Is this correct usage of lambdas in Java 8?

Question

final List<String> userIds = request.getUserIds();
final List<String> keys = userIds.stream().map(p -> { 
    return removePrefix(p); 
}).collect(Collectors.toList());

Basically, every key in the list of userIds contains a prefix "_user" which I want to remove for every key. So, I am invoking the removePrefix function on each item of the list and storing that result in another list called "keys"

score 8 · Accepted Answer · answered Apr 14 '15 at 21:09

8

Yes it's fine although you could make it a little shorter and more readable with a method reference and a static import:

final List<String> keys = userIds.stream()
                                 .map(this::removePrefix)
                                 .collect(toList());

answered Apr 14 '15 at 21:09

assylias

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Is there any difference between `.map(p -> removePrefix(p))` and `.map(this::removePrefix)`. Yeah, the first one is longer with 2 characters, but anything else? – Nagy Vilmos Apr 14 '15 at 21:12
5

No functional difference - but I find method references more readable. It's obviously a matter of taste... – assylias Apr 14 '15 at 21:14
2

@NagyVilmos That can easily be verified... And here is more on the subject: http://stackoverflow.com/questions/24487805/lambda-expression-vs-method-reference – assylias Apr 14 '15 at 21:17
2

@Nagy Vilmos: no, they are slightly different. A method reference does not need a synthetic method to hold the lambda’s code but refers to the target method directly. Well, in most cases; sometimes it requires a helper method which is exactly like the lambda expression, e.g. for varargs methods or array constructors. – Holger Apr 15 '15 at 08:06

Alexis C. · Answer 2 · 2015-04-15T06:37:06.900

The answer of @assylias is nice, but if you are not worried about modifying the list in place (and that you are allowed to modify it via its ListIterator#set method), a good alternative could be to use replaceAll:

final List<String> userIds = request.getUserIds();
userIds.replaceAll(this::removePrefix);

replaceAll works here because your function is a function from T to T (where T is String in your case), so basically a UnaryOperator, which is a Function<T,T>.

If the mapping you wanted to apply would have been from a type T to U, then getting the list's stream and doing the mapping via the Stream#map method (like you did) is the standard idiom.

Is this correct usage of lambdas in Java 8?

2 Answers2