BAEL-1261 Polymorphism in Java (#2762)

* Define beans for handling different message types in a lean chat app

* Add class based spring beans configuration

* Define spring configuration in XML for constructor based bean injection

* Refactor package structure to separate constructor based bean injection code set from setter based bean injection code set

* Define configuration and classes specific to setter-based bean injection.

* Implement tests for constructor-based and setter-based bean injections

* develop codes for explaining type erasure

* Write unit tests for type erasure examples

* Remove evaluation article code

* Modify type erasure examples and unit tests

* Modify type erasure examples and unit tests

* Add expected exception in TypeErasureUnitTest

* Correct grammar in class name

* Implement File Manager app to demonstrate Polymorphism. Develop unit tests for Polymorphism article code

* Add examples for static polymorphism

* Change sysout statments to slf4j log info statements

* Add assertions and expected errors check on Test

* Add assertions and expected errors check on Test

* Correct compile time error of symbol not found

* Removed commented out non-compiling test.

core-java/src/test/java/com/baeldung/polymorphism/PolymorphismUnitTest.java

@@ -0,0 +1,34 @@
+package com.baeldung.polymorphism;
+
+import static org.junit.Assert.*;
+
+import java.awt.image.BufferedImage;
+
+import org.junit.Ignore;
+import org.junit.Test;
+
+public class PolymorphismUnitTest {
+
+    @Test
+    public void givenImageFile_whenFileCreated_shouldSucceed() {
+        ImageFile imageFile = FileManager.createImageFile("SampleImageFile", 200, 100, new BufferedImage(100, 200, BufferedImage.TYPE_INT_RGB).toString()
+            .getBytes(), "v1.0.0");
+        assertEquals(200, imageFile.getHeight());
+    }
+
+    // Downcasting then Upcasting
+    @Test
+    public void givenTextFile_whenTextFileCreatedAndAssignedToGenericFileAndCastBackToTextFileOnGetWordCount_shouldSucceed() {
+        GenericFile textFile = FileManager.createTextFile("SampleTextFile", "This is a sample text content", "v1.0.0");
+        TextFile textFile2 = (TextFile) textFile;
+        assertEquals(6, textFile2.getWordCount());
+    }
+
+    // Downcasting
+    @Test(expected = ClassCastException.class)
+    public void givenGenericFile_whenCastToTextFileAndInvokeGetWordCount_shouldFail() {
+        GenericFile genericFile = new GenericFile();
+        TextFile textFile = (TextFile) genericFile;
+        System.out.println(textFile.getWordCount());
+    }
+}