title: Are Java random UUID's predictable?
body: <p>I would like to use a cryptographically secure primary key for sensitive data in a database - this cannot be guessable/predictable and it cannot be generated by the database (I need the key before the object is persisted).</p>
  
  <p>I understand Java uses a type 4 UUID with a cryptographically secure random number generator, however I know the UUID isn't completely random so my question is how safe is it to assume that uuids cannot be predicted from a set of existing ones?</p>