JavaExercism/exercises/collatz-conjecture/src/example/java/CollatzCalculator.java

class CollatzCalculator {

    int computeStepCount(final int start) {
        if (start <= 0) throw new IllegalArgumentException("Only natural numbers are allowed");

        if (start == 1) return 0;

        final int next;

        if (start % 2 == 0) {
            next = start / 2;
        } else {
            next = 3 * start + 1;
        }

        return 1 + computeStepCount(next);
    }

}