What is a method reference in Java?

A method reference is a shorthand notation for a lambda expression that calls an existing method. Instead of x -> x.toUpperCase(), write String::toUpperCase. There are four kinds: static method reference (ClassName::staticMethod), instance method reference on a specific instance (instance::method), instance method reference on an arbitrary instance of a type (ClassName::instanceMethod), and constructor reference (ClassName::new). Method references are more readable than lambdas when the lambda just calls an existing method without any additional logic: list.forEach(System.out::println) vs list.forEach(x -> System.out.println(x)).