Chapter 4

1. Chapter 4 Object Oriented Concepts

2. Objects • Object is an intuitive concept • We think of things when thinking of ‘object’ • In OOP there is more to object • We will learn that objects in OOP have both attributes and behavior • Examples: a box has a shape, size, dimensions (attributes). It can be moved, opened, closed etc (behavior)

3. OO Concepts • Objects hold state and behavior • State is the value of data attributes • Behavior is set of operations to do • Data and operations are packaged together • Thus in one reference both are covered

4. Abstraction and Encapsulation • Abstraction expresses important features of entity • Encapsulation is what the user sees, but the details are hidden - groups things together • Encapsulation implements an abstraction • Access to the data is through special functions only • Thus data is protected from corruption • And users are protected from system changes

5. Classes • A class groups together objects with common behavior, structure • A new instance of an object can be created – called instantiation

6. Inheritance • Classes pass on their characteristics to derived (subordinate) classes • Example: • Base class Account • Derived classes are Savings and Checking • Savings has balance and interest • Checking has balance and fee • They are special types of Account

7. Methods • These are the actions the OBJECTS perform • Methods could be called programs too • Methods are “called” by other programs or methods • Sometimes they have “arguments” like: • public void getname (stringinvar) • Here string invar is an argument of getname • Ex: getname(“John Smith”)

8. Polymorphism • A method has multiple behaviors • The behavior depends on the object calling the method • Example: • A method in each (savings and checking) gets user input. • That input is private to the method, so it is secure

9. Example from the book pg 108 using System; namespace CritterMenu { class Menu { static void Main(string[] args) { bool keepGoing = true; int choice;

10. continued while (keepGoing){ choice = showMenu(); // call method showmenu switch (choice){ // showMenu returns a value contained in choice case 0: keepGoing = false; break; case 1: Console.WriteLine("you talk to the critter"); break; case 2: Console.WriteLine ("You have fed the critter"); break; case 3: Console.WriteLine("You have played with the critter"); break; case 4: Console.WriteLine("You have renamed the critter"); break; default: Console.WriteLine("That was not a valid input"); break; } // end switch } // end while loop } // end main

11. static int showMenu() { int input = 1; Console.WriteLine(); Console.WriteLine("0) Exit"); Console.WriteLine("1) Listen to Critter"); Console.WriteLine("2) Feed Critter"); Console.WriteLine("3) Play with Critter"); Console.WriteLine("4) Rename Critter"); try { input = Convert.ToInt32(Console.ReadLine()); } catch (FormatException) { Console.WriteLine("Incorrect input"); input = 1; } // end try return input; } // end showMenu

12. Method example using System; class sumfunctions { public static decimal getsum(decimal num1, decimal num2) { decimal total1; total1=num1+num2; return total1; }// end method static void Main () {

13. Main static void Main () { decimal salary, bonus, net; salary=5000.00; bonus=1000.00; net=getsum(salary,bonus); Console.Write(“Net Sal = ”+net); }// end Main The values for salary and bonus are sent to getsum. The result is returned to Main.

14. Classes and Methods (more..)

15. Theory of Classes • You have used them before • When code is written, a class is not created, it is only declared • As in arrays to create the class: • You use new as class Building { public int floors; // number of floors public int area; // total square footage of bldg public int occupants; // number of occupants } Building house = new Building(); • house is a new instance of Building

16. Every instance of Building creates a new copy of the variables • To access the variables, use the . • Ex: house.floors = 3; • The dot operator is used for both variables and methods • The rest of the topic:

17. class Building { public int floors; // number of floors public int area; // total square footage of bldg public int occupants; // number of occupants public void areaPerPerson() { double area1 = area / occupants; Console.WriteLine(" "+ area1 +" area per person"); } public void getvalues(string txt, string txt2, string txt3, out int var1,out int var2,out int var3) { string entt; Console.Write(txt); entt=Console.ReadLine(); var1=Convert.ToInt32(entt); Console.Write(txt2); entt=Console.ReadLine(); var2=Convert.ToInt32(entt); Console.Write(txt3); entt=Console.ReadLine(); var3=Convert.ToInt32(entt); }

18. class Buildingrunner { public static void Main() { Building house = new Building(); Building office = new Building(); // assign values to fields in house house.occupants = 4; house.area = 2500; house.floors = 2; Console.WriteLine("house has:\n " + house.floors + "floors\n "+ house.occupants + "occupants\n "+ house.area + " total area"); house.areaPerPerson(); Console.WriteLine();

19. //assign values to fields in office office.getvalues ("Enter # of occpnts", "Enter area", "Enter # floors", out office.occupants, out office.area, out office.floors); Console.WriteLine("office has:\n " + office.floors + "floors\n "+ office.occupants + "occupants\n "+ office.area + " total area"); office.areaPerPerson();

20. Calls

21. Call by Value • You have called objects by passing parameters from Main. • EX: sums (salary, tax) • The VALUE of salary and tax are sent to Main • The object perhaps returns a value to Main

22. Calls by Reference • This makes it possible to return more than one value to Main • A keyword ref is used to signal call by reference • The memory address of the variable is passed • Let’s examine an example

23. Call by Reference public static void get_tax(double v1, ref double out1, ref double out2) { if ( v1>15000.00) out1 = v1 * .05; else out1 = v1 * .01; out2 = v1 – out1; } In Main… Net =get_tax(salary, ref tax, ref net);