public String lastChars(String a, String b) {
  String first = "@";
  String last = "@";
  if (!a.isEmpty()) {
    first = a.substring(0,1);
  }
  // NOTE: no "else if" here.  You want to do both "if" statements!
  if (!b.isEmpty()) {
    last = b.substring(b.length()-1);
  }
  return first + last;
}

public String lastChars2(String a, String b) {
  if (a.isEmpty() && b.isEmpty()) {
    return "@@";
  }
  else if (a.isEmpty()) {
    return "@" + b.substring(b.length()-1);
  }
  else if (b.isEmpty()) {
    return a.substring(0,1) + "@";
  }
  else {
    return a.substring(0,1) + b.substring(b.length()-1);
  }
}