1. CIS162AD Constructors Part 2 08_constructors.ppt

2. CIS162AD2 Overview of Topics  Class vs Object  ReadOnly Properties  Instance vs. Static (Shared)  Constructors and Destructors

3. CIS162AD3 Class and Objects  A class is a definition of a data type.  A class definition includes members variables and methods.  An object is a variable declared using a class definition as the data type.  A class is the definition, and an object is an instance of the class.

4. CIS162AD4 DateMDY Class DateMDY - month (now private) - day - year +setMonth +getMonth +setDay +getDay +setYear +getYear +getDate Notation: - private + public # protected

5. CIS162AD5 DateMDY Definition public class DateMDY { private int intMonth, intDay, intYear; public int Month { get { return intMonth; } set { intMonth = value;//value required keyword } //need property procedures for Day and Year public string getDate( ) As String { return ( intMonth.ToString(“N0”) + “/” + intDay.ToString(“N0”) + “/” + intYear.ToString(“N0”) ); } }

6. CIS162AD6 Public vs Private Members  Public variables can be referenced and altered in methods that declare an object using the class definition.  Public methods can be called directly in methods that declare an object using the class definition.  Private variables can only be referenced and altered by methods defined inside of the class.  Private methods can only be called by methods defined inside of the class.  Private is the default if not specified.  We must provide Property Blocks for private variables that allow programmers to set and get the values stored in the private variables.

7. CIS162AD7 Read-Only Properties  This is used for variables that are maintained by the class, but may need to provide the value to the program at some time.  This could be used on running totals, counts, or constants used in the class.  The getDate function could be converted to a Read-Only Property.  To create a Read-Only property, only code the get method of the Property Block.

8. CIS162AD8 Defining Read-Only public Class DateMDY { //Need to return the date for all instances of DateMDY. //Convert getDate to Property Date. public string Date { get { return ( intMonth.ToString(“N0”) + “/” + intDay.ToString(“N0”) + “/” + intYear.ToString(“N0”) ); } //No Set because it is read only. } }

9. CIS162AD9 Using Read-Only Property Block DateMDY bday = new DateMDY; bday.Month = 10;//use property block names bday.Day = 31;//Automatically calls Set procedure bday.Year = 1980; txtDate.Text = bday.Date( ); //Calls the Get method for Date

10. CIS162AD10 Instance vs. Static (Shared) Variables  Variables defined in a class are normally considered Instance variables.  Each object created would have separate memory allocations for their own variables.  A static variable is a single variable for all objects.  It can be used to keep a running total, count, or it could be a constant.  Static members can be accessed without instantiating an object of the class.  Static members must be referenced using the ClassName even when an instance of the class has been created (ClassName.member)  Methods that reference a static variable should also be declared as static so they can referenced without creating an instance.  Methods that are declared as static, must only reference class variables that are static and/or local variables.

11. CIS162AD11 Defining Static public class DateMDY { //The current date is needed for all instances of DateMDY //We would need to assign the system date to these variables private static intCurrMonth, intCurrDay, intCurrYear; public static string CurrentDate { get { return (intCurrMonth.ToString(“N0”) + “/” + intCurrDay.ToString(“N0”) + “/” + intCurrYear.ToString(“N0”) ); } //No Set because it is read only. } }

12. CIS162AD12 Using Static Variables DateMDY bday = new DateMDY; DateMDY dueDate = new DateMDY; //Static methods are called using class name. lblCurrDate.Text = DateMDY.CurrentDate //There would be one CurrentDate for both bday. and dueDate. //and both have the same value

13. CIS162AD13 Passing Objects to Methods  Individual variables can be passed to methods using get Methods: bday.Month.ToString(“N0”)  Entire objects can be passed to methods: private void add30Days(DateMDY ref dueDate)  The reference (address) is passed to the method, so the local object actually points to the location of the original object.  Any changes performed in the method will effect the original object.  In a call-by-value the changes will be local to the method.

14. CIS162AD14 Constructor Characteristics  A special method of the class.  The name of the method is the same is the class name.  Can NOT return a value.  Automatically called when an object of that class type is declared.  Must be Public.  Primarily used to initialize member variables.

15. CIS162AD15 Default Constructor  Default constructor has no parameters.  Always define a default constructor, even if it will not be doing any initialization.  If one is not defined,the compiler will generate one.  Usually used to assign a default value. public DateMDY ( ) { intMonth = 1;//assign default date intDay = 1; intYear = 1900; }

16. CIS162AD16 Overloaded Constructors  Constructors can be overloaded. –Same method name but a different number or type of parameters.  Initial values can be passed at declaration if there is a parameterized constructor defined. public DateMDY (int m, int d, int y) { intMonth = m; intDay = d; intYear = y; }

17. CIS162AD17 Property Blocks and Constructors  Use property blocks by using the property name, because any validation would be performed in the set method. public DateMDY (int m, int d, int y) { Month = m;//calls set procedure Day = d; Year = y; }

18. CIS162AD18 Keyword This  Sometimes we declare variables in class methods using the same names as of the class variables or property names.  Local variables override class variables and properties.  Use this. to reference the class variables or properties. public DateMDY (int Month, int Day, int Year) { this.Month = Month; //calls set procedure this.Day = Day; this.Year = Year; }

19. CIS162AD19 Date Property  In the next two slides, you are presented with some questions, and you will need to recall what the Date property does, so it is presented here as a reminder. public string Date { get { return ( intMonth.ToString(“N0”) + “/” + intDay.ToString(“N0”) + “/” + intYear.ToString(“N0”) ); } //No Set because it is read only. }

20. CIS162AD20 Using Default Constructors public DateMDY ( ) { intMonth = 1;//assign default date intDay = 1; intYear = 1900; }  In the declaration below determine why is the default constructor called and what is returned by Date( ). DateMDY bday = new DateMDY; txtDate.Text = bday.Date( );

21. CIS162AD21 Using Overloaded Constructors public DateMDY(int m, _int d, int y) { Month = m;//calls set procedure Day = d; Year = y; }  In the declaration below determine why the overloaded constructor is called and what is returned by Date( ). DateMDY payDate = new DateMDY(2, 26, 2002); txtDate.Text = payDate.Date( );

22. CIS162AD22 Destructor  A method that is automatically called when object goes out of scope.  Must be public.  The name of method is the class name preceded with a tilde (~) ~DateMDY( )  Can NOT return a value.  Can be used to “cleanup” dynamic variables.  It is recommended that we do not declare one unless it is required, because the.Net Framework performs it own garbage collection (releases memory).

23. CIS162AD23 Class Outline - Expanded  Class Name  Properties (variables)  Operations (methods) –Constructors –Destructors –Property Blocks (set and get) –Operations (getDate, Date)

24. CIS162AD24 Data Validation in Property Blocks  In the set methods of property blocks is where data validation would usually be included.  When invalid data is sent in, the class can rejected the data by Throwing an exception.  That means that the declaration of the object and usage of it’s property methods should be inside an Try/Catch block.  In our assignments we will NOT be including data validation

25. CIS162AD25 Data Validation Definition public string Description { get { return mstrDescription; } set { if (value ! = “”) mstrDescription = value; else throw new System.Exception(“Description is missing.”); } }

26. CIS162AD26 Data Validation Usage private void btnProcess(…) { try { objOrder = new clsOrder; objOrder.Description = txtDescription.Text; //… additional processing here } catch ex As Exception { MessageBox.Show("Error: " + ex.Message); } }

27. CIS162AD27 Summary  Class vs Object  Read-Only Properties  Instance vs. Static (Shared)  Constructors and Destructors