Data Structures & Algorithms in Java — Free Course
A free, full-year second course in computer science, taught in Java. Build twelve data structures from scratch — linked lists, hash maps, BSTs, tries, heaps, union-find, graphs — then compose them. Runs in the browser, no JDK install.
Common questions
What do I need before taking this course?
AP Computer Science A or a full-year introductory Java course. You should be comfortable with variables, control flow, methods, arrays, 2D arrays, ArrayList, classes, and objects. Unit 1 rebuilds the parts of Java the AP CSA syllabus no longer covers — interfaces, generics, iterators, and the equals/hashCode contract — so you do not need those going in.
Is this an AP course?
No. AP Computer Science AB, the College Board data-structures course, was discontinued after the May 2009 exam and nothing replaced it. AP CSA is the only AP Java course. This course is built to the ACM/IEEE CS2023 "CS2: Data Structures" outline instead, which is what college data-structures courses follow. It exceeds the core CSTA standards — no CSTA level expects structures implemented from scratch or Big-O reasoning — while aligning directly with the 2026 CSTA Software Development specialty standards, and every lesson carries a CSTA standards mapping. See the syllabus for the full picture.
Do I need to install anything?
No. Everything runs in the browser Java IDE, including generics, HashMap, TreeMap, ArrayDeque, PriorityQueue, Comparator, and classes you write that implement Iterable. It works on Chromebooks, iPads, and locked-down school devices.
How is the work graded?
On cost, not just correctness. Assignments check that your solution produces the right answer, that you built the structure yourself rather than wrapping a library class, and that it runs within an operation budget — so a solution that is correct but the wrong complexity does not pass.
How long is the course?
A full year — 12 units, 81 lessons, about 165 class days across 33 teaching weeks. Teachers get a published compression ladder that says exactly what to cut, and in what order, when the calendar slips.
Does it cost anything?
Every lesson is free with a free CS++ account, and so are the printable handouts and the editable slide decks. A one-time $9.99 Student Membership unlocks exams 2 through 5, the full question banks, and AI features. There is no subscription.