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

map

A collection of key-value pairs in which each key leads to one value.

A java.util.Map<K, V>; keys are unique and found with hashCode/equals (HashMap, LinkedHashMap) or compareTo (TreeMap).