JAVA - exponential distribution

I need to find formula for exponential distribution of probability, but i don´t know how to find it :( This formula have to have a powerful statistical properties(it can´t throw away any result from random to keep indistructed seek of random instance)

i am trying to find formula, which will work in method like this:

rand.getNextDuobleExpDistrib();

I have this code for now, but according to "input analyser" it doesn´t work correctly

public double getNext() {
        return -lampda * Math.log(rand.nextDouble());
 }