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

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.

It's :

numbers.forEach(i -> {System.out.println(i);});

or even simpler :

numbers.forEach(i -> System.out.println(i));