invoking methods higher up the hierarchy level ( grandparent) without using an instance

Question

Hi i have three classes as follows ,Class A which is parent ,Class B which is a subclass of A and does not have anything in it , Class C which is a subclass of B and a Test Class with the main function:

public class A {    
  double pay =0.0;    
  public double calculatePay(int hoursWorked,double rate) {    
    pay = hoursWorked*rate;      
    return pay;
  }    
}

public class B extends A {
}

public class C extends B { 
  public double show() {
    double paid = calculatePay(10, 1.3); 
     return paid;
  }
}

public class Test {
  public static void main(String[] args) {        
    C c = new  C();        
    System.out.println(c.show());           
  }

}

My question is how is it that i can call the calculatePay() method of the grandparent Class A from the Class C without using an instance/object.From my understanding of inheritance i would have thought that i would need an instance of A or C in order to invoke the grandparent method like this even though i know i have inherited the calculatePay method from A i would still require an instance of A or C ?

A a = new A();
a.calculatePay;

or

C c = new C();
c.calculatePay;

I couldnt use the super keyword because A is not a parent to C but a grandparent.Instead i just called it like this and stored it in a double variable:

  double paid = calculatePay(10, 1.3); 

And when i run the program from main () it works and gives me the correct output according to the logic in the grandparent.Can someone please explain this ? I have seen this in android programming where you call getMethods() from a grandparent within classes which are like 3 levels down the hierarchy

Answer 1

As the method is inherited, both would work - you just need the correct syntax:

A a = new A();
a.calculatePay(10, 1.3);

Very same if you used C c = new C() instead.

You don't have to do anything special in Java - as (non-private) methods declared in the A class can be invoked on instances of subclasses.

Beyond that: as of now, there is no sense in having 3 different classes. There is no point in B extending A and being empty. You don't write down B extends A because you can. You do such things because it makes sense to do so - because it helps you solving your problem.

Your problem doesn't require three classes and inheritance at all.

And regarding your actual question: you have to understand that OO languages like Java or C++ typically a "table" approach. Meaning: when you invoke a method on an object, what happens is that "the system" looks into a table for the class of that object - and that table tells it "which" method to call (see here for further reading). In that sense, it doesn't make any difference if you do

this.foo() ... within class Foo

or

someFooObject.foo()

And that is exactly what your code is doing - by just saying calculatePay(10, 1.3) within C.show() ... you are invoking that method on the this reference implicitly.

Answer 2

When you are making an instance of class C, you are already invoking the method calculatePay from class A as it is not overridden in any other class.

Assuming that you have some functionality defined in class B and the calculatePay() method is overridden in class C, so that the code makes sense, You can override the calculatePay in B and call A's calculatePay using super as shown:

public class A {    
  double pay =0.0;    
  public double calculatePay(int hoursWorked,double rate) {    
    pay = hoursWorked*rate;      
    return pay;
  }    
}

public class B extends A {
  public double calculatePay(int hours,double rate) {    
    return super.calculatePay(hours,rate);
  }
}

public class C extends B { 

  public double calculatePay(int hoursWorked,double rate) {    
          //overridden implementation
  }    

  public double show() {

    //calling calculatePay from B will ultimately call it from A
     double paid = super.calculatePay(10, 1.3);
     return paid;
 }

For the confusion regarding How calculatePay is invoked without using instance of class C or A. To invoke the method you can use Instance of the class or this or simply the name of method.

public class Test {

    String s;

    private String hey() {
        return s;
    }

    public String getS(){
        String sm = this.hey(); // or simply hey();
        // here I could just write hey(); without this

        return sm;
    }
}