Visual Basic: An Object Oriented Approach

1. Visual Basic: An Object Oriented Approach 3 – Making Objects Work

2. Object Structure • An Object • has an internal structure that defines its state or identity • has an interface through which other objects can work with it

3. Internal Structure and Operations • The state of an object is the set of values of its attributes • these are stored in variables, whose value can change over time • they can be altered or manipulated by operations, known as program statements

4. Operators • Operators work on values and variables to perform • simple arithmetic (+, -, *, / …) • string (text) manipulation • date manipulation • other forms of calculation • assignment (=) • comparison (=, >, <, >=, <=, Like) • Values, variables and operators can be put together (using a form of grammar) to form expressions

5. Expressions • An expression • is a combination of values, variables and operators • has a value (found by calculating the expression) • has a type of value (e.g. number, string, date…) • can be a direct replacement for a value of the appropriate type x+2 “Joe” & “ Bloggs” Date + 7 (1 week from today) z * (x + 2) (=z * 3 if x is 1)

6. Conditions • Conditions are expressions that evaluate to True or False • Use conditions to control what happens next in a program • select statements to execute • repeat certain statements until a condition is True If age >= 18 Then buy beer Else buy soft drink End If Do sit driving test Loop Until Result=“Pass”

7. Abstraction • To create a computer program, we must create an abstraction • Remove all unnecessary details • Retain all info necessary to the calculations

8. Object Orientation • In Object-Oriented programs, Objects are abstractions of real-world things • We often refer to this a Modelling • An object is a model of something we represent in a program • The object Interface is how we manipulate it

9. Statements • A statement is the fundamental syntactical element of a program • smallest piece of code that the language will accept in its own right • A statement can be used to • set aside space for data storage (variables) • assign a value to a variable • perform a calculation and assign its result to a variable • execute a previously-defined operation • control other statements

10. Example statements Declares variables Dim x As Integer, y As Single x = 2 x = x + 4 If x > 3 Then y = Sqr(x) Print y End If Assigns a value to the variable Assigns the result of an expression Controls other statements Executes a pre-defined operation

11. Statements in Objects • Operations in objects are sequences of statements • Can change the state of the object by changing the value of one or more variables • Can retrieve data from the object • Could do both • In general • A Sub is a sequence of statements that does something to the state of an object • A Function is a sequence of statements that retrieves a value based on the values of the object’s member variables • A Property is a set of statements that reflect an attribute of the object, allowing it to be read or changed • Sometimes, these distinctions can become blurred

12. Inter-object communication • One object (the Client) makes use of another object (the Server) by sending it a message • The message is a statement within one of the Client’s methods (operations). (ATM) (Account) Deposit (100.00) Account.Deposit 100.00 This statement is an operation in the ATM object