About Polymorphism

[Song Guo]

public class A {
    private String name;
    public A(){
        System.out.println("In A's default constructor: "
           + this);   // In A's constructor: B@...    why 'this' is 'B@...' 
    }
    public A(String n) {
        name = n;
    }
    public String getName() {
        System.out.println("In A's getName() method: " + this);   // Also 'this' is 'B@...'
        return this.name;
    }
}

public class B extends A {
    private String name;
    public B(String n) {
        name = n;
    }
    public static void main(String[] args) {
        B b = new B("Jim");
        System.out.println(b.getName());        // null,  why? 
    }
}

a) When a B object is created, first the default constructor of A will be called, but why the output is "In A's default constructor: B@...", not 'A@...' ?
b) Though A's default constructor is called and its name field is null, in B's constructor it has 'name = n'. Why b.getName() is null ?
Thank you!

[Mohamed Sanaulla]

For a) The object in the context is B and not A, hence "this" refers to the instance of B class.
For b) As the name is private in A, its not visible in class B though B extends A. Private members are only visible within the class they are declared. The constructor in B sets the value for B's name and not A's name.
Try declaring the name variable in class A to be protected and remove the name declaration from class B.

[Campbell Ritchie]

I know you will call me pedantic, but you do not have a default constructor. You only get default constructors when you don’t write a constructor yourself.

[Song Guo]

Mohamed Sanaulla wrote:For a) The object in the context is B and not A, hence "this" refers to the instance of B class.
For b) As the name is private in A, its not visible in class B though B extends A. Private members are only visible within the class they are declared. The constructor in B sets the value for B's name and not A's name.
Try declaring the name variable in class A to be protected and remove the name declaration from class B.

In the context above, 'this' is the reference of object B, and in B's constructor it has name = n statement. So why b.getName() gets A's name null , not b's name 'Jim' ?'
Thank you !

[Mohamed Sanaulla]

Song Guo wrote:

Mohamed Sanaulla wrote:For a) The object in the context is B and not A, hence "this" refers to the instance of B class.
For b) As the name is private in A, its not visible in class B though B extends A. Private members are only visible within the class they are declared. The constructor in B sets the value for B's name and not A's name.
Try declaring the name variable in class A to be protected and remove the name declaration from class B.

In the context above, 'this' is the reference of object B, and in B's constructor it has name = n statement. So why b.getName() gets A's name null , not b's name 'Jim' ?'
Thank you !

As I mentioned in b) that the name variable is private to the class, which means the changes you make to the name variable in class B is limited to that only.

[Winston Gutkowski]

Song Guo wrote:In the context above, 'this' is the reference of object B, and in B's constructor it has name = n statement. So why b.getName() gets A's name null , not b's name 'Jim' ?'

Actually, you're wrong on two counts:

1. The only this that has any bearing on events is the this defined in A.getName(), and in that context it is a reference to an object of type A (a B object IS an A, remember).

2. When you call b.getName(), you are actually calling A.getName(), since that's the only method you've defined; therefore polymorphism doesn't even come into the picture.

Winston

[Song Guo]

Mohamed Sanaulla wrote:

Song Guo wrote:

Mohamed Sanaulla wrote:For a) The object in the context is B and not A, hence "this" refers to the instance of B class.
For b) As the name is private in A, its not visible in class B though B extends A. Private members are only visible within the class they are declared. The constructor in B sets the value for B's name and not A's name.
Try declaring the name variable in class A to be protected and remove the name declaration from class B.

In the context above, 'this' is the reference of object B, and in B's constructor it has name = n statement. So why b.getName() gets A's name null , not b's name 'Jim' ?'
Thank you !

As I mentioned in b) that the name variable is private to the class, which means the changes you make to the name variable in class B is limited to that only.

Modify the private in both A and B to public, the output is the same.

[Winston Gutkowski]

Song Guo wrote:In the context above...

Song,

Please DontWriteLongLines (←click) inside code blocks. I've broken up the one of yours that was causing problems this time.

Thanks.

Winston

[Song Guo]

Song Guo wrote:

Mohamed Sanaulla wrote:

Song Guo wrote:

Mohamed Sanaulla wrote:For a) The object in the context is B and not A, hence "this" refers to the instance of B class.
For b) As the name is private in A, its not visible in class B though B extends A. Private members are only visible within the class they are declared. The constructor in B sets the value for B's name and not A's name.
Try declaring the name variable in class A to be protected and remove the name declaration from class B.

In the context above, 'this' is the reference of object B, and in B's constructor it has name = n statement. So why b.getName() gets A's name null , not b's name 'Jim' ?'
Thank you !

As I mentioned in b) that the name variable is private to the class, which means the changes you make to the name variable in class B is limited to that only.

Modify the private in both A and B to public, the output is the same.

Problem solved. Thanks.