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
set
A collection that never holds two equal elements.
A java.util.Set; HashSet promises no order, LinkedHashSet insertion order, TreeSet sorted order.