Local classes with Lambda expressions

Question

As I tested, the below code executes without any issues. But I could not understand the logic. Can someone please explain?

public static void main(String[] args) {
    List<String> london = new ArrayList<>(Arrays.asList("Chinatown","Croydon","Eden Park"));
    class City{
        private String name;
        City(String name){
            this.name = name;
        }

        public void printCity(){
            System.out.println(name);
        }
    }

    london.stream().map(City::new).forEach(City::printCity); //Chinatown,Croydon,Eden Park
}

In the above example code, I have following questions.

The foreach method always takes a consumer object. In here printCity method is not a method that takes an argument. Still it works. How?
printCity method is not a static method here. How a City itself can call a instance method?

Consumer takes the object, which is passed via the method reference. It could also be expanded to c -> c.printCity() — ifly6
– ifly6, Commented Feb 20, 2018 at 3:36
It’s funny that you asked two questions, which answer each other… — Holger
– Holger, Commented Feb 20, 2018 at 13:47

curlyBraces · Accepted Answer · 2018-02-20 03:37:40Z

5

In your last statement you have currently used method references. This can be alternatively written as following:

london.stream().map(name -> new City(name)).forEach(city -> city.printCity());

As you can see above, the map returns City instances. So forEach gets City instances and calls printCity on each instance.

When using method references, City::printCity is not the same as calling City.printCity. See Method References documentation

Method references are just easy-to-read lambda expressions.

answered Feb 20, 2018 at 3:37

curlyBraces

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1 Comment

Shamran Siddique Over a year ago

Thanks! Good explanation

ifly6 · Accepted Answer · 2018-02-20 21:31:27Z

2

The consumer is equivalent to something like c -> c.printCity(), (City c) -> c.printCity(), or some long anonymous class that is a massive pain to type out.

The city instances are the subject of invocation. There is just syntactical sugar in the form of City::printCity which passes the method to the expression, where the instance is called.

edited Feb 20, 2018 at 21:31

answered Feb 20, 2018 at 3:37

ifly6

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