Javaランダム生成定長純数字または数字アルファベット混合数

package randomdata;
import java.util.Random;
public class RandomCharData {
    //          int  ，   ASCII 
    public static void main(String[] args) {
        //       
        for(int i=0;i<15;i++)
            createData(20);
        System.out.println("---------------");
                          
        //      
        for(int i=0;i<15;i++)
        createRandomCharData(20);
    }
    //                 
    //0~9 ASCII 48~57
    //A~Z ASCII 65~90
    //a~z ASCII 97~122
    public static void createRandomCharData(int length)
    {
        StringBuilder sb=new StringBuilder();
        Random rand=new Random();//            
        Random randdata=new Random();
        int data=0;
        for(int i=0;i<length;i++)
        {
            int index=rand.nextInt(3);
            //           ，     
            switch(index)
            {
            case 0:
                 data=randdata.nextInt(10);//     0~9
                 sb.append(data);
                break;
            case 1:
                data=randdata.nextInt(26)+65;//      65~90     
                sb.append((char)data);
                break;
            case 2:
                data=randdata.nextInt(26)+97;//      97~122     
                sb.append((char)data);
                break;
            }
        }
        String result=sb.toString();
        System.out.println(result);
    }
                      
    //               
    public static void createData(int length) {
        StringBuilder sb=new StringBuilder();
        Random rand=new Random();
        for(int i=0;i<length;i++)
        {
            sb.append(rand.nextInt(10));
        }
        String data=sb.toString();
        System.out.println(length+" random data: "+data);
    }
}

97862483216351602095
70514049428709111734
58909883924816310266
57687118857245129003
61026032192487969045
24071314312917474726
33994529915727126720
96782018153746273742
63798758707067214215
07076213273633627868
48227106476502535792
59944228129822555128
20025042941676211212
70116211781380045731
88967160116193801569
---------------
0Qa0cZYe6YyMf4zrLEzEJqK6f
MwwDl6Kan92RseOPWJuf6WpnF
0H1249B0Y7qoGmbj0ZWeCliId
PIsRRAS99XIbPwBqCdV7BUt7e
X6R5cotl638K83c4fowNv0S3j
03p7YzBA0q2OlB70pL4MTQf5P
7x5Yd57SOYGoTS9ABi8RA2F5y
1XVK9wrhrFxINd4gpi4Ip05Hg
r53YoB2aBgeP2i4061A2CMNK2
X1R3VWi7423VSs0j0u6R7uMy7
uCQ19SGe4U19V9U4XVqB5QDmc
2OaJ1O3JC52617kMTH2yK9yyJ
kM85MalnsR8T299yR8B0bP5DH
rAApB65VXpj4qxp0gEj4Oh34I
3k88S216XWbK83XaCTnvin02f