6

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);
            }

        });
    }

}
mino
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  • This looks like it might be a duplicate question. Try going here http://stackoverflow.com/questions/4000062/can-we-create-an-object-of-an-interface – astidham2003 Apr 18 '12 at 00:44

3 Answers3

17

This is called an anonymous inner class.

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

SLaks
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3

Your assumption is correct, and you cannot instantiate an interface. You can instantiate an anonymous class however, which is what the code is doing.

Jim Barrows
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1

You need a Printer object for the print function of NumberPrinter. When you call that function you don't actually instantiate the Printer interface but you instantiate an implementation of it and this is why it's working.

Your assumption was correct by the way.

Chris911
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  • So, sorry, I'm confused why can you pass it an anonymous inner class? Is it an object of type Printer? – mino Apr 18 '12 at 00:34
  • Since this is anonymous inner class there's no object to refer to so I don't think there's any Printer object here. The new Printer() call creates a new instance of that class and returns it as the result of the statement. – Chris911 Apr 18 '12 at 00:43