abstract class and anonymous class

Question

abstract class Two {
    Two() {
        System.out.println("Two()");
    }
    Two(String s) {
        System.out.println("Two(String");
    }
    abstract int  display();
}
class One {
    public Two two(String s) {
        return new Two() {          
            public int display() {
                System.out.println("display()");
                return 1;
            }
        };
    }
}
class Ajay {
    public static void main(String ...strings ){
        One one=new One();
        Two two=one.two("ajay");
        System.out.println(two.display());
    }
}

we cannot instantiate an abstract class then why is the function Two two(String s) able to create an instance of abstract class Two ????

score 11 · Accepted Answer · answered Mar 01 '11 at 12:31

11

It does not create an instance of abstract Two. It creates a concrete, anonymous class that extends Two and instantiates it.

It's almost equivalent to using a named inner class like this:

class One {
    public Two two(String s) {
        return new MyTwo();
    }

    class MyTwo extends Two {
        public int display() {
            System.out.println("display()");
            return 1;
        }
    }
}

answered Mar 01 '11 at 12:31

Konrad Garus

53,145
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2

[Stop pre-reading my mind](http://stackoverflow.com/questions/5154740/abstract-class-and-anonymous-class/5154764#5154764)! – Joachim Sauer Mar 01 '11 at 12:55
Is there a way to make MyTwo Anonymous? – Usman Ismail Aug 25 '11 at 14:37
@Usman Yes, that's what the original poster's question does. – Konrad Garus Aug 26 '11 at 07:28

score 2 · Answer 2 · answered Mar 01 '11 at 12:32

2

Because it implements the missing function display(). It returns an anonymous subclass of Two. You can see this if you look at the compiled files. You will have a One$1.class there, which extends Two.class!

answered Mar 01 '11 at 12:32

Daniel

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abstract class and anonymous class

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