?調用Collections.sort()方法，為什么sort會報錯？

是創建Stringbuilder的原因嗎

package com.imooc.collection;

import java.util.ArrayList;

import java.util.List;

import java.util.Random;

public class Collections {

public void testsort1(){

//給List中添加泛型

List <String> stringlist=new ArrayList <String> ();

//創建字符

String str="qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM1234567890";

//創建Random對象

Random random=new Random();

for(int i=0;i<10;i++){

//c創建一個String類型的變量K

StringBuilder sb=new StringBuilder();

for(int j=0;j<1+random.nextInt(10);j++){

char cr=str.charAt((int)(random.nextInt(62)));

sb=sb.append(String.valueOf(cr));

System.out.println("獲取的的字符串:"+sb);

stringlist.add(sb.toString());

}

}

System.out.println("----排序前----");

System.out.println("公共:"+stringlist.size()+"個元素");

for (String string : stringlist) {

System.out.println("排序前元素:"+string);

}

System.out.println("---排序后---");

System.out.println("公共:"+stringlist.size()+"個元素");

Collections.sort(stringlist);

for (String string : stringlist) {

System.out.println("排序后:"+string);

}

}

public static void main(String[] args){

Collections it=new Collections();

it.testsort1();

}

}