Lazy evaluation, collectors, parallel streams, and the readability and performance traps.
4 questions in Java & Spring.
It assumed unique keys and non-null values. Collectors.toMap throws IllegalStateException on a duplicate key and NullPointerException on a null value, so supply a merge function that encodes a real rule, use groupingBy when duplicates are legitimate, or collect into a HashMap directly if nulls must survive.
No. Since JDK 9, count() may skip the pipeline entirely when the size is known from the source and no stage adds or removes elements, so peek never runs. Stream laziness means stages run only when the terminal operation needs them, which makes side effects in peek or map unreliable rather than merely bad style.
Laziness means intermediate operations only describe a pipeline, so nothing traverses the source until a terminal operation pulls, and short-circuiting operations can stop early. Parallel helps when the source splits evenly, the work per element is CPU-bound and substantial. Use this streams answer to show the decision, trade-off, and evidence rather than a memorised definition.
A collector is a supplier of a mutable container, an accumulator that folds one element into it, a combiner that merges two containers, and a finisher. The combiner runs only in parallel, which is why an untested combiner is the usual defect, and the characteristics tell the pipeline what it may skip or reorder.