Lecture 5
Writing Classes
Richard Gesick
Figures from Lewis, “C# Software Solutions”, Addison Wesley
CSE 1301
Topics
• Adding a Class to a Project
• Defining a Class
– Attributes
– Methods
•
•
•
•
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Using a Class
Visibility
Instance Variables
Properties
Multiple Classes in a Project
• You’ve seen projects with many classes
– Unity projects can (and often do) have multiple
classes… one for each game element
• Can add classes to our project
– Keep them all in one file (~bad choice)
– One class per file (better choice)
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Why User-Defined Classes?
Primitive data types (int, double, char, .. ) are
great …
… but in the real world, we deal with more
complex objects: products, Web sites, flight
records, employees, students, ..
Object-oriented programming enables us to
manipulate real-world objects.
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User-Defined Classes
• Combine data and the methods that operate
on the data
• Advantages:
– Class is responsible for the validity of the data.
– Implementation details can be hidden.
– Class can be reused.
• Client of a class
– A program that instantiates objects and calls
methods of the class
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Classes
• A class can contain data declarations and
method declarations
int size, weight;
char category;
Data declarations
Method declarations
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Important Terminology
• Fields
– instance variables: data for each object
– class data: static data that all objects share
• Members
– fields and methods
• Access Modifier
– determines access rights for the class and its
members
– defines where the class and its members can be
used
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Using the Class
• Before we define the Die class, let’s use it
• Remember that OOP allows us to ignore the
“guts” and just presume it works
• There are 3+ people involved
– The writer of the class (knows what the “guts” are)
– The user of the class, writing the main program
(knows his code, but doesn’t know the class “guts”)
– The user of the program – sees the interface, doesn’t
see program code at all
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The “User” of a Class
• Recall we don’t want to have to expose the
“guts”
• But we do need to tell a user of our class what
it can do and what they have access to
– Define the API of the class
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The Die
• At this point, what do you know about Die
from the code?
– You can “Roll()” it
– It has “FaceValue”
– Can call “SetFaceValue()”
– Can call “GetFaceValue()”
• Now let’s look at the guts…
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The Die Class API
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Die Class
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Die Class (cont)
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Die Class (cont)
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Unified Modeling Language (UML)
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Visibility
• OO motivation: protection/security
• We need a way of selectively “publishing”
parts of a class and “hiding” other parts of the
class
• Public & private
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public vs. private
• Instance variables are usually declared to be
private
• Methods that will be called by the client of the
class are usually declared to be public
• Methods that will be called only by other methods
of the class are usually declared to be private
APIs of methods are published (made known) so that
clients will know how to instantiate objects and call
the methods of the class
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Visibility Example
class BMW_Z4 {
private int ModelYear;
public string LicensePlate;
private bool TopUp;
Note the visibility
(attributes will usually be private)
public void Drive()
{
Console.WriteLine("Roadin’ and Rockin’");
}
public void OpenTop()
{
TopUp = false;
}
}
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Method are
typically
public
Object Method & Attribute Visibility
BMW_Z4 myCar;
myCar = new BMW_Z4();
myCar.LicensePlate = "BMR4ME";
myCar.ModelYear = 2004;
myCar.Drive();
Illegal b/c private
myCar.OpenTop();
myCar.Drive();
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Interacting with Objects
• Keep private/public visibility as needed
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Visibility
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Defining Instance Variables
Syntax:
accessModifier dataType identifierList;
dataType can be primitive date type or a class type
identifierList can contain:
– one or more variable names of the same data type
– multiple variable names separated by commas
– initial values
• Optionally, instance variables can be declared as
const
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Examples of Instance Variable Definitions
private string name = "";
private const int PERFECT_SCORE = 100,
PASSING_SCORE = 60;
private int startX, startY,
width, height;
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The Auto Class
class Auto
{
private string model;
private int milesDriven;
private double gallonsOfGas;
}
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Properties
• Combines field/attribute with method
• Standard:
–
–
–
–
–
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Make attributes private
Lower-case first letter of attribute
Make properties public
Upper-case first letter of properties
Define “get” and “set” for each property (selectively)
Properties Example
class BMW_Z4 {
private int modelYear;
private string licensePlate;
private bool topUp;
public int ModelYear
{
get { return modelYear; }
set { if (value >= 2003) modelYear = value; }
}
public string LicensePlate
{
get { return licensePlate; }
set { if (value.Length() == 6) licensePlate = value;
}
} ...
}
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Auto Properties
• Now how would you write the properties for
the Auto class?
class Auto
{
private string model;
private int milesDriven;
private double gallonsOfGas;
}
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