Tutorial On Association

By: Robin Chander

What is an Association Relationship in UML modeling?

- In UML models , an Association is a relationship between such as classes , that describes the reasons for the relationship and rules to govern that relationship.

An association represents a structural relationship that connects two classifiers. Like attributes , association records the properties of classifiers.

An association is nothing but a line drawn between the participating classes.

Under what circumstances it is used?

- It can be used to show the design decisions that one make about the classes in the application that contain data and to show which of those classes need to share data.

How it looks in UML?

- In UML class diagrams , association relationships in JAVA application represents the following things:

1) A semantic relationship between two or more classes that specifies connections between their instances.

2)A structural relationship that specifies that objects of one class are connected to objects of second class.

a) Bidirectional Association Relationship:

In bidirectional association relationship , simply a line will be shown which indicates association between the classes , there will be no arrowhead sign on either sides of the line.

b) Unidirectional Association Relationship:

In unidirectional association relationship , there will be a line with arrowhead pointing towards the target. Here Class One is source and Class Two is target.

How it is implemented in code?

-In Java there are three steps to implement association relationship.

1) The Foremost thing thing is , we need to declare an instance variable of the class.

2) Assignment of existing instance to instance variable.

3) Parameter of known class in other class constructor.

Let us take Cup-Coffee example to illustrate the above mentioned points.

package tutorial;

public class Cup {

private Coffee _coffee;

public Cup(Coffee coffee) {

_coffee = coffee;

}

}

* Code in green colour is instance variable declaration and the constructor definition is in blue colour.

How is Composition Relationship different from Association Relationship?

- In UML class diagrams the Composition relationship is shown using a solid line connecting two classes involved in relationship . The one end of the line is marked with a diamond.

Here , First Class is the source and Second Class is the target.

Composition Relationship in code:

Let us assume a class named Cup to have Coffee as property.

First of all , we need to declare an instance variable.

package tutorial;

public class Cup {

private Coffee _coffee;

public Cup() {

_coffee = new tutorial.Coffee();

}

}

* The code written in Green is instance variable declaration and the code written in Blue is constructor definition.