Build a URL-shortener click report that uses groupingBy, partitioningBy, toMap, teeing and a custom Collector on Java 21, with 21 checks and the errors I hit.
Collectors.toMap throws IllegalStateException: Duplicate key when two elements map to the same key. Exact causes, JDK 8-25 message differences, and the fixes.
How to remove duplicates from a list in Java and keep order: LinkedHashSet, distinct(), removeIf and by-key dedupe, on a parcel-scan feed with 21 checks.
Turn a for loop into a Java streams pipeline that filters and transforms a list. Build a gradebook in Java 21, trip every classic mistake, and check it all.
Flatten students, courses and assignments with flatMap, flatMapToInt, Optional.stream and mapMulti on Java 21. 22 checks, forced errors and the fixes included.
Gopi Gorantala