Something

private BigInteger lueIsoLuku(Scanner nappis) {
  BigInteger paluu;
  boolean kelpo = false;

  while(!kelpo) {
    try {
      paluu = nappis.nextBigInteger();
      kelpo = true;
    } catch (InputMismatchException ime) {
      System.out.println("Epäkelpo isoluku!");
      kelpo = false;
    }
  }

  return paluu;
}