新手小白，望大神指导！

public void testSort4(){
List<String> stringList = new ArrayList<String>();
String stringBase = "0123456789abcdefghijklmnopqrstuvwxyz"

• "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  String sb ="" ;
  char[] ch;
  Random random = new Random();
  for (int i = 0; i < 10; i++) {
  do{
  int temp = random.nextInt(10);
  ch = new char[temp];
  for(int j = 0;j < temp;j++ ){
  int temp2 = random.nextInt(stringBase.length());
  char ch2 = stringBase.charAt(temp2);
  ch[j] = ch2;
  }
  sb = sb.valueOf(ch);
  }while(stringList.contains(sb));

      System.out.println("将要添加的字符串是："+sb);
      stringList.add(sb);
  }
  System.out.println("-----------排序前---------------");
  for (String string : stringList) {
      System.out.println("元素："+string);
  }
  System.out.println("-----------排序后---------------");
  Collections.sort(stringList);
  for (String string : stringList) {
      System.out.println("元素："+string);
  }

  }