1. Object-Oriented Design AND CLASS PROPERTIESPs
Module 6 – Part II
Object-Oriented Design AND CLASS PROPERTIES
6/7/2019
CSE 1321 Module 6

2. Topics Default Constructors Multiple Constructors (overloading)
Constructor “chaining”
Control of Properties
“Getters” or “Accessors”
“Setters” or “Modifiers”
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CSE 1321 Module 6

3. Default Constructors In many languages, there’s a default constructor
If you don’t create one, it’s created for you AND
IT’S INVISIBLE! AND
It takes no parameters AND
It sets variables/attributes:
To zero (0) for numbers
To FALSE for booleans
To NULL (empty) for objects like strings
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4. What it would look like (if you could see it)
CLASS Dog BEGIN
CREATE rabid
CREATE weight
CREATE name
// It’s INVISIBLE!
CONSTRUCTOR Dog ( )
rabid ← false
weight ← 0
name ← NULL
END CONSTRUCTOR
END CLASS Ps

5. Multiple Constructors (a.k.a. overloading)
We can have more than one constructor
Remember, constructors are used to initialize our objects
We can use parameters with the constructor to customize the initialization
Sometimes we have more or less data to use in the customization
We’d like to be able to pass in only what we know
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6. Implementation
Overloading the constructor involves using the same method/function name
Vary the number of parameters; AND/OR
Vary the type of parameters
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7. Example
CLASS BMW_Z4 BEGIN CONSTRUCTOR BMW_Z4() // constructor #1 BEGIN ModelYear ← 2004 TopUp ← false LicensePlate ← "DEALER" END CONSTRUCTOR CONSTRUCTOR BMW_Z4(parameter: year) // constructor #2 BEGIN ModelYear ← year TopUp ← false LicensePlate ← "DEALER" END CONSTRUCTOR END CLASS Ps
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8. Which is Called?
BMW_Z4 myCar = new BMW_Z4(); BMW_Z4 yourCar = new BMW_Z4(2007); BMW_Z4 herCar = new BMW_Z4(2008);
The constructor with the matching parameter definition will be called
If no parameters, the () constructor
If one parameter, the (int) constructor
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9. Java - Constructor Example
class BMW_Z4 {
int modelYear;
String licensePlate;
boolean topUp;
BMW_Z4 () {
modelYear = 2004; topUp = false;
licensePlate = "DEALER";
} BMW_Z4 (int year)
modelYear = year; topUp = false; }
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10. C# - Constructor Example
class BMW_Z4 {
int modelYear;
string licensePlate;
bool topUp;
BMW_Z4 () {
modelYear = 2004; topUp = false;
licensePlate = "DEALER";
} BMW_Z4 (int year)
modelYear = year; topUp = false; }
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11. Python – Overload Constructor Example
Python doesn’t support overloading, but there are ways to work around this. We can still write only one constructor, but create it so that it accept multiple arguments. Then, in the constructor, we can look for certain values and assign them accordingly, as shown below:
#construct a BMW object
def __init__(self, modelYear = 2018, TopUp = True, licensePlate = "DEALER"):
self.modelYear = modelYear
self.topUp = TopUp
self.licensePlate = licensePlate
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12. Visibility of Class Content
Problem: Imagine class “Person” with an “age” attribute
Then, a “bad guy” directly sets the age to -3!
We need protection/security
We need a way of selectively “publishing” parts of a class and “hiding” other parts of the class
Use public & private keywords
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13. Sloppy Definitions
public: anyone (or anything) can see it from anywhere! That is, anything outside the class.
private: can only be seen/called inside the class.
Note, variables, methods and even classes can be marked either public or private
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14. Implications of public and private (this is a bit hard to understand…)
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15. General Rules Variables/attributes are normally marked private
public variables can be tampered with and abused!
Methods are most often marked public
However, some methods should be marked as private
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CSE 1321 Module 4

16. Visibility Example Class Name: BMW_Z4 Attributes:
ModelYear (private visibility, integer type)
LicensePlate (public visibility, string type – why public?)
TopUp (private visibility, boolean type)
Methods:
Drive (public visibility, no return value)
OpenTop() (public visibility, returns boolean)
EmitPollutantsWhenNoOneIsLooking() (private?)
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17. Visibility and Access from main
CREATE myCar AS BMW_Z4 // Create an object called myCar
myCar = new BMW_Z4 // Bring it to life
myCar.LicensePlate = "BMR4ME” // Legal. LicensePlate is public
myCar.ModelYear = // Illegal because ModelYear is private
myCar.Drive() // Legal. Drive() is public
myCar.OpenTop() // Legal. OpenTop() is public
// Illegal for multiple reasons… myCar.EmitPollutantsWhenNoOneIsLooking()
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18. Wait! How do we change private variables?
Let’s review, shall we?
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19. Public Doggies (with private lives)
PUBLIC CLASS Dog BEGIN
PRIVATE CREATE rabid // change this?
PRIVATE CREATE weight // and this?
PUBLIC CREATE name
// Boring constructor
PUBLIC CONSTRUCTOR Dog ( )
rabid ← false
weight ← 0
name ← NULL
END CONSTRUCTOR
END CLASS Ps

20. Main idea
Use methods to access and change the variables/attributes. Instead of direct access
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21. Accessors/Modifiers public methods that enable change to attributes
Also called “getters” and “setters”
Why is this cool? Can put in protections to prevent bad data
PUBLIC setAge (parameter: newAge) BEGIN IF (newAge < 110) THEN age = newAge ELSE PRINT (“That’s too old!”) ENDIF END
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22. Properties Example Pseudocode
CLASS BMW_Z4 BEGIN PRIVATE ModelYear PRIVATE LicensePlate PUBLIC METHOD SetModelYear (Year) BEGIN ModelYear ← Year END PUBLIC METHOD GetModelYear () BEGIN RETURN ModelYear END PUBLIC METHOD SetLicensePlate (value) BEGIN LicensePlate ← value END PUBLIC METHOD GetLicensePlate () BEGIN RETURN LicensePlate END END CLASS Ps
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CSE 1321 Module 6 5

23. Class BMW_Z4
public class BMW_Z4 { private int ModelYear; private String LicensePlate; private boolean TopUp; public void SetModelYear(int Year) { ModelYear = Year; } public int getModelYear() { return ModelYear; } public void SetLicensePlate(String value) {LicensePlate = value;} public String getLicensePlate(){ return LicensePlate; } public void SetTopUp(boolean value) { TopUp = value; } public boolean getTopUp() { return TopUp; } } 6
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CSE 1321 Module 6

24. Class BMW_Z4 public class BMW_Z4
{ private int modelYear; private string licensePlate; private bool topUp; public int ModelYear { get {return modelYear;} set {modelYear = value; }} public string SetLicensePlate
{ get {return licensePlate;} set{licensePlate = value;}} public bool TopUp { get {return TopUp;} set{topUp = value; }} } 7
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CSE 1321 Module 6

25. Class BMW_Z4 class BMW: def setModelYear(self, modelYear):
self.modelYear = modelYear
def getModelYear(self):
return self.modelYear
def setTopUp(self, topUp):
self.topUp = topUp
def getTopUp(self):
return self.topUp
def setLicensePlate(self, licensePlate):
self.licensePlate = licensePlate
def getLicensePlate(self):
return self.licensePlate 8
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CSE 1321 Module 6

26. A common point of confusion
We see a lot of keywords like this and self
What is that?
A reference to something inside the class we’re coding in
Commonly used to resolve ambiguity of variables:
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27. A point of confusion
PUBLIC CLASS Dog BEGIN PRIVATE CREATE rabid PRIVATE CREATE weight PUBLIC CREATE name PUBLIC CONSTRUCTOR Dog (parameter: rabid, weight, name) rabid ← rabid // what? weight ← weight // huh? name ← name // wuh? END CONSTRUCTOR END CLASS Ps
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28. Better
PUBLIC CLASS Dog BEGIN PRIVATE CREATE rabid PRIVATE CREATE weight PUBLIC CREATE name PUBLIC CONSTRUCTOR Dog (parameter: rabid, weight, name) this.rabid ← rabid // assign attributes this.weight ← weight // the parameters this.name ← name END CONSTRUCTOR END CLASS Ps
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29. Last thing: Constructor Chaining
It’s possible for one constructor to leverages/calls another
You can do this as many times as you want
Let’s see an example
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30. PUBLIC CLASS Dog BEGIN PRIVATE CREATE rabid PRIVATE CREATE weight PUBLIC CREATE name PUBLIC CONSTRUCTOR Dog ( ) // Default constructor // Call the constructor below this (FALSE, 4, “Fluffy”) END CONSTRUCTOR PUBLIC CONSTRUCTOR Dog (parameter: rabid, weight, name) this.rabid ← rabid this.weight ← weight this.name ← name END CLASS Ps
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31. Summary A class is made of variables and methods.
Variables represent the state of the class objects;
Variables should (almost) always be declared private
Method provide controlled access to variables
Methods intended for class client should be public.
Other methods for local use should be private.
Controlled access to the variables promotes encapsulation.
6/7/2019
CSE 1321 Module 6