CODEWITHSUNDEEP
java
zarcel
zarcel

Reputation: 1241

Assinging object of inherited class to reference of super class

Given the follwoing Code:

public class A {
     int at=2;

    public int m(int i){return at+i;}
}

class B extends A {
     int at=3;

        public int m(int i){return at+5*i;}
}

public class Main {

    public static void main(String args[]){
        A x = new B();

        System.out.println("Output "+x.m(x.at));
    }
}

The output is 13.

How does it work? I know that it takes method from B, but what about arguments?

Upvotes: 3

Views: 64

Answers (3)

Sergey Fedorov
Sergey Fedorov

Reputation: 2169

Actually you have two at fields one declared in A and other in B. Method m in A refers to field at in A; method m in B refers to filed at in B. Method is overridden, field is not. x is declared as instance of A so x.at is 2. When you call method m the overridden method is called (B.m(2)) so you get 3+5*2 = 13.

In such cases Java Language specification should tell you what's right and why. Answer to this question can be found in 8.3. Field Declarations

If the class declares a field with a certain name, then the declaration of that field is said to hide any and all accessible declarations of fields with the same name in superclasses, and superinterfaces of the class.

A hidden field can be accessed by using a qualified name (§6.5.6.2) if it is static, or by using a field access expression that contains the keyword super (§15.11.2) or a cast to a superclass type.

cast to a superclass type is your case.

Upvotes: 3

Deepak Agrawal
Deepak Agrawal

Reputation: 1301

Variables does not override like methods. So variables value depends on Reference Variable not on the instance being referenced by this reference variable.

example: in your case. In main() method

A x =new B();

reference variable x is of type A and it hold a reference of an instance of type B.

So as X is of type A. WhenEver you write x.at it being refer Class A variable.

Upvotes: 1

kajacx
kajacx

Reputation: 12939

Look at expression: x.m(x.at). Since x is cdeclared as A, x.at will refer to at field in A.

However, it gets more complicated when it comes to methods. Although x is declared as A, x in fact is of type B, so method m will be called from x's acctual class, B.

Maybe this coud help.

Upvotes: 6

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