1. Lecture 4
Using Classes
Richard Gesick

2. Topics
Class Basics and Benefits
String class
Random Class
Math Class

3. 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.

4. 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

5. What’s in a Class Class contains Members are Fields Instance variables
Methods
Instance variables
Class variables

6. 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

7. 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

8. 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

9. Date Class Constructor Summary
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

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

11. 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

12. 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.

13. 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

14. 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.

15. 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

16. 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

17. 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!

18. When calling a method, include only expressions in your argument list
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

19. 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

20. 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”

21. 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

22. 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=“ello”

23. Base Class Libraries and The C# API
This link will take you to the .Net framework class library.
On that page most of the classes you will need are in the System namespace.

25. 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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26. Random API
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27. The upper bound in the Random class is exclusive
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28. 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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29. 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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