Day 21, Java Holiday Calendar 2016, Concatenate Java Streams

Today's tip is about concatenating streams. The task of the day is to construct a concatenated stream that lazily consumes a number of underlying streams. So, dumping the content from the various streams into a List and then stream from the list or using the Stream.builder() will not do.

As an example, we have three streams with words that are relevant to the US history and constitution:

        // From 1787
        final Stream preamble = Stream.of(
            "We", "the", "people", "of", "the", "United", "States"
        );

        // From 1789
        final Stream firstAmendment = Stream.of(
            "Congress", "shall", "make", "no", "law", 
            "respecting", "an", "establishment", "of", "religion"
        );

        // From more recent days
        final Stream epilogue = Stream.of(
            "In", "reason", "we", "trust"
        );

Creating a concatenated stream can be done in many ways including these:

        // Works for a small number of streams
        Stream.concat(
            preamble,
            Stream.concat(firstAmendment, epilogue)
        )
            .forEach(System.out::println);

        
        // Works for any number of streams
        Stream.of(preamble, firstAmendment, epilogue)
            .flatMap(Function.identity())
            .forEach(System.out::println);

Both methods will produce the same result and they will also close the underlying streams upon completion. Personally, I prefer the latter method since it is more general and can concatenate any number of streams. This is the output of the program:

We
the
people
of
the
United
States
Congress
shall
make
no
law
respecting
an
establishment
of
religion
In
reason
we
trust