Using Stream StreamSupplier

Question

I'm using following code to reuse Stream but getting

java.lang.IllegalStateException: stream has already been operated upon or closed

Code

public static void main(String[] args) {

    try {
        String[] array = { "a", "b", "c", "d", "e" };
        Stream<String> ss = Stream.of(array);
        Supplier<Stream<String>> streamSupplier = () -> ss;

        long count = streamSupplier.get().count();
        // get new stream
        streamSupplier.get().forEach(x -> System.out.println(x));

        // get another new stream

        System.out.println(count);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Answer 1

Stream::count is the terminal operation which closes the Stream, so no more pipelines can proceed.

You create the Stream<String> before you put it to the Supplier. If you want the Supplier to provide a new Stream witch each call, create the Stream<String> directly in the Supplier.

Supplier<Stream<String>> streamSupplier = () -> Stream.of(array);

Then streamSupplier.get() will always work with the new Stream<Stream>.

---

Yet, you might achieve the same result with Stream::peek which works the same like Stream::forEach with an exception that it doesn't close the Stream but returns the same unmodified one (@Lino was faster).

long count = streamSupplier.get().peek(System.out::println).count();

Answer 2

Don't assign Stream.of(array) to an intermediate variable, just return it directly in the Supplier:

Supplier<Stream<String>> streamSupplier = () -> Stream.of(array);

That is because previously you would just always supply the same reference when calling supplier.get() but in fact you wanted to return a new Stream.

Also as @Eugene suggested, using Arrays.stream() over Stream.of() is preferred. As the latter is a varargs method, but just delegates to the former.

---

Also your current approach can be simplified using the Stream.peek() method:

long count = Arrays.stream(array)
    .peek(System.out::println)
    .count();