Ryfujk

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);

}

Any idea if I can do this on a stream (Stream<T>) with stream api? Thanks.

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

Stream in Java is not reusable. This means, that consuming stream can be done only once. If you get the first element from a stream, you can iterate over it one more time.

Workaround would be to create another stream same as the first one or getting the first item and then creating a stream, something like that:

Stream<E> stream = (List<E>) itemIter).stream();

E firstElement = itemIter.next();

stream.foreach(...);

There is StreamEx library that extend standard java stream api. Using StreamEx.of(Iterator) and peekFirst :

StreamEx.of(itemIter.iterator())

        .peekFirst(e -> System.out.println(e.getClass().getSimpleName()))

        .forEach(System.out::println);

You can abuse reduction:

Stream<E> stream = ...;

System.out.println(stream

                   .reduce("",(out,e) -> 

                   out + (out.isEmpty() ? e.getClass().getSimpleName()+"n" : "")

                   + e));

Using streams possibly in two steps as:

((List<E>) itemIter).stream()

        .findFirst()

        .ifPresent(e -> System.out.println(e.getClass().getSimpleName()));

((List<E>) itemIter)

        .stream()

        .skip(1)

        .forEach(System.out::println);

or alternatively as :

IntStream.range(0, ((List<E>) itemIter).size()).mapToObj(a -> {

    if (a == 0) {

        return ((List<E>) itemIter).get(a).getClass().getSimpleName();

    } else {

        return ((List<E>) itemIter).get(a);

    }

}).forEach(System.out::println);

You could use peek for that:

AtomicBoolean first = new AtomicBoolean(true);

itemIter.stream()

    .peek(e -> {

        if(first.get()) {

            System.out.println(e.getClass().getSimpleName());

            first.set(false);

        }

    })

    ...

One workaround is to do it like this -

import java.util.*; 

import java.util.stream.Collectors;

public class MyClass {

   static int i = 0;

   static int getCounter(){

       return i;

   }

   static void incrementCounter(){

       i++;

   }

    public static void main(String args[]) {

        List<String> list = Arrays.asList("A", "B", "C", "D", "E", "F", "G"); 

        List<String> answer = list.stream().filter(str -> {if(getCounter()==0) {System.out.println("First Element : " + str);} incrementCounter(); return true;}). 

        collect(Collectors.toList()); 

        System.out.println(answer); 

    }

}

Output :

First Element : A

[A, B, C, D, E, F, G]