What is the equivalent lambda expression for System.out::println

java lambda java-8 method-reference
Steve picture Steve · Jan 19, 2015 · Viewed 19.5k times · Source

I stumbled upon the following Java code which is using a method reference for System.out.println:

class SomeClass {
    public static void main(String[] args) {
           List<Integer> numbers = Arrays.asList(1,2,3,4,5,6,7,8,9);
           numbers.forEach(System.out::println);
        }
    }
}

What is the equivalent lambda expression for System.out::println?

Answer

Holger picture Holger · Jan 19, 2015

The method reference System.out::println will evaluate System.out first, then create the equivalent of a lambda expression which captures the evaluated value. Usually, you would use
o -> System.out.println(o) to achieve the same as the method reference, but this lambda expression will evaluate System.out each time the method will be called.

So an exact equivalent would be:

PrintStream p = Objects.requireNonNull(System.out);
numbers.forEach(o -> p.println(o));

which will make a difference if someone invokes System.setOut(…); in-between.

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