How to create an array of 20 random bytes?

Try the Random.nextBytes method:

byte[] b = new byte[20];
new Random().nextBytes(b);

If you want a cryptographically strong random number generator (also thread safe) without using a third party API, you can use SecureRandom.

Java 8 (more secure than previous versions):

byte[] bytes = new byte[20];
SecureRandom.getInstanceStrong().nextBytes(bytes);

Java 6 & 7:

SecureRandom random = new SecureRandom();
byte[] bytes = new byte[20];
random.nextBytes(bytes);

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