Lecture 4
Using Classes
Richard Gesick
Figures from Lewis, “C# Software Solutions”, Addison Wesley
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Topics
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Class Basics and Benefits
Creating Objects
.NET Architecture and Base Class Libraries
Random Class
Math Class
Object-Oriented Programming
• Classes combine data and the methods (code)
to manipulate the data
• Classes are a template used to create specific
objects
• All C# programs consist of at least one class.
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Example
• Student class
– Data: name, year, and grade point average
– Methods: store/get the value of each piece of
data, promote to next year, etc.
• Student Object: student1
– Data: Maria Gonzales, Sophomore, 3.5
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Some Terminology
• Object reference: identifier of the object
• Instantiating an object: creating an object of a
class
• Instance of the class: the object
• Methods: the code to manipulate the object
data
• Calling a method: invoking a service for an
object.
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Class Data
• Members of a class: the class's fields and
methods
• Fields: instance variables and class variables
– Fields can be:
• any primitive data type (int, double, etc.)
• objects
• Instance variables: variables defined in the
class and given values in the object
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What’s in a Class
Class contains
Members are
Fields
Instance variables
Class variables
Methods
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Encapsulation
• Instance variables are usually declared to be
private, which means users of the class must
reference the data of an object by calling
methods of the class.
• Thus the methods provide a protective shell
around the data. We call this encapsulation.
• Benefit: the class methods can ensure that the
object data is always valid.
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Naming Conventions
• Class names: start with a capital letter
• Object references: start with a lowercase
letter
• In both cases, internal words start with a
capital letter
• Example: class: Student
objects: student1, student2
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1. Declare an Object Reference
Syntax:
ClassName objectReference;
or
ClassName objectRef1, objectRef2…;
• Object reference holds address of object
• Example:
–
Date d1;
• d1 contains the address of the object, but the
object hasn’t been created yet
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2. Instantiate an Object
• Objects MUST be instantiated before they can be
used
• Call a constructor using new keyword
• Constructor has same name as class.
• Syntax:
objectReference = new ClassName( arg list );
• Arg list (argument list) is comma-separated list of
initial values to assign to object data, and may be
empty
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Date Class API
Constructor: special method that creates an
object and assigns initial values to data
Date Class Constructor Summary
Date( )
creates a Date object with initial month, day, and year
values of 1, 1, 2000
Date( int mm, int dd, int yy )
creates a Date object with initial month, day, and year
values of mm, dd, and yy
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Instantiation Examples
Date independenceDay;
independenceDay = new Date(7,4, 1776 );
Date graduationDate = new Date(5,15,2008);
Date defaultDate = new Date( );
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Objects After Instantiation
Object
Instances
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Object Reference vs. Object Data
• Object references point to the location of
object data.
• An object can have multiple object references
pointing to it.
• Or an object can have no object references
pointing to it. If so, the garbage collector will
free the object's memory
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Creating Aliases
Date hireDate = new Date( 2, 15, 2003 );
Date promotionDate = new Date( 9, 28, 2004 );
promotionDate = hireDate;
int x = 5, y = 3;
x = y;
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Two References to an Object
• After program runs, two object references
point to the same object
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null Object References
• An object reference can point to no object. In
that case, the object reference has the value
null
• Object references have the value null when
they have been declared, but have not been
used to instantiate an object.
• Attempting to use a null object reference
causes a run time exception.
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NullReference
Date aDate;
aDate.setMonth( 5 );
Date independenceDay = new Date( 7, 4, 1776 );
// set object reference to null
independenceDay = null;
// attempt to use object reference
independenceDay.setMonth(5);
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String and StringBuilder
• string is like a primitive data type but creates
an immutable object
– Once created, cannot be changed
– Does not need to be instantiated
• Stringbuilder is a class
– Must be instantiated
– Can be changed
• Use StringBuilder when many concatenations
are needed
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Reusability
• Reuse: class code is already written and
tested, so you build a new application faster
and it is more reliable
• Example: A Date class could be used in a
calendar program, appointment-scheduling
program, online shopping program, etc.
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How To Reuse A Class
• You don't need to know how the class is
written.
• You do need to know the application
programming interface (API) of the class.
• The API is published and tells you:
– How to create objects
– What methods are available
– How to call the methods
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The Argument List in an API
• Pairs of “dataType variableName”
• Specify
– Order of arguments
– Data type of each argument
• Arguments can be:
– Any expression that evaluates to the specified
data type
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Method Classifications
• Accessor methods
– Gets the values of object data
• Mutator methods
– Writes/changes values of object data
• Others to be defined later
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Dot Notation
• Use when calling method to specify which
object's data to use in the method
• Syntax:
objectReference.methodName( arg1, arg2, … )
• Note: no data types are specified in the
method call; arguments are values only!
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Calling a
Method
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• When calling a method, include only
expressions in your argument list. Including data
types in your argument list will cause a compiler
error.
• If the method takes no arguments, remember to
include the empty parentheses after the
method's name. The parentheses are required
even if there are no arguments.
• The following examples use string class
properties and methods
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Length Property
public int Length { get; }
The number of characters in the current string.
Remarks
The Length property returns the number of Char
objects in this instance, not the number of Unicode
characters.
Example:
string h= “hello”;
int len = h.Length;
len has a value of 5
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To upper and lower case
public string ToLower()
Return Value: A string in lowercase.
public string ToUpper()
Return Value: A string in uppercase.
Example:
string myString = “good luck”;
myString = myString.ToUpper();
myString now has the value “GOOD LUCK”
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IndexOf methods
public int IndexOf( char value )
The zero-based index position of value if that character
is found, or -1 if it is not.
public int IndexOf( string value)
The zero-based index position of value if that string is
found, or -1 if it is not.
string myString= “hello world”;
int e_index=myString.IndexOf(‘e’); // e_index= 1
int or_index= myString.IndexOf(“or”); //or_index=7
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Substring methods
public string Substring( int startIndex, int length )
A string that is equivalent to the substring of length length
that begins at startIndex in this instance
public string Substring( int startIndex)
A string that begins at startIndex and continues to the
end of the source string
string h= “hello”;
string s= h.Substring(1,3); //s = “ell”
string t = h.Substring (2); //t=“llo”
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.NET Architecture
• Framework
• When you press F5
– source code compiled into IL
– submitted to .NET engine for execution
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Base Class Libraries and The C# API
• This link will take you to the .Net framework
class library.
• https://msdn.microsoft.com/enus/library/gg145045%28v=VS.110%29.aspx
• On that page most of the classes you will need
are in the System namespace.
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using Declaration
• Must have using statement to use values in
library:
using System.Text;
• Or you can fully qualify:
System.Text.StringBuilder phrase =
new System.Text.StringBuilder
(“Change is inevitable”);
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Random Class
• To generate random numbers
• Generates a pseudorandom number
(appearing to be random, but mathematically
calculated based on seed value)
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Random API
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The upper
bound in
the Random
class is
exclusive
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Math Class
• Basic mathematical functions
• All methods are static methods (class
methods)
– invoked through the name of the class
– no need to instantiate object
• Two static constants
– PI = the value of pi
– E = the base of the natural logarithm
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Calling static Methods
• Use dot syntax with class name instead of
object reference
• Syntax:
ClassName.methodName( args )
• Example:
int absValue = Math.Abs( -9 );
• abs is a static method of the Math class that
returns the absolute value of its argument
(here, -9).
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Summary
• What did you learn?
• Muddiest Point
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