Hamlet says to Rosencrantz and Guildenstern about their coming to see him, “I will tell you why; so shall my anticipation prevent your discovery.” What does this last phrase mean?

The last phrase means that his friends don't have to share what has passed between them and the King and Queen. In essence, he's exposing his knowledge that they've been sent for and willing to tell them what he will just because..... he really doesn't want to know the whys and the whats.