You’d use Random to generate a random number, then test it against a literal to match the probability you’re trying to achieve.
So given:
boolean val = new Random().nextInt(25)==0;
val will have a 1/25 probability of being true (since nextInt() has an even probability of returning any number starting at 0 and up to, but not including, 25.)
You would of course have to import java.util.Random; as well.
As pointed out below, if you’re getting more than one random number it’d be more efficient to reuse the Random object rather than recreating it all the time:
Random rand = new Random();
boolean val = rand.nextInt(25)==0;
..
boolean val2 = rand.nextInt(25)==0;