生成随机字符串并排序

public void testSort2(){
List<String> stringList=new ArrayList<String>();
Random r1=new Random();
Random r2=new Random();
String base="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
int length=base.length();
for(int i=0;i<10;i++){
StringBuffer sb=new StringBuffer();
do{

        for(int j=0;j<r1.nextInt(10);j++){

                int num=r2.nextInt(length);
                sb.append(base.charAt(num));

        }
        }while(stringList.contains(sb.toString()));
        stringList.add(sb.toString());
    }
    System.out.println("-----排序前------");
    for(String string:stringList){
        System.out.println("元素："+string);
    }
    Collections.sort(stringList);
    System.out.println("------排序后----——-");
    for(String string:stringList){
        System.out.println("元素："+string);
    }
}