wap to generate one time password otp?

import java.util.Random;

public class OtpGenerate
{

public static char[] otp(int len)
{
System.out.println("Generating OTP using Random class : ");
        System.out.print("Your  OTP Number  is : ");
String num="0123456789";
Random r1=new Random();
char [] otp=new char[len];
for(int i=0;i<len;i++)
{
otp[i]=num.charAt(r1.nextInt(num.length()));
}
return otp;

}

public static void main(String[] args) {
System.out.println(OtpGenerate.otp(4));
}
}

o/p:-

Generating OTP using Random class : 

Your  OTP Number  is : 3599