BAEL-1734 add different methods to find file extension (#4122)

* BAEL-1734 add different methods to find file extension

* fix camel case formatting for variables

* fix variable name

Remove camel casing for variable name as it is a single word.

core-java/src/test/java/com/baeldung/extension/ExtensionTest.java

@@ -0,0 +1,29 @@
+package com.baeldung.extension;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class ExtensionTest {
+    private Extension extension = new Extension();
+
+    @Test
+    public void getExtension_whenApacheCommonIO_thenExtensionIsTrue() {
+        String expectedExtension = "txt";
+        String actualExtension = extension.getExtensionByApacheCommonLib("jarvis.txt");
+        Assert.assertEquals(expectedExtension, actualExtension);
+    }
+
+    @Test
+    public void getExtension_whenStringHandle_thenExtensionIsTrue() {
+        String expectedExtension = "java";
+        String actualExtension = extension.getExtensionByStringHandling("Demo.java");
+        Assert.assertEquals(expectedExtension, actualExtension);
+    }
+
+    @Test
+    public void getExtension_whenGuava_thenExtensionIsTrue() {
+        String expectedExtension = "class";
+        String actualExtension = extension.getExtensionByGuava("baeldung/Demo.class");
+        Assert.assertEquals(expectedExtension, actualExtension);
+    }
+}