CSE 1301

Lecture 4

Using Classes

Figures from Lewis, “C# Software Solutions”, Addison Wesley

Richard Gesick

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Topics

• Class Basics and Benefits• Creating Objects• .NET Architecture and Base Class Libraries• Random Class• Math Class

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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

Methods

Instance variables

Class variables

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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 APIConstructor: special method that creates an

object and assigns initial values to dataDate 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 nullindependenceDay = null;

// attempt to use object referenceindependenceDay.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

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public int Length { get; } The number of characters in the current string.RemarksThe 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

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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

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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= 1int or_index= myString.IndexOf(“or”); //or_index=7

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Substring methods

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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/en-us/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