutput = producer.produce(arbitraryInput);
+
+        assertEquals(arbitraryInput, objectOutput);
+        assertEquals(String.class, objectOutput.getClass());
+    }
+
+    @Test
+    public void whenInputIsArbitrary_thenIntegerProducerFails() {
+        String arbitraryInput = "just a random text";
+        Producer producer = new IntegerProducer();
+
+        assertThrows(NumberFormatException.class, () -> producer.produce(arbitraryInput));
+    }
+
+    @Test
+    public void whenInputIsSupported_thenProducerCreatesInteger() {
+        String integerAsString = "42";
+        Producer producer = new IntegerProducer();
+
+        Object result = producer.produce(integerAsString);
+
+        assertEquals(Integer.class, result.getClass());
+        assertEquals(Integer.parseInt(integerAsString), result);
+    }
+
+    @Test
+    public void whenInputIsSupported_thenIntegerProducerCreatesIntegerWithoutCasting() {
+        String integerAsString = "42";
+        IntegerProducer producer = new IntegerProducer();
+
+        Integer result = producer.produce(integerAsString);
+
+        assertEquals(Integer.parseInt(integerAsString), result);
+    }
+}