Free public beta until December 4, 2026:
every course is open, no card needed.
After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept.
How the beta works
Comparable
The interface whose compareTo method gives a type its natural order.
java.lang.Comparable with int compareTo(T other), negative, zero or positive; TreeSet, TreeMap and sorting use it when no Comparator is given.