W3Schools Tryit Editor

import java.io.FileOutputStream;
import java.io.IOException;
public class Main {
 public static void main(String[] args) {
 String text = "\nAppended text!";
 // true = append mode
 try (FileOutputStream output = new FileOutputStream("filename.txt", true)) {
 output.write(text.getBytes());
 System.out.println("Successfully appended to file.");
 } catch (IOException e) {
 System.out.println("Error writing file.");
 e.printStackTrace();
 }
 }
}