Java SecureRandom generate secure random number

Question

How to generate a secure random number bit length is 256 bits using Java. What is the different between:

SecureRandom random = new SecureRandom(); 
                and
SecureRandom prng = SecureRandom.getInstance("SHA1PRNG");

Answer 1

With the first line, you let the JVM use the "secure random" implementation which is registered as default implementation in that JVM, whereas the second line forces the use of a specific implementation.

It is recommended to use the default (new SecureRandom()) because a specific Java-compatible platform could have a dedicated provider which is "better" (e.g. if the platform has a hardware RNG, the JVM might have been configured to use it with a specific implementation of the SecureRandomSpi interface, which you get by selecting the "default provider" but not by enforcing the use of "SHA1PRNG").