Java：通过主方法的抽象类调用

Question

Okay, people are probably going to run to flag this as a duplicate, just by reading the title and without really reading the question. So please know that I HAVE tried to look at other questions on this platform, but have not found something that clears my doubts exactly. Kindly allow me to reach out and ask my question. Thanks in advance.

Interface is absolutely abstract and cannot be instantiated; A Java abstract class also cannot be instantiated, but can be invoked if a main() exists.

I do not completely understand the latter part of the statement. Is this talking about the main method being directly within the abstract class itself ? Is it talking about invoking the abstract class via a child's main method ? Or both ?

Secondly, I have seen examples like the following.

abstract class Printer
{
     public void print() { … };
}

public class TestPrinter
{
     public static void main( String[] args )
     {
          // use of anonymous class
          final Printer p = new Printer()
          {
               @override
               public void print()
               {
                    ...
               }
          }
     }
}

And have been told that an anonymous class is at work. But, I seriously do not understand how, since the variable 'p' is clearly being assigned to... and it's an abstract class variable!! How is that even possible? I thought abstract classes can not be instantiated or initialized.

Any help would be appreciated.

Answer 1

 final Printer p = new Printer()
      {
           @override
           public void print()
           {
                ...
           }
      }

This means that an anonymous class is created which extends Printer and the variable p is referring to subclass instance.

This is simply polymorphism in action. By creating anonymous class here, you are creating a subclass of Printer and using polymorphism you are using the superclass reference variable p to refer to object of subclass which is anonymous but extends Printer because of the syntax below

Printer p = new Printer(){...}

and this is not only limited to abstract class, you can also create an anonymous class which implements and interface. Consider below example.

package com.test;

public interface SomeInterface {

    public void SomeMethod();

}

and a class below package com.test;

public class TestAnonymous {

    public static void main(String[] args) {

        SomeInterface obj = new SomeInterface() {

            @Override
            public void SomeMethod() {

                System.out
                        .println("Yaayy!!! Creating an anonymous class which implements SomeInterface ");

            }
        };

        obj.SomeMethod();

    }

}

which prints out

Yaayy!!! Creating an anonymous class which implements SomeInterface

What is means that the syntax creates an anonymous class which either extends the abstract class or implements the interface of which reference variable is used to instantiate. It calls method of the subclass.

You can refer jsl. https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.9.1

Now your question

Interface is absolutely abstract and cannot be instantiated; A Java abstract class also cannot be instantiated, but can be invoked if a main() exists

What I understand from your questions is whether you want to know whether main method can be run in abstract class without instantiating it, The answer is YES

package com.test;

public abstract class AbstractClass {

    public static void main(String[] args) {

        System.out.println("main method in abstract class");

    }

}

compile and invoke it using java AbstractClass and it should print

main method in abstract class

IF you want to know whether the abstract class constructor is invoked when instantiating the anynomous class or not, then also the answer is YES. Whenver a subclass constructor is called, it invokes super class constructor by making a super() call. So abstract class constructor will also get called. http://www.thejavageek.com/2013/07/21/initialization-blocks-constructors-and-their-order-of-execution/

Answer 2

An abstract class is just like any other class - except for the fact that it cannot be instantiated directly. I presume you know the use for such a facility. Hence it can very well have a main() , which is a static method, not an instance method

The anonymous class in your example, extend s the abstract class (or implement s an interface, if one is specified). So p is not assigned to an abstract class instance, but to an instance of a class which extends the abstract class

Answer 3

A Java abstract class also cannot be instantiated, but can be invoked if a main() exists.

Since main() method is static an can be invoked without instantiation. eg

abstract class AbstractClass{
    public static void main(String[] args){

    }
}

The same was not true for interface till Java7, With Java8 you can have static main method inside interface hence same would be true for Java8

You second question, while creating an instance of Printer you have defined the sub-class of Printer as well but you can not use this defined implementation of Printer again(since there is no name associated with the implementation), Hence, its anonymous.

Answer 4

A Java abstract class also cannot be instantiated, but can be invoked if a main() exists.

It is nonsense. Any static method of an abstract class can be invoked. Not just main().

Secondly, I have seen examples like the following ... and have been told that an anonymous class is at work.

That is correct.

But, I seriously do not understand how, since the variable 'p' is clearly being assigned to... and it's an abstract class variable!! How is that even possible? I thought abstract classes can not be instantiated or initialized.

It is the anonymous class that is being instantiated here, which extends the abstract class, and provides an implementation of the abstract method. The reference to that class is being stored into Person p , because Person is a superclass of the anonymous class. You can do that for any other class or interface. There's nothing new here.