I am thinking about this java oop problem . I don't exactly know exactly what is really happening there . Can someone make me understand ?

abstract class A {
    public int proc (A p){
        return 98;
    }
}

class B extends A {
    public int proc(A p) {
        return 17;
    }
}

class C extends A {
    public int proc (C p) {
        return 65;
    }
} 

public class HelloWorld{

     public static void main(String []args){
        C x = new C(); // here x is C type and is an instance of C ?
        A y=new B();  // here y is A type and is an instance of B?
        C z=new C();  // here z is C type and is an instance of C ?
        System.out.println(y.proc(x) z.proc(x)); /* y is A type so it is looking for proc function  in A ,but doesn't return 98 
, z is C type and it is looking for proc function in C and return 65 .*/
        
     }
}

Can someone tell me how should I tackle theese instances ?

CodePudding user response：

In Java, you can create base class objects which hold the child classes objects except for abstract classes.

A is an abstract class, you cannot instantiate it, but you can assign an object of the child class to it which holds the child class object's features.

You can think, we are assigning the reference of B to y which has a type of A.

System.out.println(y.proc(x));

The code above will print 17, which is the return value of the proc in class B returns.

Whenever you call the methods of y, the compiler will give you class B's methods automatically. So, you are creating an instance of class B in the memory and assigning it to y which points to the same memory location. For that reason, you can use the methods of class B. Same goes for z as well.

CodePudding user response：

Y is an A but contains a reference to a B so when we call y.proc it is the proc in B that is called, and returns 17, not 98. You need to look at the instance that is created and not the type of variable.