Antwort
File datei = new File("telefon.txt");
try {
System.out.println(datei.isFile());
FileWriter writer = new FileWriter(datei, true); // das "true" verhindert das
writer.write(vorname.getText() + "; "); // überschreiben
writer.write(nachname.getText() + "; ");
writer.write(strasse.getText() + "; ");
writer.write(plz.getText() + "; ");
writer.write(ort.getText() + "; ");
writer.write(geburtstag.getText() + "; ");
writer.write(telefonnummer.getText() + "; ");
writer.write(today + "; \n");
writer.flush();
} catch (IOException e) {
System.out.println("Datei konnte nicht bearbeitet werden");
}