How to Implement Association in Java

The association relationship indicates that a class knows about, and holds a reference to, another class. Associations 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 composition are types of association relationships.

Associations join one or more of one thing against one or more of another thing. A professor might be associated with a college course (a one-to-one relationship) but also with each student in her class (a one-to-many relationship). The students in one section might be associated with the students in another section of the same course (a many-to-many relationship) while all the sections of the course relate to a single course (a many-to-one relationship).

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Aggregation in Java: Definition and Examples

By Paul Leahy

Association Example

Imagine a simple war game with an AntiAircraftGun class and a Bomber class. Both classes need to be aware of each other because 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.