Implement sorting with Java Streams

Problem Sort a collection of objects using the Java Streams API, then extend it to multi-key ordering. Be able to fall back to implementing a sort manually if asked.

Input / Output

• Input: a List<T> (e.g. employees) and one or more sort keys.
• Output: a new sorted list (Streams are non-mutating).

Expected approach

• list.stream().sorted(Comparator.comparing(T::key)).collect(toList()).
• Multi-key: Comparator.comparing(...).thenComparing(...), reversed(), and nullsFirst/nullsLast for null handling.
• Discuss stability of the sort, parallelStream() and when it helps/hurts, and the underlying O(n log n) cost.

Discussion points

• Difference between sorted() and sorting the source list in place (Collections.sort).
• Comparator vs Comparable (natural ordering).
• Pitfalls of mutating shared state inside a stream pipeline.