CODEWITHSUNDEEP
javainheritance
Akshay Katoch
Akshay Katoch

Reputation: 75

Object class inheritance

We know that class A extends Object class implicitly but if we instantiate Object class object with class A constructor and try to access print() method of class A it shows a compile time error while in other case where class B extends class A and Class A object instantiated with class B constructor and try to access print() method works fine. 

class A{
    public void print(){
        System.out.println("A");
    }
}
class B extends A{
    public void print(){
        System.out.println("B");
    }
}
public class MyClass {
    public static void main(String args[]) {
        Object o = new A();
        o.print(); // error
        A a = new B();
        a.print();  // print B
    }
}

Upvotes: 0

Views: 59

Answers (3)

GhostCat
GhostCat

Reputation: 140613

This line:

o.print(); // error

You know that o is actually of type A.

The compiler could know, but in Java, it does not know. The compiler only knows that there is some variable named o, of type Object.

Thus the compiler slaps your fingers for invoking a method on o that the declared type Object doesn't support!

Other languages are "smarter" about such things, but in Java, that is how things are.

Upvotes: 2

Amongalen
Amongalen

Reputation: 3131

You example with class B extends A isn't the same as the case of A extends Object. The difference is that in the first case the superclass (A) contains the print() method, in the second case the superclass (Object) doesn't contain the print(). If you remove the print() method from the A class, the second part won't work either:

class A{
}

class B extends A{
    public void print(){
        System.out.println("B");
    }
}

public class MyClass {
    public static void main(String args[]) {
        Object o = new A();
        //o.print(); // error
        A a = new B();
        a.print();  // error
    }
}

Upvotes: 1

dn_c
dn_c

Reputation: 614

Because Object class does not have print() method and the reference o is of type Object, the compiler will throw an error. In case 2 : class A has print() method thats why no compilation error and at runtime it calls class B's print() method because we assigned class B's reference at runtime.

Upvotes: 0

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