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();
    }
  }
}