Preparing for the SCJA

[Tuesday Ganal]

Hi guys, I'm preparing for SCJA and I saw this exam topic about Concrete Classes. What is a concrete class. I can't seem to find it in the Java documentation, and even in my Java complete curriculum. Anybody can help me in finding a site where I can get info on concrete classes? Thanks so much.

[Triguna Mutuguppe Sripathi]

http://www.brpreiss.com/books/opus5/html/page610.html

Hope this helps.

[Cameron Wallace McKenzie]

Really, a concrete class is one that is not abstract. If you can create an instance of a class by invoking its contstructor, it's concrete!

[Mala Gupta]

The following is an example of a concrete class:

class Person {
       // variables and methods
}

You can create instances of this class as follows:

Person p = new Person();

However, if you prefix the definition of class Person with the keyword abstract, it will then be termed as an abstract class, as follows:

abstract class Person {
       // variables and methods
}

Now, the following code will not compile:

Person p = new Person();

The compiler will complaint that it cannot create objects of an abstract class. An abstract class, as its name suggests, is an incomplete representation of a concept. Hence, creation of its objects isn't allowed.

cheers
Mala

[Tuesday Ganal]

Cameron Wallace McKenzie wrote:Really, a concrete class is one that is not abstract. If you can create an instance of a class by invoking its contstructor, it's concrete!

Thanks, Cameron. BTW, I'm reading your book SCJA Certification Guide.

[Tuesday Ganal]

Triguna Mutuguppe Sripathi wrote:http://www.brpreiss.com/books/opus5/html/page610.html

Hope this helps.

Thanks, Triguna

[Tuesday Ganal]

Mala Gupta wrote:The following is an example of a concrete class:

class Person {
       // variables and methods
}

You can create instances of this class as follows:

Person p = new Person();

However, if you prefix the definition of class Person with the keyword abstract, it will then be termed as an abstract class, as follows:

abstract class Person {
       // variables and methods
}

Now, the following code will not compile:

Person p = new Person();

The compiler will complaint that it cannot create objects of an abstract class. An abstract class, as its name suggests, is an incomplete representation of a concept. Hence, creation of its objects isn't allowed.

cheers
Mala

Thanks, Mala for differetiating an abstract from a concrete class. It helps

[Mala Gupta]

I am glad, it does. A simple example is much more effective than reading 2 pages of theory.

cheers
Mala