程序编译通过，但是运行后报异常，求大神帮忙

package com.studet.course;

import java.util.ArrayList;

import java.util.Collections;

import java.util.List;

import java.util.Random;

public class ListSort {

public ListSort()

{

}

public void testCollections() {

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

Random random = new Random();

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

String temp = "";

do{

int length = random.nextInt(9)+1;

temp = "";

for(int j = 0;j < length; j++){

temp = temp + randomString();

}

}while(listStrings.contains(temp));

listStrings.add(temp);

System.out.println("添加成功"+temp);

}

System.out.println("-------排序前---------");

for (String string : listStrings) {

System.out.println(string);

}

Collections.sort(listStrings);

System.out.println("---------排序后--------");

for (String string : listStrings) {

System.out.println(string);

}

}

public char randomString() {

String str = "0123456789qwertyuioplkjhgfdsamnbvcxzQWERTYUIOPLKJHGFDSAZXCVBNM";

Random random = new Random();

char c = str.charAt(random.nextInt(64));

return c;

}

/**

* @param args

*/

public static void main(String[] args) {

// TODO Auto-generated method stub

    ListSort test = new ListSort();

    test.testCollections();

    //test.randomString();

}

}