Question

So I just found this code example online a while ago and I’m going over it again but quite confused.

From looking at it, what I gather (and it might be wrong) is that it passes to the print method in the NumberPrinter class a Printer object. However, the interface is also called Printer, so aren’t we instantiating an anonymous class of the Printer interface, defining the methods and then passing it?

My basic question is, is my initial assumption correct? And if so I thought you could not instantiate an interface?

public class NumberPrinter {

    public interface Printer {
        public void print (int idx);
    }

    public static void print (Printer p) {
        for (int i = 0; i < 4; i++) {
            p.print(i);
        }
    }

    public static void main(String[] args) {
        print(new Printer() {

            @Override
            public void print(int idx) {
                System.out.println(idx);
            }

        });
    }

}

Answer 1

This is called an anonymous inner class.

It creates an un-named class that implements the Printer interface.