Best way to generate unique Random number in Java

I have to generate unique serial numbers for users consisting of 12 to 13 digits. I want to use random number generator in Java giving the system. Time in milliseconds as a seed to it. Please let me know about the best practice for doing so. What I did was like this

Random serialNo = new Random(System.currentTimeMillis());
System.out.println("serial number is "+serialNo);

Output came out as: serial number is java.util.Random@13d8cc98