How do I get the first element while continue streaming?

Question

I have a stream of generic items. I'd like to print the class name of the first item + the toString() of all the items.

If I had an Iterable, it would look like this:

Iterable<E> itemIter = ...; boolean first = true; for (E e : itemIter) {     if (first) {         first = false;         System.out.println(e.getClass().getSimpleName());     }     System.out.println(e); } 

Can I do this on a stream (Stream<T>) with the stream API?

* Please note that it's a question about streams - not about iterators. I have a stream - not an iterator.

Answer 1

There is StreamEx library that extends standard Java's Stream API. Using StreamEx.of(Iterator) and peekFirst :

StreamEx.of(itemIter.iterator())         .peekFirst(e -> System.out.println(e.getClass().getSimpleName()))         .forEach(System.out::println); 

Answer 2

If your starting point is a Stream and you want to retain all of its properties and the laziness, the following solution will do:

public static <E> Stream<E> forFirst(Stream<E> stream, Consumer<? super E> c) {     boolean parallel = stream.isParallel();     Spliterator<E> sp = stream.spliterator();     return StreamSupport.stream(() -> {         if(sp.getExactSizeIfKnown() == 0) return sp;         Stream.Builder<E> b = Stream.builder();         if(!sp.tryAdvance(b.andThen(c))) return sp;         return Stream.concat(b.build(), StreamSupport.stream(sp, parallel)).spliterator();     }, sp.characteristics(), parallel); } 

E.g. when you use it with

List<String> list = new ArrayList<>(List.of("foo", "bar", "baz")); Stream<String> stream = forFirst(         list.stream().filter(s -> s.startsWith("b")),         s -> System.out.println(s+" ("+s.getClass().getSimpleName()+')')     ).map(String::toUpperCase); list.add(1, "blah"); System.out.println(stream.collect(Collectors.joining(" | "))); 

it will print

blah (String) BLAH | BAR | BAZ 

demonstrating that the processing will not start before commencing the terminal operation (collect), hence reflecting the preceding update to the source List.