CSE 1301 Lecture 4 Using Classes Figures from Lewis, “C# Software Solutions”, Addison Wesley Richard Gesick

CSE 1301 Topics Class Basics and Benefits Creating Objects.NET Architecture and Base Class Libraries Random Class Math Class

CSE 1301 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.

CSE 1301 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

CSE 1301 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.

CSE 1301 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

CSE 1301 What’s in a Class Class contains Members are Fields Methods Instance variables Class variables

CSE 1301 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.

CSE 1301 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

CSE 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

CSE 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

CSE 1301 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

CSE 1301 Instantiation Examples Date independenceDay; independenceDay = new Date(7,4, 1776 ); Date graduationDate = new Date(5,15,2008); Date defaultDate = new Date( );

CSE 1301 Objects After Instantiation Object Instances

CSE 1301 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

CSE 1301 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;

CSE 1301 Two References to an Object After program runs, two object references point to the same object

CSE 1301 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.

CSE 1301 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);

CSE 1301 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

CSE 1301 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.

CSE 1301 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

CSE 1301 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

CSE 1301 Method Classifications Accessor methods – Gets the values of object data Mutator methods – Writes/changes values of object data Others to be defined later

CSE 1301 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!

CSE 1301 Calling a Method

CSE 1301 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

CSE 1301 Length Property 28 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

CSE 1301 To upper and lower case 29 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”

CSE 1301 IndexOf methods 30 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

CSE 1301 Substring methods 31 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 ”

CSE 1301.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. us/library/gg145045%28v=VS.110%29.aspx us/library/gg145045%28v=VS.110%29.aspx On that page most of the classes you will need are in the System namespace. 34

CSE 1301 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) 3-37

CSE 1301 Random API 3-38

CSE The upper bound in the Random class is exclusive

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CSE 1301 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 3-41

CSE 1301 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). 3-42

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CSE 1301 Summary What did you learn? Muddiest Point