面向对象中的多态

在说多态之前，我们先看一段代码：

package JavaInterview;
class A {
        int i;
    public A() {
        i = 3;
        System.out.println(i);
    }
        void display() {
            System.out.println(i);
    }
}    
class B extends A {
    int j;
    public B() {
        j = 4;
        System.out.println(j);
    }
    void display() {
        System.out.println(j);
    }
    
}    
public class InheritanceDemo {
    public static void main(String[] args)
    {
        A obj = new B();
        obj.i=1;
        obj.j=2; // can"t reach it
        obj.display();     
    }
}

package JavaInterview;
class A {