MarcG MarcG - 1 year ago 80
Java Question

Adding two Java 8 streams, or an extra element to a stream

I can add streams or extra elements, like this:

Stream stream = Stream.concat(stream1, Stream.concat(stream2, Stream.of(element));

And I can add new stuff as I go, like this:

Stream stream = Stream.concat(
stream1.filter(x -> x!=0), stream2)
.filter(x -> x!=1),
.filter(x -> x!=2);

But this is ugly, because
is static. If
were an instance method, the above examples would be much easier to read:

Stream stream = stream1.concat(stream2).concat(element);


Stream stream = stream1
.filter(x -> x!=0)
.filter(x -> x!=1)
.filter(x -> x!=2);

My question is:

1) Is there any good reason why
is static? Or is there some equivalent instance method I'm missing?

2) In any case, is there a better way of doing this?

Answer Source

If you add static imports for Stream.concat and Stream.of, the first example could be written as follows:

Stream<Foo> stream = concat(stream1, concat(stream2, of(element)));

It's a bad idea to import static methods with names, that are used for different purposes. So it might be better to create your own static methods with more meaningful names. However, for demonstration I will stick with this name.

public static <T> Stream<T> concat(Stream<? extends T> lhs, Stream<? extends T> rhs) {
    return Stream.concat(lhs, rhs);
public static <T> Stream<T> concat(Stream<? extends T> lhs, T rhs) {
    return Stream.concat(lhs, Stream.of(rhs));

With these to static methods (optionally in combination with static imports), the two examples could be written as follows:

Stream<Foo> stream = concat(stream1, concat(stream2, element));

Stream<Foo> stream = concat(
                         concat(stream1.filter(x -> x!=0), stream2).filter(x -> x!=1),
                     .filter(x -> x!=2);

The code is now significantly shorter. However, I agree that the readability hasn't improved. So I have another solution.

In a lot of situations, Collectors can be used to extend the functionality of streams. With the two Collectors at the bottom, the two examples could be written as follows:

Stream<Foo> stream = stream1.collect(concat(stream2)).collect(concat(element));

Stream<Foo> stream = stream1
                     .filter(x -> x!=0)
                     .filter(x -> x!=1)
                     .filter(x -> x!=2);

The only difference between your desired syntax and the syntax above is, that you have to replace concat(...) with collect(concat(...)). The two static methods can be implemented as follows (optionally used in combination with static imports):

private static <T,A,R,S> Collector<T,?,S> combine(Collector<T,A,R> collector, Function<? super R, ? extends S> function) {
    return Collector.of(
public static <T> Collector<T,?,Stream<T>> concat(Stream<? extends T> other) {
    return combine(Collectors.toList(),
        list -> Stream.concat(, other));
public static <T> Collector<T,?,Stream<T>> concat(T element) {
    return concat(Stream.of(element));

Of course there is a drawback with this solution that should be mentioned. collect is a final operation that consumes all elements of the stream. On top of that, the collector concat creates an intermediate ArrayList each time it is used in the chain. Both operations can have a significant impact on the behaviour of your program. However, if readability is more important than performance, it might still be a very helpful approach.