What is the difference between map and forEach in Java streams in this case?

Question

I know map is not a terminal operation, but could someone explain this in more detail?

userList.stream().map(i -> {
System.out.println(i);
return i;
}).collect(Collectors.toList());

List<User> arrayList = new ArrayList<>();
userList.stream().forEach(i -> {
    System.out.println(i);
    arrayList.add(i);
});

Answer 1

Since map is not a terminal operation, it will be executed only when the terminal operation collect is executed.

So we should compare the two terminal operations of the two snippets - collect vs. forEach.

collect(Collectors.toList()) creates a List instance and adds all the elements of the Stream to it, after applying the Function passed to map on each of them. Applying the map Function prints each element in your case, and returns the element unchanged.

forEach applies the Consumer pass to it to it element of the Stream, so in your case it prints each element and adds it to the ArrayList you created prior to the stream pipeline.

Both snippets produce the same output, but in different ways. Note that Collectors.toList() currently produce an ArrayList, but it gives no guarantee regarding the type of List returned, so that may change in the future.