Read a real prompt
Every question is grounded in the same dataset of people records, so you spend your time on the Java, not on parsing new context.
Maps · Arrays · Lists · Collections
Learn Java the way you'll actually use it.
Build practical Java skills with realistic data problems: filter users, group records, create lookup maps, calculate summaries, and sort results.
40
Questions
5
Levels
How it works
Three steps, every question.
Write your solution
Use the built-in browser IDE to write real Java against arrays, lists, maps, sets, and streams — no setup required.
Run & compare
Your code compiles and runs server-side. See expected versus actual output instantly, with diagnostics on failures.
What you'll practice
Real collections. Real application patterns.
Arrays
Indexing, iteration, and transforming raw arrays before they ever become a collection.
Lists
ArrayList mechanics, iteration order, mutation pitfalls, and converting to and from streams.
Maps
Lookups, grouping with Collectors.groupingBy, merging values, and building derived keys.
Sets
Deduplication, membership checks, and set operations across records in the dataset.
Streams
filter, map, distinct, reduce, and multi-step pipelines that chain cleanly together.
Sorting & Comparators
Comparator chains, reversed ordering, thenComparing, and custom sort keys.
Curriculum
Beginner to master.
One dataset
Same records. Harder questions as you go.
Every challenge, from your first filter to a multi-step grouped summary, runs against the same set of people records. Less time parsing new data on every question, more time practicing Java.
Server-side Java compile checks
Expected versus actual output diffing
Shared dataset for every single question
Go To Practice
Sample rows
| name | age | language |
|---|---|---|
| Ava | 14 | Java |
| Cleo | 17 | Python |
| Emi | 29 | JavaScript |
| Gia | 31 | Java |
| Moe | 35 | Go |