In this case, use FilenameUtils.getExtension from Apache Commons IO

Here is an example of how to use it (you may specify either full path or just file name):

import org.apache.commons.io.FilenameUtils;

// ...

String ext1 = FilenameUtils.getExtension("/path/to/file/foo.txt"); // returns "txt"
String ext2 = FilenameUtils.getExtension("bar.exe"); // returns "exe"

Maven dependency:

<dependency>
  <groupId>commons-io</groupId>
  <artifactId>commons-io</artifactId>
  <version>2.6</version>
</dependency>

Gradle Groovy DSL

implementation 'commons-io:commons-io:2.6'

Gradle Kotlin DSL

implementation("commons-io:commons-io:2.6")

Others https://search.maven.org/artifact/commons-io/commons-io/2.6/jar

java File.java I don't think this is doing what you think it's doing. Recently, java allowed you to compile and run in the same step. That's what is going on here. If you pass java a .java file it will compile and then run it. Traditionally, all .java files went to javac and all .class files went to java (but omitting the extension).

"The correct answer is .java. Key Points Java code files have the extension .java. This extension indicates that the file contains Java source code, which can be compiled into bytecode by the Java compiler. After compilation, the bytecode is stored in files with the .class extension, which can be executed by the Java Virtual Machine (JVM). Additional Information .js: This extension is used for JavaScript files, which are scripts that run in web browsers. .txt: This extension is used for plain text files, which contain unformatted text and can be opened with any text editor. .class: This extension is used for Java bytecode files that result from compiling Java source code files. These files are executed by the Java Virtual Machine (JVM)."