Example 1: Java Program to get the file extension

import java.io.File;

class Main{

  public static void main(String[] args){
    File file = new File("Test.java");

    // convert the file name into string
    String fileName = file.toString();

    int index = fileName.lastIndexOf('.');
    if(index > 0) {
      String extension = fileName.substring(index + 1);
      System.out.println("File extension is " + extension);
    }
  }
}

In the above example,

• file.toString() – Converts the File object into a string.
• fileName.lastIndexOf(‘.’) – Returns the last occurrence of character. Since all file extension starts with ‘.’, we use the character ‘.’.
• fileName.substring() – Returns the string after character ‘.’.

Example 2: Get the file extension of all files present in a directory

Now, suppose we want to get the file extension of all the files present in a directory. We can use the above process in the loop.

import java.io.File;

class Main{

  public static void main(String[] args){
    File directory = new File("Directory");

    // list all files present in the directory
    File[] files = directory.listFiles();
    System.out.println("FilestttExtension");

    for(File file : files) {
      // convert the file name into string
      String fileName = file.toString();

      int index = fileName.lastIndexOf('.');
      if(index > 0) {
        String extension = fileName.substring(index + 1);
        System.out.println(fileName + "t" + extension);
      }
    }
  }


}

Output

Files                   Extension
Directoryfile1.txt     txt
Directoryfile2.svg     svg
Directoryfile3.java    java
Directoryfile4.py      py
Directoryfile5.html    html

Note: The output of the program depends on the directory you use and the files in the directory.

• If you are using the Gauva Library, you can directly use the getFileExtension() method to get the file extension. For example,

  String fileName = "Test.java";
  String extension = Files.getFileExtension(fileName);

• And, also the Apache Commons IO provides the FilenameUtils class provides the getExtension method to get the extension of the file.

  String extension = FilenameUtils.getExtension("file.py") // returns py