How do I get the file extension of a file in Java?

Just to be clear, I'm not looking for the MIME type.

Let's say I have the following input: /path/to/file/foo.txt

I'd like a way to break this input up, specifically into .txt for the extension. Is there any built in way to do this in Java? I would like to avoid writing my own parser.

Solution 1:

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

Solution 2:

Do you really need a "parser" for this?

String extension = "";

int i = fileName.lastIndexOf('.');
if (i > 0) {
    extension = fileName.substring(i+1);
}

Assuming that you're dealing with simple Windows-like file names, not something like archive.tar.gz.

Btw, for the case that a directory may have a '.', but the filename itself doesn't (like /path/to.a/file), you can do

String extension = "";

int i = fileName.lastIndexOf('.');
int p = Math.max(fileName.lastIndexOf('/'), fileName.lastIndexOf('\\'));

if (i > p) {
    extension = fileName.substring(i+1);
}

Solution 3:

private String getFileExtension(File file) {
    String name = file.getName();
    int lastIndexOf = name.lastIndexOf(".");
    if (lastIndexOf == -1) {
        return ""; // empty extension
    }
    return name.substring(lastIndexOf);
}

Solution 4:

If you use Guava library, you can resort to Files utility class. It has a specific method, getFileExtension(). For instance:

String path = "c:/path/to/file/foo.txt";
String ext = Files.getFileExtension(path);
System.out.println(ext); //prints txt

In addition you may also obtain the filename with a similar function, getNameWithoutExtension():

String filename = Files.getNameWithoutExtension(path);
System.out.println(filename); //prints foo

Solution 5:

If on Android, you can use this:

String ext = android.webkit.MimeTypeMap.getFileExtensionFromUrl(file.getName());