Does a sequential stream in Java 8 use the combine

问题:

If I call collect on a sequential stream (eg. from calling Collection.stream()) then will it use the combiner parameter I pass to collect? I presume not but I see nothing in the documentation. If I'm correct, then it seems unfortunate to have to supply something that I know will not be used (if I know it is a sequential stream).

回答1:

Keep in mind to develop against interface specifications -- not against the implementation. The implementation might change with the next Java version, whereas the specification should remain stable.

The specification does not differentiate between sequential and parallel streams. For that reason, you should assume, that the combiner might be used. Actually, there are good examples showing that combiners for sequential streams can improve the performance. For example, the following reduce operation concatenates a list of strings. Executing the code without combiner has quadratic complexity. A smart execution with combiner can reduce the runtime by magnitudes.

List<String> tokens = ...;
String result = tokens.stream().reduce("", String::concat, String::concat);