Write a program to rename all .txt file in a directory?

A program to rename all .txt file in a directory is as follows:

import java.io.File;
public class Demo {
   public static void main(String[] args) {
      try {
         File file1 = new File("demo1.txt");
         File file2 = new File("demo2.txt");
         file1.createNewFile();
         file2.createNewFile();
         boolean flag = file1.renameTo(file2);
         System.out.print("File renamed? " + flag);
      } catch(Exception e) {
         e.printStackTrace();
      }
   }
}