Steve Steve - 2 months ago 6
Java Question

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

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

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.