Java Generating Secure Random

SecureRandom class in Java provides cryptographically strong random number generator (RNG). A cryptographically strong random number minimally complies with the statistical random number generator tests specified in FIPS 140-2. The seed material passed to the random number must be unpredictable. 

A caller obtains a SecureRandom  via a no argument constructor or one of the getInstance methods. 

SecureRandom random = new SecureRandom();

byte bytes[] = new byte[20];

random.nextBytes(bytes);

Many random number generators are in the form of pseudo - random number generator (PRNG), which means they use deterministic algorithm to produce a pseudo-random sequence from a true random seed. Other implementations may produce true random numbers, and yet others may use combination of these two techniques. 

Also, if the requirement is only to generate a unique identifier which is not securely strong, below approach can be used as well.  

 String uuid = UUID.randomUUID().toString();

 uuid = uuid.replaceAll("-", "z");

 return uuid;

References:

https://docs.oracle.com/javase/7/docs/api/java/security/SecureRandom.html