public class Immutable {
    public static String upcase(String s) {
        return s.toUpperCase();
    }

    public static void main(String[] args) {
        String q = "howdy";
        System.out.println(q);
        String qq = upcase(q);
        System.out.println(qq);
        System.out.println(q);
    }
}

public class UsingStringBuilder {
    public static Random rand = new Random(47);

      public String toString() {
        StringBuilder result = new StringBuilder("[");
        for(int i = 0; i < 25; i++) {
          result.append(rand.nextInt(100));
          result.append(", ");
        }
        result.delete(result.length()-2, result.length()); //去掉start到end的字符串
        result.append("]");
        return result.toString();
      }

      public static void main(String[] args) {
        UsingStringBuilder usb = new UsingStringBuilder();
        System.out.println(usb);
      }
}