Java生成含字母和数字的6位随机字符串

package com.amos.tools; import java.util.ArrayList;import java.util.List;import java.util.Random; public class InvertCodeGenerator {   public static void main(String[] args) {    // TODO Auto-generated method stub    List<String> results=genCodes(6,100);    System.out.println(results.size());  }   public static List<String> genCodes(int length,long num){         List<String> results=new ArrayList<String>();         for(int j=0;j<num;j++){      String val = "";                    Random random = new Random();         for(int i = 0; i < length; i++)         {           String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; // 输出字母还是数字                        if("char".equalsIgnoreCase(charOrNum)) // 字符串           {             int choice = random.nextInt(2) % 2 == 0 ? 65 : 97; //取得大写字母还是小写字母             val += (char) (choice + random.nextInt(26));           }           else if("num".equalsIgnoreCase(charOrNum)) // 数字           {             val += String.valueOf(random.nextInt(10));           }         }      val=val.toLowerCase();      if(results.contains(val)){        continue;      }else{        results.add(val);      }    }    return results;                       }      }