introduction_OOP for the java courses [Autosaved].pptx

Introduction To Object-Oriented
Programming
Basic Object-Oriented principles such as encapsulation,
overloading as well the object-oriented approach to design.

Reminder: What You Know
There are different approaches to writing computer programs.
They all involve decomposing your programs into parts.
What is different between the approaches (how the decomposition occurs)/(criteria used for breaking
things down”)
There approach to decomposition you have been introduced to thus far:
Procedural
Object-Oriented (~2 weeks for CPSC 231)

An Example Of The Procedural Approach
(Presentation Software)
Break down the program by what it does (described with
actions/verbs)
Filing Editing Helping
…
Creating
new
document
Opening a
document
Saving a
document
… Exiting
program
PowerPoint

What You Will Learn
• How to break your program down into objects (New term: “Object-Oriented
programming”)
• This and related topics comprise most of the remainder of the course

An Example Of The Object-Oriented Approach
(Simulation)
• Break down the program into entities (classes/objects - described with nouns)
Zoo
Animals Buildings
Visitors
Staff
Admin
Animal
care
Lions
Tigers
Bears (oh
my!)
ETC.

Classes/Objects
• Each class of object includes descriptive data.
• Example (animals):
• Species
• Color
• Length/height
• Weight
• Etc.
• Also each class of object has an associated set of actions
• Example (animals):
• Sleeping
• Eating
• Excreting
• Etc.

Example Exercise: Basic Real-World Alarm Clock
• What descriptive data is needed?
• What are the possible set of actions?

Additional Resources
A good description of the terms used in this section (and terms used in some of the later sections).
http://docs.oracle.com/javase/tutorial/java/concepts/
A good walk through of the process of designing an object-oriented program, finding the candidate
objects e.g., how to use the “find a noun’”approach and some of the pitfalls of this approach.
http://archive.eiffel.com/doc/manuals/technology/oosc/finding/page.html

Types In Computer Programs
• Programming languages typically come with a built in set of types that are known to
the translator
int num;
// 32 bit whole number (e.g. operations: +, -, *, /, %)
String s = "Hello";
// Unicode character information (e.g. operation: concatenation)
• Unknown types of variables cannot be arbitrarily declared!
Person tam;
// What info should be tracked for a Person
// What actions is a Person capable of
// Compiler error!

A Class Must Be First Defined
• A class is a new type of variable.
• The class definition specifies:
• What descriptive data is needed?
• Programming terminology: attributes = data (New definition)
• What are the possible set of actions?
• Programming terminology: methods = actions (new definition)
• A method is the Object-Oriented equivalent of a function

Defining A Java Class
Format:
public class <name of class>
{
attributes
methods
}
Example (more explanations coming shortly):
public class Person
{
private int age; // Attribute
public Person() { // Method
age = in.nextInt();
}
public void sayAge() {// Method
System.out.println("My age is " + age);
}
}

The First Object-Oriented Example
• Program design: each class definition (e.g., public class <class name>) must
occur its own “dot-java” file).
• Example program consists of two files in the same directory:
• (From now on your programs must be laid out in a similar fashion):
• Driver.java
• Person.java

The Driver Class
public class Driver
{
public static void main(String [] args)
{
Person aPerson = new Person();
aPerson.sayHello();
}
}
// Class person
public void
sayHello()
{
...
}

Class Person
public class Person
{
public void sayHello()
{
System.out.println("I don't wanna say hello.");
}
}

New Concepts: Classes Vs. Objects
• Class:
• Specifies the characteristics of an entity but is not an instance of that entity
• Much like a blue print that specifies the characteristics of a building (height, width, length etc.)
www.colorbox.com

New Concepts: Classes Vs. Objects (2)
• Object:
• A specific example or instance of a class.
• Objects have all the attributes specified in the class definition
Images: James Tam

main() Method
• Language requirement: There must be a main() method - or equivalent –
to determine the starting execution point.
• Style requirement: the name of the class that contains main() is often
referred to as the “Driver” class.
• Makes it easy to identify the starting execution point in a big program.
• Do not instantiate instances of the Driver1
• For now avoid:
• Defining attributes for the Driver1
• Defining methods for the Driver (other than the main() method)1
1 Details may be provided later in this course

Compiling Multiple Classes
• One way (safest) is to compile all code (dot-Java) files when any code
changes.
• Example:
• javac Driver.java
• javac Person.java
• (Alternatively use the ‘wildcard’): javac *.java

Why Must Classes Be Defined
• Some classes are already pre-defined (included) in a programming language with a
list of attributes and methods e.g., String
• Why don’t more classes come ‘built’ into the language?
• The needs of the program will dictate what attributes and methods are needed.

Defining The Attributes Of A Class In Java
• Attributes can be variable or constant (preceded by the ‘final’ keyword), for now
stick to the former.
• Format:
<access modifier>1
<type of the attribute> <name of the attribute>;
• Example:
public class Person
{
private int age;
}
1) Although other options may be possible, attributes are almost always set to private (more on this
later).

New Term: Object State
• Similar to how two variables can contain different data.
• Attributes: Data that describes each instance or example of a class.
• Different objects have the same attributes but the values of those attributes can vary
• Reminder: The class definition specifies the attributes and methods for all objects
• Example: two ‘monster’ objects each have a health attribute but the current value of
their health can differ
• The current value of an object’s attribute’s determines it’s state.
Age: 35
Weight: 192
Age: 50
Weight: 125
Age: 0.5
Weight: 7
www.colourbox.com

Defining The Methods Of A Class In Java
Format:
<access modifier>1
<return type2
> <method name> (<p1 type> <p1 name>, <p2 type> <p2 name>…)
{
<Body of the method>
}
Example:
public class Person
{
// Method definition
public void sayAge() {
}
}
1) For now set the access modifier on all your methods to ‘public’ (more on this later).
2) Return types: includes all the built-in ‘simple’ types such as char, int, double…arrays
and classes that have already been defined (as part of Java or third party extras)

Parameter Passing: Different Types
Parameter type Format Example
Simple types <method>(<type> <name>) method(int x, char y)
{ ... }
Objects <method>(<class> <name>) method(Person p) { ... }
Arrays <method>(<type> []…
<name>)
method(Map [][] m) { ... }
When calling a method, only the names of the parameters must be
passed e.g., System.out.println(num);

Return Values: Different Types
Return type Format Example
Simple types <type> <method>() int method()
{ return(0); }
Objects <class> <method>() Person method() {
Person p = new
Person();
return(p);
}
Arrays <type>[]... <method>() Person [] method() {
Person [] p = new
Person[3];
return(p);
}

What Are Methods
• Possible behaviors or actions for each instance (example) of a class.
Walk()
Talk()
Walk()
Talk()
Fly()
Swim()

Instantiation
• New definition: Instantiation, creating a new instance or example of a class.
• Instances of a class are referred to as objects.
• Format:
<class name> <instance name> = new <class name>();
• Examples:
Person jim = new Person();
Scanner in = new Scanner(System.in);
Creates new object
Variable names: ‘jim’,
‘in’

Constructor
• New term: A special method to initialize the attributes of an object as the objects are instantiated (created).
• The constructor is automatically invoked whenever an instance of the class is created e.g., Person aPerson = new
Person();
• Constructors can take parameters but never have a return type.
Constructor
Call to constructor
(creates
something ‘new’)
Object
x
y
z
Object
x = 1
y = 2
z = 3
class Person {
// Constructor
public
Person() {
…
}
}

New Term: Default Constructor
• Takes no parameters
• If no constructors are defined for a class then a default constructor comes ‘built-into’
the Java language.
• e.g.,
class Driver {
main() {
}
}
class Person {
private int age;
}
Do
previous
example
but with
constructo
r setting
arguments
show how
attributes
can be set
at run time
rather than
fixed

Calling Methods (Outside The Class)
• You’ve already done this before with pre-created classes!
• First create an object (previous slides)
• Then call the method for a particular variable.
• Format:
<instance name>.<method name>(<p1 name>, <p2 name>…);
• Examples:
jim.sayName();
// Previously covered example, calling Scanner class method
System.out.print("Enter your age: ");
age = in.nextInt();
Scanner
variable Calling
method

Calling Methods: Outside The Class You’ve Defined
• Calling a method outside the body of the class (i.e., in another class definition)
• The method must be prefaced by a variable (actually a reference to an object – more
on this later).
public class Driver {
public static void main(String [] args) {
Person bart = new Person();
Person lisa = new Person();
// Incorrect! Who ages?
becomeOlder();
// Correct. Happy birthday Bart!
bart.becomeOlder();
}
}

Calling Methods: Inside The Class
• Calling a method inside the body of the class (where the method has been defined)
• You can just directly refer to the method (or attribute)
public class Person {
private int age;
public void birthday() {
becomeOlder(); // access a method
}
public void becomeOlder() {
age++; // access an attribute
}

Second Object-Oriented Example
• Learning concepts:
• Attributes
• Constructors
• Accessing class attributes in a class method

Class Driver
public class Driver
{
{
jim.sayAge();
}
}
public Person() {
Scanner in = new
Scanner(System.in);
System.out.print("Enter
age: ");
age = in.nextInt();
}
public void sayAge()
{
System.out.println
("My age is "
+ age);
}

Class Person
public class Person
{
private int age;
public Person()
{
System.out.print("Enter age: ");
age = in.nextInt();
}
public void sayAge()
{
}
}

Creating An Object
• Two stages (can be combined but don’t forget a step)
• Create a variable that refers to an object e.g., Person jim;
• Create a *new* object e.g., jim = new Person();
• The keyword ‘new’ calls the constructor to create a new object in memory
• Observe the following
Person jim;
jim = new Person(12);
jim = new Person(22);
jim null
Jim is a reference to a Person
object
age =22
age =12

Terminology: Methods Vs. Functions
• Both include defining a block of code that be invoked via the name of the method or
function (e.g., print() )
• Methods a block of code that is defined within a class definition (Java example):
public class Person
{
public Person() { ... }
public void sayAge() { ... }
}
• Every object that is an instance of this class (e.g., jim is an instance of a Person)
will be able to invoke these methods.
jim.sayAge();

Terminology: Methods Vs. Functions (2)
• Functions a block of code that is defined outside or independent of a class (Python example – it’s largely
not possible to do this in Java):
# Defining method sayBye()
class Person:
def sayBye(self):
print("Hosta lavista!")
# Method are called via an object
jim = Person()
jim.sayBye()
# Defining function: sayBye()
def sayBye():
print("Hosta lavista!")
# Functions are called without creating an object
sayBye()

Methods Vs. Functions: Summary & Recap
Methods
• The Object-Oriented approach to
program decomposition.
• Break the program down into classes.
• Each class will have a number of
methods.
• Methods are invoked/called through an
instance of a class (an object).
Functions
• The procedural (procedure = function)
approach to program decomposition.
• Break the program down into functions.
• Functions can be invoked or called
without creating any objects.

Second Example: Second Look
Calls in Driver.java
jim.sayAge();
Person.java
private int age;
public Person() {
age = in.nextInt();
}
System.out.println("My age
is " + age);
}
}
More is needed:
•What if the attribute ‘age’ needs to
be modified later?
•How can age be accessed but not
just via a print()?

Viewing And Modifying Attributes
1) New terms: Accessor methods: ‘get()’ method
• Used to determine the current value of an attribute
• Example:
public int getAge()
{
return(age);
}
2) New terms: Mutator methods: ‘set()’ method
• Used to change an attribute (set it to a new value)
• Example:
public void setAge(int anAge)
{
age = anAge;
}

Class Person
• Notable differences: constructor is redesigned, getAge() replaces sayAge(),
setAge() method added
public class Person
{
private int age;
public Person() {
…
age = in.nextInt();
}
System.out.println("My
age
is " + age);
}
}
public class Person
{
private int age;
public Person() {
age = 0;
}
public int getAge() {
return(age);
}
public void setAge
(int anAge){
age = anAge;
}
}

Class Driver
public class Driver
{
{
System.out.println(jim.getAge());
jim.setAge(21);
System.out.println(jim.getAge());
}
}

Constructors
• Constructors are used to initialize objects (set the attributes) as they are
created.
• Different versions of the constructor can be implemented with different
initializations e.g., one version sets all attributes to default values while
another version sets some attributes to the value of parameters.
• New term: method overloading, same method name, different parameter list.
public Person(int anAge) { public Person() {
age = anAge; age = 0;
name = "No-name"; name = "No-name";
} }
// Calling the versions (distinguished by parameter list)
Person p1 = new Person(100); Person p2 = new Person();

Example: Multiple Constructors
• Location:
/home/219/examples/intro_OO/fourth_constructorOverloading

Class Person
public class Person
{
private int age;
private String name;
public Person()
{
System.out.println("Person()");
age = 0;
name = "No-name";
}

Class Person(2)
public Person(int anAge) {
System.out.println("Person(int)");
age = anAge;
name = "No-name";
}
public Person(String aName) {
System.out.println("Person(String)");
age = 0;
name = aName;
}
public Person(int anAge, String aName) {
System.out.println("Person(int,String)");
age = anAge;
name = aName;
}

Class Person (3)
public int getAge() {
return(age);
}
public String getName() {
return(name);
}
public void setAge(int anAge) {
age = anAge;
}
public void setName(String aName) {
name = aName;
}
}

Class Driver
Person jim1 = new Person(); // age, name default
Person jim2 = new Person(21); // age=21
Person jim3 = new Person("jim3"); // name=“jim3”
Person jim4 = new Person(65,"jim4");
// age=65, name = “jim4”
System.out.println(jim1.getAge() + " " +
jim1.getName());
jim2.getName());
jim3.getName());
jim4.getName());
}
}

New Terminology: Method Signature
• Method signatures consist of: the type, number and order of the
parameters.
• The signature will determine the overloaded method called:
Person p1 = new Person();
Person p2 = new Person(25);

Overloading And Good Design
•Overloading: methods that implement similar but not
identical tasks.
•Examples include class constructors but this is not the
only type of overloaded methods:
System.out.println(int)
System.out.println(double)
etc.
For more details on class System see:
• http://java.sun.com/j2se/1.5.0/docs/api/java/io/PrintStream.html
• Benefit: just call the method with required parameters.

Method Overloading: Things To Avoid
Distinguishing methods solely by the order of the parameters.
m(int,char);
Vs.
m(char,int);
Overloading methods but having an identical implementation.
Why are these things bad?

Method Signatures And Program Design
Unless there is a compelling reason do not change the
signature of your methods!
class Foo
{
void
fun()
{
}
}
Before:
class Foo
{
void fun(int
num)
{
}
}
After:
public static void main
()
{
Foo f = new Foo();
f.fun()
}
This change
has broken
me! 

Graphical Summary Of Classes
• UML (Unified modeling language) class diagram
• Source “Fundamentals of Object-Oriented Design in UML” by Booch, Jacobson, Rumbaugh
(Dorset House Publishing: a division of Pearson) 2000
• UML class diagram provides a quick overview about a class (later you we’ll talk about
relationships between classes)
• There’s many resources on the Safari website:
• http://proquest.safaribooksonline.com.ezproxy.lib.ucalgary.ca/
• Example “Sams Teach Yourself UML in 24 Hours, Third Edition” (concepts)
• Hour 3: Working with Object-Orientation
• Hour 4: Relationships
• Hour 5: Interfaces (reference for a later section of notes “hierarchies”)

UML Class Diagram
<Name of class>
-<attribute name>: <attribute type>
+<method name>(p1: p1type; p2 : p2 type..) :
<return type>
Person
-age:int
+getAge():int
+getFriends():Person []
+setAge(anAge:int):void

Why Bother With UML?
It combined a number of different approaches and has become the standard notation.
It’s the standard way of specifying the major parts of a software project.
Graphical summaries can provide a useful overview of a program (especially if relationships
must be modeled)
Just don’t over specify details

Back To The ‘Private’ Keyword
• It syntactically means this part of the class cannot be accessed outside of the class definition.
• You should always do this for variable attributes, very rarely do this for methods (more later).
• Example
private int age;
public Person() {
age = 12; // OK – access allowed here
}
}
aPerson.age = 12; // Syntax error: program won’t
// compile!
}
}

New Term: Encapsulation/Information Hiding
Protects the inner-workings (data) of a class.
Only allow access to the core of an object in a controlled fashion (use the public parts
to access the private sections).
Typically it means public methods accessing private attributes via accessor and mutator methods.
Controlled access to attributes:
Can prevent invalid states
Reduce runtime errors
private
data
public
method
public
method
public
method
set data
(mutator
method)
get data
(accessor
method)

How Does Hiding Information Protect Data?
Protects the inner-workings (data) of a class
e.g., range checking for inventory levels (0 – 100)
Location of the online example:
/home/219/examples/intro_OO/fifth_noProtection
Driver Inventory
+stockLevel: int
+Inventory()

Class Inventory
public class Inventory
{
public int stockLevel;
public Inventory()
{
stockLevel = 0;
}
}

Class Driver
public class Driver
{
{
Inventory chinook = new Inventory();
chinook.stockLevel = 10;
System.out.println("Stock: " + chinook.stockLevel);
chinook.stockLevel = chinook.stockLevel + 10;
chinook.stockLevel = chinook.stockLevel + 100;
chinook.stockLevel = chinook.stockLevel - 1000;
}
}

Utilizing Information Hiding: An Example
+MIN: int
+MAX: int
+CRITICAL: int
-stockLevel: int
+inventoryTooLow():boolea
n
+add(amount : int)
+remove(amount : int)
+showStockLevel()
Inventory
Driver

Class Inventory
public class Inventory
{
public final int CRITICAL = 10;
public final int MIN = 0;
public final int MAX = 100;
private int stockLevel = 0;
public boolean inventoryTooLow()
{
if (stockLevel < CRITICAL)
return(true);
else
return(false);
}

Class Inventory (2)
public void add(int amount)
{
int temp;
temp = stockLevel + amount;
if (temp > MAX)
{
System.out.println();
System.out.print("Adding " + amount +
" item will cause stock ");
System.out.println("to become greater than " + MAX + "
units (overstock)");
}
else
{
stockLevel = temp;
}
}

Class Inventory (3)
public void remove(int amount)
{
int temp;
temp = stockLevel - amount;
if (temp < MIN)
{
System.out.print("Removing " + amount +
System.out.println("to become less than " + MIN + " units
(understock)");
}
else
{
stockLevel = temp;
}
}
public String showStockLevel ()
{ return("Inventory: " + stockLevel); }
}

The Driver Class
public class Driver
{
public static void main (String [] args)
{
Inventory chinook = new Inventory();
chinook.add(10);
System.out.println(chinook.showStockLevel());
chinook.add (10);
chinook.add (100);
chinook.remove (21);
// JT: The statement below won't work and for
good reason!
// chinook.stockLevel = -999;
}
}

Add(): Try Adding 100 items to 20 items
public void add(int amount)
{
int temp;
temp = stockLevel + amount;
if (temp > MAX)
{
System.out.println();
System.out.print("Adding " + amount +
System.out.println("to become greater than " + MAX +
" units (overstock)");
}
else
{
stockLevel = temp;
}
} // End of method add

Remove(): Try To Remove 21 Items From 20 Items
public void remove(int amount)
{
int temp;
temp = stockLevel - amount;
if (temp < MIN)
{
System.out.print("Removing "
+ amount +
" item will cause
stock ");
System.out.println("to become less
than " + MIN + " units
(understock)");
}
else
{
stockLevel = temp;
}
}
public String showStockLevel ()
{ return("Inventory: " + stockLevel); }
}

New Terms And Definitions
• Object-Oriented programming
• Class
• Object
• Class attributes
• Class methods
• Object state
• Instantiation
• Constructor (and the Default constructor)
• Method
• Function

New Terms And Definitions (2)
• Accessor method (“get”)
• Mutator method (“set”)
• Method overloading
• Method signature
• Encapsulation/information hiding
• Multiplicity/cardinality

After This Section You Should Now Know
• How to define classes, instantiate objects and access different part of an
object
• What is a constructor and how is it defined and used
• What are accessor and mutator methods and how they can be used in
conjunction with encapsulation
• What is method overloading and why is this regarded as good style
• How to represent a class using class diagrams (attributes, methods and access
permissions) and the relationships between classes
• What is encapsulation/information-hiding, how is it done and why is it important to
write programs that follow this principle

Copyright Notification
• “Unless otherwise indicated, all images in this presentation are used with permission
from Microsoft.”
slide 71

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