CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall

Association No necessary lifetime link Two implementations: –The first is very similar to composition, but differs in one crucial respect: where the target class is instantiated. –The second, which decouples lifetimes completely, is a bit more complex but also more flexible.

Revisiting Clifford Dog-Tail relationship is COMPOSITION –Dog takes responsibility for creating a Tail Dog-Collar relationship is ASSOCIATION –Dog takes NO responsibility for creating Collar

First implementation 3 changes to source class: 1.Declaration of instance variable 2.Assignment of existing instance to the instance variable 3.Parameter of constructor is of same type as instance variable 1 2 3

Dog – Collar example in Java public class Dog { private Collar _collar; public Dog(Collar collar) { _collar = collar; } 1 2 3

Essence of association one object can communicate with another object there is no necessary lifetime link between the two objects the objects in the relationship can change over time

Lifetime issue in constructor implementation the source class (Dog) does not create an instance of the target (Collar) class there is a lifetime dependency: the target (Collar) object must exist before the source (Dog) object can be created: – a Collar object must be provided in the constructor of the Dog object.

This occurs in this particular implementation of the relationship, but is not an essential characteristic of the relationship. Lifetime issue (continued)

Changing a property value We want to be able to set a new value for the property (e.g. give Clifford a new collar). How can we do that? –Using a “mutator” method.

Association relationship (take 2) We’ve seen one implementation of “knows a” – using a parameter in constructor: public class Dog { private Collar _collar; public Dog(Collar collar) { _collar = collar; } Now we will see a more flexible implementation.

Association via constructor and mutator method public class Dog { private Collar _collar; public Dog(Collar c) { _collar = c; } public void setCollar(Collar c) { _collar = c; }

Constructor/mutator similarity Similarity: both set the value of the instance variable. Difference: constructor sets value when Dog object is created mutator changes the value at some later point in time public class Dog { private Collar _collar; public Dog(Collar c) { _collar = c; } public void setCollar(Collar x) { _collar = x; }

On blackboard object diagram & memory diagram for: clifford.Collar smallCollar; smallCollar = new clifford.Collar(); clifford.Dog fido; fido = new clifford.Dog(smallCollar); clifford.Collar mediumCollar; mediumCollar = new clifford.Collar(); fido.setCollar(mediumCollar);