为什么我不能从超类对象调用子类方法

Question

Consider below two classes

class Parent {
public void parentMethod(){
System.out.println("inside parentMethod");
}
}

public class child extends Parent {
public void childMethod(){
System.out.println("inside childMethod");
}

public static void main(String [] args){
Parent p =new Child();
p.childMethod();
}
}

Why cannot i call the child class method with following syntax? Parent p =new Child(); p.childMethod();

The type of object that p holds is that of Child but still i cannot call child class method. Why such a call cannot complete in java?

Answer 1

Because the variable is of type Parent . Which means it can hold any object of type Parent , be it a Child or anything else that implements the Parent class. The compiler can't guarantee that the variable will hold a Child , because you told it to make the variable a Parent .

To invoke a method on Child , make the variable that type:

Child p = new Child();
p.childMethod();

Alternatively, if you can yourself guarantee that the variable is pointing to a Child object, you can cast it:

Parent p = new Child();
Child c = (Child)p;
c.childMethod();

Though there's an argument to be made that if you can always guarantee that the variable will be a Child then it should have been of type Child in the first place.

Answer 2

You cannot call that method because Parent#childMethod is not a method. Child#childMethod is a method.

Think about this:

Parent p = new Parent();
p.childMethod();

Should you be able to do this? No, because you have an instance of Parent .

Your l-value is Parent so you will only be able to call methods that are defined in Parent . To do what you want, you'd need to cast Parent to a Child , but you need to make sure that it actually is an instance of Child :

if(p instanceof Child) {
  ((Child)p).childMethod()  
}

Answer 3

This is because the reference variable type is of type Parent. Therefore, the variable only has access to the attributes and behaviors of the reference class. Note, you can override methods in the child class... For example:

public class child extends Parent {

  @Override
  public void parentMethod(){
    System.out.println("inside childMethod");
  }

  public static void main(String [] args){
    Parent p =new Child();
    p.childMethod(); // PRINTS "inside childMethod"
  }
}