method reference vs lambda expression

Question

I want to replace lambda expression by method reference in the below example :

 public class Example {

        public static void main(String[] args) {
            List<String> words = Arrays.asList("toto.", "titi.", "other");
         //lambda expression in the filter (predicate)
            words.stream().filter(s -> s.endsWith(".")).forEach(System.out::println);
        }
   }

I want to write a something like this :

words.stream().filter(s::endsWith(".")).forEach(System.out::println);

is it possible to transform any lambda expression to method reference.

Possible duplicate of Method references for non-empty arguments? — fafl
– fafl, Commented Oct 24, 2016 at 10:01
You can't use :: if the argument is on the instance call. You can replace s -> "hi".equals(s) with "hi"::equals but not if it's s -> s.equals("hi") — Peter Lawrey
– Peter Lawrey, Commented Oct 24, 2016 at 10:07
under the hood MethodHandle can shuffle arguments, but this is not exposed in the java method reference syntax and I wonder if it would actually be any faster after inling. scala on the other hand can do that with partially applied functions — the8472
– the8472, Commented Oct 24, 2016 at 10:52
@the8472: MethodHandle can shuffle arguments, but the result is not a direct method handle anymore and LambdaMetaFactory does support direct method handles only. Partially applied functions, on the other hand, would work, as they don’t shuffle arguments, and LMF supports left-to-right parameter binding. So for .endsWith("."), where the right parameter ought to be bound, no chance… — Holger
– Holger, Commented Oct 24, 2016 at 11:21
The syntax for a method reference would be String::endsWith, plus some hypothetical way to bind the "." argument. What’s the advantage over s -> s.endsWith(".")? — Holger
– Holger, Commented Oct 24, 2016 at 11:24

Holger · Accepted Answer · 2016-10-24 11:39:42Z

4

There is no way “to transform any lambda expression to method reference”, but you can implement a factory for a particular target type, if this serves recurring needs:

public static <A,B> Predicate<A> bind2nd(BiPredicate<A,B> p, B b) {
    return a -> p.test(a, b);
}

with this, you can write

words.stream().filter(bind2nd(String::endsWith, ".")).forEach(System.out::println);

but actually, there’s no advantage. Technically, a lambda expression does exactly what you want, there’s the minimum necessary argument transformation code, expressed as the lambda expression’s body, compiled into a synthetic method and a method reference to that synthetic code. The syntax
s -> s.endsWith(".") also is already the smallest syntax possible to express that intent. I doubt that you can find a smaller construct that would still be compatible with the rest of the Java programming language.

answered Oct 24, 2016 at 11:39

Holger

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2 Comments

midy62 Over a year ago

why when i use : Supplier<String> x=String::toUpperCase; i get "Cannot make a static reference to the non-static method toUpperCase() from the type String" but when i use BiPredicate<String,String> x=String::endsWith; this work fine

Holger Over a year ago

How is toUpperCase supposed to be a Supplier? It requires a String to convert to upper case, so you have to use a function type that consumes a String, e.g. Function<String,String> or UnaryOperator<String>. Or you bind an instance like Supplier<String> x="foo"::toUpperCase, but there is not much use in a supplier that always supplies "FOO"…

Nikhil Nanivadekar · Accepted Answer · 2016-10-27 02:12:10Z

You can use selectWith() from Eclipse Collections. selectWith() takes a Predicate2 which takes 2 parameters instead of a Predicate. The second parameter to selectWith() gets passed as the second parameter to the Predicate2 every time it's called, once per item in the iterable.

MutableList<String> words = Lists.mutable.with("toto.", "titi.", "other");
words.selectWith(String::endsWith, ".").each(System.out::println);

By default Eclipse Collections is eager, if you want to iterate lazily then you can use asLazy()

words.asLazy().selectWith(String::endsWith, ".").each(System.out::println);

If you can't change from List:

List<String> words = Arrays.asList("toto.", "titi.", "other");
ListAdapter.adapt(words).selectWith(String::endsWith, ".").each(System.out::println);

Eclipse Collections' RichIterable has several other *With methods which work well with method references, including rejectWith(), partitionWith(), detechWith(), anySatisfyWith(), allSatisfyWith(), noneSatisfyWith(), collectWith()

Note: I am a contributor to Eclipse Collections.

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