Agregation vs Normal Association in Java?

Hello,

Thanks for your answer. In fact, I think the same, but it's very confuse for myself construct different UML diagrams and in the code became all the same.

My doubt is how to represent these structures in the code, for example:

public class Tyre{

private size;

private radius;

private pressure;

...

}

public class Car{

private Tyre myTyres; // Is this an aggregation?

...

}

public class Shopper{

public buyACar(Car aCar){ // Is this an association

...

}

}

This kind of question is because, the relationship between the classes are different. I've ever had this doubt? Is an attribute considered ever an aggregation relation and when I passed an object by reference to a function is an simple association relation.

Best regards.

--

Clayton

claytonvaldoa at 2007-7-16 2:47:33 >

hi, you can think association is a super relationship of aggreagation.. something like: Composition extends Aggregations, and Agregations extends Association, Association extends Dependency.

So, if 2 class has Aggregations relationship, the two class has Assoctiation relationship, and have Dependancy, but may not have Composition.

Association is only a mark that a class has another class, eg: employer has employee, full stop. And it doesn't describe further what is the relationship between employer and employee.

Aggregations is if you have a classroom which has bunch of students. But the students may also go to sport hall, swimming pool, concert hall, etc. Thus, if there is no classroom, the student are still there, live and kicking.

Composition is like water, it has Hidrogen and Oxygen. If it doesn't have Oxygen, it is not "water"

Generally, the more specific relationship you use highlight in the diagram, the better and more readable is your design. So, telling "these class has Aggregation" is more useful than "these class has dependency"..

Alex

lexzeusa at 2007-7-16 2:47:33 >

>Do you have examples of Java for the concepts that you listted for me? I thank you so much, because it's very difficult to get some Java code in the Internet explaining these concepts

Aggregation example:

You have product dual LCD TV-Monitor which can function as TV and pc monitor. Now you want to put this product into 2 categories: Color TV and LCD monitor because it makes sense that the product is belong both categories.

The relationship is aggregation, because the category Color TV may contain a product, which is LCD TV-Monitor, but not necessery, because without LCD TV-Monitor, the category Color TV is exist on its own.

Composition example:

For an instance of ArticleBean (it is a simple javabean) to be displayed properly by jsp, it consists of 3 parts: header, content, and footer (which all are objects).

Here, the relation is composition, ArticleBean must have header, content, and footer in order to be displayed correctly. You can't have an article if there is no title. And generally, no article exists with a same title. So, it is safe to consider if you delete the ArticleBean, the header, content and footer are also deleted.

lexzeusa at 2007-7-16 2:47:33 >

Hi,

Association example:

UML-wise:

Again, using DP as an example, the Adapter pattern (Version: An object adapter relies on object composition.)

There is an association between the Adapter and Adaptee.

http://www.tml.hut.fi/~pnr/GoF-models/html/Adapter.html

Gentlemen,

Correct me if I'm wrong in Aggregation example and/or Association example.

PaulPhuoca at 2007-7-16 2:47:33 >

>Gentlemen,

>Correct me if I'm wrong in Aggregation example and/or Association example.

You are correct, adaptor and adaptee use aggregation + association...

If 2/more classes use aggregation, the classes use association because association is super relationship of aggregation.

Alex

lexzeusa at 2007-7-16 2:47:33 >