UML Relationships

Question

In regards to implementation of UML Relationships using Java, how are Aggregation and Dependency relationships typically implemented. I have Association and Generalization but am missing the other two.
Association - in Java, implemented as member variables in participating classes
Generalization - in Java, implemented using inheritance in source code (using extends keyword)
Aggregation/Composition - ? member variables also ?
Dependency - ?

Answer 1

So, Aggregation and Composition are special types of Associations. Put simply yes, though you will often find that people use associations during the early part of the analysis then resovlved this to either Aggregation or Composition as the class diagram design emerges from the domain model.
I describe aggregation as a dynamic association and composition as a static association.
So I would describe a Car as a composition which includes an Engine, without an Engine it is not really a Car. Composition is implicitly a bi-directionally realationship.
class Car {
Engine engine = New  Engine( this ) ;
class Engine {
Car car = null ;
public Engine(Car car) {
this.car = car;
}I could describe a Car and it's Driver as aggregation, a car is still a car without a driver and the lifetime of an instance of a car and the driver are not linked (well not usually :). A car has many drivers linked to it at various times during its lifetime.
class Car {
Driver driver;
public setDriver( Driver driver) {
this.driver = driver;
driver.drive( this ) ;
class Driver {
Car car;
public drive(Car car) {
this.car = car;
They too are implemented via class variables. They are more properly described as object references.
These may be Associative Classes, between the two Classes. In this example a Hashtable.
class Car {
Hashtable passengers;
public setPassenger( Passenger passenger) {
this.passengers.add( passenger ) ;
And those special rules you mention are
contingent upon how the developer writes the
application. Or in another way, it is the
responsibility of the developer to enfore the rules of
Association and Composition. There is nothing in the
Java 2 language that governs this behavior.True, mapping the association types from UML notation to code is the programmers job, and the exact implementation may vary with language semantics.. Though the OMG are defining the MDA to define the relationship bewteen UML model and implementation code.