dynamic change letter java

import java.util.Random;

public class alphanumber {

private static Random rand = new Random();

public static String generate(String characters, int n) {
char[] array = characters.toCharArray();
shuffle(array);
char[] temp = new char[n];
System.arraycopy(array, 0, temp, 0, n);
return new String(temp);
}

public static void shuffle(char[] array) {
int N = (100 + rand.nextInt(1000)) * array.length;
for(int i = 0; i < N; i++) {
int idx1 = rand.nextInt(array.length);
int idx2 = rand.nextInt(array.length);
char temp = array[idx1];
array[idx1] = array[idx2];
array[idx2] = temp;
}
}

public static void main(String[] args) {
for(int i = 0; i < 1000; i++) {
StringBuffer sb=new StringBuffer();
String s = generate("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789", 7);
char data[]=s.toCharArray();
sb.append("{'").append(data[0]).append("','").append(data[1]).append("','").append(data[2]).append("','");
sb.append(data[3]).append("','");
sb.append(data[4]).append("','");
sb.append(data[5]).append("','");
sb.append(data[6]).append("'},");

System.out.println(sb.toString());
}
}
}