How to parallely iterate a collection in java?

The Stream API in Java 1.8 was designed with parallelism in mind, so you can convert any Stream to a parallel stream. for example:

 Stream.of(1,2,3,4,5,6,7).parallel().forEach(System.out::println); 

And In fact every Stream can be converted into a parallel one

Stream<Integer> parallel = stream.parallel(); 

And Collection provides a method that return a parallelStream.

Stream<Integer> parallel = Arrays.asList(1,2,3).parallelStream(); 

Now, this does not mean every operation can be efficiently parallelised, it depends on the nature of the source collection and the task you doing. Check this out

Assuming you're on Java 8, you should use streams. If you're not on Java 8, there's no baked-in equivalent.

sandwichCollection.parallelStream().forEach((sandwich) -> { sandwich.eat(); });