How do I trim a file extension from a String in Java?

java

file-extension

edge-cases

unit-testing

byAlex Kataev·Aug 14, 2024

To trim a file extension, you can utilize the substring method of String class and lastIndexOf to zero in on the final period, denoting the extension.

String fileName = "photo.jpeg";
String baseName = fileName.substring(0, fileName.lastIndexOf('.'));

In this case, baseName now contains "photo"; the .jpeg extension is discarded.

Edge cases: The thorny instances

Beyond the quick solution, it's crucial we handle edge cases efficiently.

Concealed files & missing extensions

Files starting with a dot (.htaccess) or lacking an extension should be left untouched.

// .htaccess remains, nothing after first dot to regard as an extension
String hiddenFile = ".htaccess";
String base = (hiddenFile.indexOf('.') == 0) ? hiddenFile : hiddenFile.substring(0, hiddenFile.lastIndexOf('.'));

// README stays as is, since no extension present to trim
String noExtension = "README";
base = (noExtension.contains(".")) ? noExtension.substring(0, noExtension.lastIndexOf('.')) : noExtension;

Filenames with multiple dots

Handle filenames with multiple dots wisely – only the last part should be considered an extension.

// .tar is part of the filename, not part of extension ".gz"
String complexName = "archive.tar.gz";
String simplified = complexName.substring(0, complexName.lastIndexOf('.'));

Universal path trimming

Use System.getProperty("file.separator") for portable path separator handling.

// No more hardcoding path separators! Your OS will thank you.
String pathSeparator = System.getProperty("file.separator");

Bonus power-up: Using external libraries

To handle even thornier issues, Apache Commons IO comes to the rescue with FilenameUtils.removeExtension().

// A swiss army knife for filename manipulations
String completeFileName = FilenameUtils.removeExtension(fileName);

Cutting edge strategies

Fluent code: The beauty of chained methods

Chaining methods can yield concise and expressive code.

// You can code fluently when you chain Java methods like this.
String edited = fileName.replaceAll("(?<=.).+$", "").concat("_edited");

Verify before you cut

The endsWith method allows you to verify an extension's presence before removing it.

// Lets not cut an innocent string that never had an extension.
if (fileName.endsWith(".jpeg")) {
  // Safe to process, the poor thing really had an extension!
}

Evaluating and scaling your operations

Unit tests: Your safety net

Don't just trust your code, always test thoroughly for different scenarios.

// "Hey JUnit, we trust, but we also test."
assert "recipe".equals(removeExtension("recipe.txt"));
assert "multi.part".equals(removeExtension("multi.part.tar.gz"));