[BAEL-11402] - Moved articles out of core-java (part 3)

core-java-io/src/main/java/com/baeldung/extension/Extension.java

@@ -0,0 +1,23 @@
+package com.baeldung.extension;
+
+import com.google.common.io.Files;
+import org.apache.commons.io.FilenameUtils;
+
+import java.util.Optional;
+
+public class Extension {
+    //Instead of file name we can also specify full path of a file eg. /baeldung/com/demo/abc.java
+    public String getExtensionByApacheCommonLib(String filename) {
+        return FilenameUtils.getExtension(filename);
+    }
+
+    public Optional<String> getExtensionByStringHandling(String filename) {
+        return Optional.ofNullable(filename)
+                       .filter(f -> f.contains("."))
+                      .map(f -> f.substring(filename.lastIndexOf(".") + 1));
+    }
+
+    public String getExtensionByGuava(String filename) {
+        return Files.getFileExtension(filename);
+    }
+}