CSE115: Introduction to Computer Science I

CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall 645-4739 alphonce@buffalo.edu

Phones off Signs out

Announcements • Exam 2 in a week and a half (10/21) • Exam 1 solution will be posted today • Installation session • next week (10/13 and 10/14) • will send e-mail with details

Agenda • association relationship • constructor form • accessor/mutator form • variation on a theme • capture the domain

Revisiting Clifford

Another relationship • One collar for its life…? • A dog has-a tail; the tail is a part of the dog. • We need something different to model the relationship between a dog and a collar.

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: • Declaration of instance variable • Assignment of existinginstance to the instance variable • 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 3 2

UML • See Eclipse example

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 inconstructor 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.

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

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”: public class Dog { private Collar _collar; public Dog(Collar collar) { _collar = collar; } } • Now we will see a more flexible implementation.

Association viaconstructor and mutatormethod 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 mutatorchanges the value at some later point in time public class Dog { private Collar _collar; public Dog(Collar c) { _collar = c; } public void setCollar(Collar c) { _collar = c; } }

Retrieving a property value • Suppose an object wants to let other objects know the value of one of its properties? • How can we do that? • We can define an “accessor” method.

accessor method public class Dog { private Collar _collar; public Dog(Collar collar) { _collar = collar; } public void setCollar(Collar collar) { _collar = collar; } public Collar getCollar() { return _collar; } }

accessor/mutatordifferencesin function Information flowing in to method public void setCollar(Collar collar){ _collar = collar; } public Collar getCollar() { return _collar; } Information flowing out from method

accessor/mutator differencesin form public void setCollar (Collar collar) a.c.m. r.t.s. name parameter list public Collar getCollar () a.c.m. r.t.s. name parameter list a.c.m. = access control modifier r.t.s. = return type specification void = no value is returned by method (note difference with constructors: no r.t.s.)

Example 1 Shape s1 = new Shape(java.awt.Color.BLUE); Shape s2 = new Shape(java.awt.Color.RED); Shape BLUE _color s1 Shape RED _color s2

Example 1 Shape s1 = new Shape(java.awt.Color.BLUE); Shape s2 = new Shape(java.awt.Color.RED); s2.setColor(s1.getColor()); Shape BLUE _color s1 Shape RED _color s2

Example 2 Dog s1 = new Dog(new Collar()); Dog s2 = new Dog(new Collar()); Dog _collar s1 Dog _collar s2

Example 2 Dog s1 = new Dog(new Collar()); Dog s2 = new Dog(new Collar()); s2.setCollar(s1.getCollar()); Dog _collar s1 ??? Dog _collar s2

What could we do instead?

CSE115: Introduction to Computer Science I