为什么在创建抽象类的引用类型对象和访问自己的成员时存在这种差异？

Question

Consider i have an abstract class with a non-abstract method in it. i tried creating reference type object and access that non-abstract method.. it gives me a compilation error. The following program demonstrates it.

abstract class A
{
    public void print()
    {
        System.out.println("this is the print method");
    }
}

class test
{
    public static void main(String args[])
    {
        A ob;
        ob.print();
    }
}

then this is the output i am getting,

Variable ob might not have been initialized

If i assume the above as true, then why is it allowing the same in the following program. In the following Applet, i use the paint() method as an entry point. So, here i create a reference type object of predefined class Graphics (an abstract class), and try accessing the drawString() method of it, then no compilation error.. like this-->

import java.applet.*;
import java.awt.*;

public class apptest extends Applet
{
    public void paint(Graphics g)// Graphics is an abstract class
    {
        String s="this is an applet";
        g.drawString(s,20,20);
    }
}

How is this Applet allowing me to do the same ?

Answer 1

You have understood it wrong, in the below code Graphics is a parameter to a method, even though its an abstract class

    public void paint(Graphics g)// Graphics is an abstract class
    {
    String s="this is an applet";
    g.drawString(s,20,20);
    }

To invoke this method, the caller should definitely create a concrete instance of type Graphics . Something like below

Graphics g = new TwoDGraphics(); // TwoDGraphics extends Graphics, and a non abstract class
applet.paint(g);

Coming back to your example.. its an error to use an uninitialized variable

A ob; --> its not initialized
ob.print();

a below code should have worked.

A ob = new A() {
  //created an anonymous implementation of abstract A
}
ob.print();

Answer 2

Two things:

1. You cannot instantiate the class
2. You have only created the reference for your abstaract class and not an object. For caling any instance method you need to have an object not just reference. But in this case you will not be able to create the instance.

So in short you will not be able to achieve what you are trying to do. There two ways to call the non-abstract method of an abstrac class:

1. Create a child class and using its object you can call the non-abstract method of your abstract class. You may use the reference of either your child class or your parent class, if you have not overridden the method, then call to non-abstract method will always call the parent class method.

2. Make the method static and call it using the reference of parent clas.