Java determines whether the string is empty and whether the string is a number

//这是对的
if (selection != null && !selection.equals("")) {
      whereClause += selection;
  }

//这是错的
if (!selection.equals("") && selection != null) {
      whereClause += selection;
  }

// 调用java自带的函数
public static boolean isNumeric(String number) {
  for (int i = number.length(); --i >= 0;) {
      if (!Character.isDigit(number.charAt(i))) {
          return false;
      }
  }
  return true;
}
// 使用正则表达式
public static boolean isNumeric(String number) {
  Pattern pattern = Pattern.compile("[0-9]*");
  return pattern.matcher(str).matches();
}
// 利用ASCII码
public static boolean isNumeric(String number) {
  for (int i = str.length(); --i >= 0;) {
      int chr = str.charAt(i);
      if (chr < 48 || chr > 57)
          return false;
  }
  return true;
}