The association relationship is a way of describing that a class knows about and holds a reference to another class. This can be described as a "has-a" relationship because the typical implementation in Java is through the use of an instance field. The relationship can be bi-directional with each class holding a reference to the other. Aggregation and compositionare types of association relationships.
Examples:
Imagine a simple war game where there is a AntiAircraftGun class and a Bomber class. Both classes need to be aware of each other as they are designed to destroy each other:
public class AntiAirCraftGun {
private Bomber target;
private int positionX;
private int positionY;
private int damage;
public void setTarget(Bomber newTarget)
{
this.target = newTarget;
}
//rest of AntiAircraftGun class
}
public class Bomber {
private AntiAirCraftGun target;
private int positionX;
private int positionY;
private int damage;
public void setTarget(AntiAirCraftGun newTarget)
{
this.target = newTarget;
}
//rest of Bomber class
}
The AntiAirCraftGun class has-a Bomber object and the Bomber class has-a AntiAirCraftGun object.
