Is it possible to instantiate abstract class using anonymous inner class and mechanism to access method within it

Question

There is no any way to create object using abstract class. But when using anonymous inner class it is possible to run the following code. And not only that start() method is not accessible, therefore What is the reason for running following program without giving any compile error and what is the mechanism to access start() method.

abstract class Vehicle{
        abstract void park();
    }   

    class Demo{
        public static void main(String args[]){
            Vehicle v1=new Vehicle(){
                int speed;
                void park(){
                    System.out.println("Parking for cars...");
                }
                void start(){
                    System.out.println("Start...");
                }
            };
            v1.park();
        }
    }

Anonymous Coward · Accepted Answer · 2015-09-05T08:01:20.953

Consider the following piece of code (code 2).

abstract class Vehicle{
    abstract void park();
}   

class Demo{
  static class Car extends Vehicle {
       int speed;
       void park(){
                System.out.println("Parking for cars...");
       }
   }

   public static void main(String args[]){
        Vehicle v1=new Car();
        v1.park();
   }
}

It is no surprise that the previous piece of code gives no compiler error.
Car extends Vehicle and Car is not abstract. Hence you can instantiate it.

Lets see what happens when code 2 is compiled :

javac *.java
list .class files
Demo$Car.class  Demo.class  Vehicle.class

And this is what happens when the code in the question (code 1) is compiled :

javac *.java
list .class files
Demo$1.class  Demo.class  Vehicle.class

We get the same except we get Demo$1.class in code 1 and Demo$Car.class in code 2.
That happens because code 1 is actually creating a class which inherits from Vehicle. A so called anonymous class. The class has no name(*) but it is still a full fledged class which inherits from Vehicle and can be instantiated just as Car can be instantiated.

This code :

Vehicle v1=new Vehicle(){

is NOT instantiating an object of class Vehicle. It is instantiating an object of a class which inherits from Vehicle which has no name.

(*) Actually it does have a name. Its name is "1 in Demo" aka Demo$1. The JVM needs a name to be able to run it, you can't just tell the JVM to run something without telling it what to run. The name is one which is not a sintactically valid name for a class; can't name a class as 1. This is by dessign as it ensures that annonymous class names won't clash with normal class names.

You may want to include that your Terminal commands are for *nix systems only. — saagarjha, Sep 05 '15 at 06:35
Ok, will clarify that. javac is common to any platform with JDK though. — Anonymous Coward, Sep 05 '15 at 08:00

score 4 · Answer 2 · answered Sep 05 '15 at 06:15

this is exactly the idea in abstract classes. you can't instanciate the abstract class, but you can instanciate any sub-class of it that implements the abstract methods. Such sub classes can be concrete classes, or anonymous classes like you did here, so no reason for this code not to compile

score 1 · Answer 3 · answered Sep 05 '15 at 06:18

Anonymous class means you have declared local class in your Demo. Anonymous class is an expression which can have the implementation of abstract methods. Mainly anonymous class is a local implementation of interface or abstract class which must contain implementation of abstract methods and same way implementation of abstract methods of abstract class. So, in short you can instantiate abstract class with anonymous class.

Here, you are actually declaring your whole class Vehicle as anonymous class in your Demo with required implementation. Note here that you will have Demo$1.class file after compiling the Demo class which is an anonymous class and implementation of your Vehicle in Demo.

score 1 · Answer 4 · answered Sep 05 '15 at 06:19

1

Though you are writing new Vehicle() but its not creating an instance of Vehicle class. Its creating an instance of subtype of Vehicle which does not have any name

answered Sep 05 '15 at 06:19

Rahman

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user3437460 · Answer 5 · 2015-09-05T06:42:32.980

There is no any way to create object using abstract class. But when using anonymous inner class it is possible to run the following code. What is the reason for runinng following program without giving any compile error.

The code runs because you are not instantiating an object directly from the abstract class. When you create an anonymous class from abstract class or interface you are actually creating a new class extending/implementing to the abstract class or interface.

It does not really matters whether you are using an inner class or not.

In short, when you create an anonymous class, it is the new "nameless" class which extends the abstract class gets instantiated !

Is it possible to instantiate abstract class using anonymous inner class and mechanism to access method within it

5 Answers5