forEach vs forEachOrdered in Java 8 Stream

Question

I understand that these methods differ the order of execution but in all my test I cannot achieve different order execution.

Example:

System.out.prin         


        

Answer 1

Although forEach shorter and looks prettier, I'd suggest to use forEachOrdered in every place where order matters to explicitly specify this. For sequential streams the forEach seems to respect the order and even stream API internal code uses forEach (for stream which is known to be sequential) where it's semantically necessary to use forEachOrdered! Nevertheless you may later decide to change your stream to parallel and your code will be broken. Also when you use forEachOrdered the reader of your code sees the message: "the order matters here". Thus it documents your code better.

Note also that for parallel streams the forEach not only executed in non-determenistic order, but you can also have it executed simultaneously in different threads for different elements (which is not possible with forEachOrdered).

Finally both forEach/forEachOrdered are rarely useful. In most of the cases you actually need to produce some result, not just side-effect, thus operations like reduce or collect should be more suitable. Expressing reducing-by-nature operation via forEach is usually considered as a bad style.