Bael 1734 get file extension in java (#4522)

* change method to return Optionals

* add check for empty Optional

* replace ifPresent() with get()

* remove extra check

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

@@ -19,8 +19,7 @@ public class ExtensionUnitTest {
     public void getExtension_whenStringHandle_thenExtensionIsTrue() {
         String expectedExtension = "java";
         Optional<String> actualExtension = extension.getExtensionByStringHandling("Demo.java");
-        Assert.assertTrue(actualExtension.isPresent());
-        actualExtension.ifPresent(ext -> Assert.assertEquals(expectedExtension,ext));
+        Assert.assertEquals(expectedExtension, actualExtension.get());
     }
 
     @Test