A truly random set of numbers could have the name number repeat many times. Similar to flipping 100 pennies and getting 99 heads and one tail. Probability theory is, in fact, concerned with calculating the odds on such an event (whuch incidentally is something like ½*½**½... for a grand total of 100 terms. And there's a much more compact mathematical notation for that, but it's hard enough to display fractions here.
However, the case where a number must
not repeat is a common one. For example, you cannot draw more than one Ace of Spades from an honest standard playing-card deck.
I don't think there's anything in the standard
Java repertoire to implement that directly, so probably the best you can do is set up a Map, save each drawn number as a key, then when you pull the next random number see if it's already in the map, and if so, keep pulling until you get a value that isn't.
Some people, when well-known sources tell them that fire will burn them, don't put their hands in the fire.
Some people, being skeptical, will put their hands in the fire, get burned, and learn not to put their hands in the fire.
And some people, believing that they know better than well-known sources, will claim it's a lie, put their hands in the fire, and continue to scream it's a lie even as their hands burn down to charred stumps.