Java Program to Find File Extension

While doing file I/O operations in Java you may need to know the extension of the file. In this post we’ll see a Java program to get the file extension. Since there is no direct Java File API method to get the file extension you will have to use the Java String class methods to do that.

Another option is to use Apache IO where FilenameUtils has a method getExtension() to get the file name. But that requires Apache IO jar in the class path.

Getting file extension Java program

1. In String class there is a lastIndexOf(int ch) method that gives the index of the last occurrence of the specified character, using that you can get the index of the last '.' and using subString() you can get the string after that last dot. That way you can get the extension of the passed file.

public class FileExtension {
  public static void main(String[] args) throws IOException {
    File file = new File("F:\\knpcode\\links.txt");
    String extension = getFileExtension(file);
    System.out.println("Extension is- " + extension);
    
    file = new File("F:\\knpcode\\Ubuntu Page.html");
    extension = getFileExtension(file);
    System.out.println("Extension is- " + extension);
  }
	
  private static String getFileExtension(File file) {
    // null and file exist check
    if(file == null || !file.exists()){
      return "File not found";
    }
    
    String fileName = file.getName();
    int extIndex = fileName.lastIndexOf(".");
    // -1 is returned if index is not found
    if(extIndex == -1) {
      return "";
    }else {
      return fileName.substring(fileName.lastIndexOf("."));
    }
  }
}

Output

Extension is- .txt
Extension is- .html

If you don’t want the accompanying '.' then use

fileName.substring(fileName.lastIndexOf(".") + 1

2. Using Apache IO library which provides FilenameUtils class with method getExtension() which returns the textual part of the filename after the last dot. If there is no extension then empty string is returned, if file is null then null is returned.

import java.io.File;
import java.io.IOException;
import org.apache.commons.io.FilenameUtils;

public class FileExtension {
  public static void main(String[] args) throws IOException {
    File file = new File("F:\\knpcode\\links.txt");
    String extension = getFileExtensionApache(file);
    System.out.println("Extension is- " + extension);
    
    file = new File("F:\\knpcode\\Ubuntu Page.html");
    extension = getFileExtensionApache(file);
    System.out.println("Extension is- " + extension);
  }
		
  private static String getFileExtensionApache(File file) {
    return FilenameUtils.getExtension(file.getName());
  }
}

Output

Extension is- txt
Extension is- html

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KnpCode

March 20, 2022

Java Program to Find File Extension

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