public static void main(String[] args) {
    List<String> list=new ArrayList<String>();
    Random r=new Random();
    StringBuffer s;
    char c;     
    for(int i=0;i<10;i++){
        do{
            s=new StringBuffer();   
            int length=r.nextInt(10)+1;  
            for(int j=0;j<length;j++){

                do{
                    c=(char)(r.nextInt('z'-'0'+1)+'0');         
                }while(!((c>='0'&&c<='9')||(c>='A'&&c<='Z')||(c>='a'&&c<='z')));
                s.append(c);
            }
        }while(list.contains(s.toString()));    
        list.add(s.toString());         
    }
    System.out.println("-----------排序前---------");
    for (String string : list) {
        System.out.println(string);
    }

    Collections.sort(list);
    System.out.println("-----------排序后---------");
    for (String string : list) {
        System.out.println(string);
    }   
}