为什么以下代码给出错误？

Question

I am trying to learn about inheritance and I came across this problem.

Here is the code:

import java.util.*;
class Parent
{
    void show()
    {
        System.out.println("show from parent");
    }
}
class Child extends Parent
{
    public static void main(String s[])
    {
        Parent p=new Child();
        p.show();
        p.display();
    }
    void show()
    {
        System.out.println("show from child");
    }
    void display()
    {
        System.out.println("display from child");
    }
}

And the error is:

G:\javap>javac Child.java
Child.java:15: error: cannot find symbol
                p.display();
                 ^
  symbol:   method display()
  location: variable p of type Parent
1 error

If I'm able to access show() then why am I not able to access display() knowing that display() is inherited and is also present in the class definition of Child class.

Answer 1

You must understand the distinction between the run time type and the compile time type .

At run time your variable p holds a reference to a Child instance. So calling the show method will run the code in Child#show because this overrides the method Parent#show .

At compile time, the compiler can only know about the declared type of the variable. And this is Parent . So the compiler can only allow access to the fields and methods of type Parent , but not of type Child .

The display method simply isn't declared in Parent , hence the error.

Answer 2

if u want to call the display method of client then you must need to create object of child class. eg. Child child=new Child();

otherwise you need to write display method in parent class.

the rules is reference of parent class cant call member of child.

Answer 3

Display（）方法不在父类中，这是错误。您正在访问父类show方法而不是子类。如果您尝试使用对象访问父类中的方法，则不需要子类中的方法