Java How to Program, Early Objects, 11th Edition

... System.out::println

in this case is a method reference—a shorthand notation for a lambda that calls the specified method. A method reference of the form

objectName::instanceMethodName

is a bound instance method reference—“bound” means the specific object to the left of the :: (System.out) must be used to call the instance method to the right of the :: (println). The compiler converts System.out::println into a one-parameter lambda like

x -> System.out.println(x)

that passes the lambda’s argument—the current stream element (represented by x)—to the System.out object’s println instance method, which implicitly outputs the String representation of the argument. The stream pipeline of lines 12–13 is equivalent to the following for loop:

for (int i = 1; i <= 10 ...