Lecture 1 Classes and Objects

8/18/2019 Lecture 1 Classes and Objects

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Classes and Objects

Prepared by: Prof. Irysh Paulo R. Tipay, MSCS











Quote for the Day

“ Any fool can write code that a computer canunderstand. Good programmers write codethat humans can understand. ” - Martin Fowler



Recap!

What you already know… Classes are the fundamental building blocks

of a Jaa program

From Classes! we can create Objects"

#n Jaa! Objects are basically referencevariables

A Class can hae its own Method/s $eg. %he&tring 'lass hae the methods length$(!

charAt$(! inde)*f$( etc".(

%here are a lot of built-in 'lasses in Jaa $eg.&tring 'lass! &canner 'lass! etc(



User-Defined Classes

A class that is created by a programmer.



Objects

Objects in Object-Oriented Programming!represents something in the real world

+g. 'ats! ,ogs! %rees! &chools! een umaneings are considered *b/ects

#n OOP, +erything under the sun isconsidered as an Object.



Objects

*J+'%&

A%%0#1%+& +A2#*0&



Objects

Gala)y 3ote

A%%0#1%+&'olor 4 red

5hone number 4678988

rand 4 &amsung3etwork &ubscriber -

&mart

+A2#*0&&end %e)t Message

'all5lay games

&urf 3et



Objects – Try this one….

,og

A%%0#1%+& : +A2#*0& :



Objects – Try this one….

&tudent

A%%0#1%+& : +A2#*0& :



Classes

Blue-print / model that is used to create anObject.

Just like an *b/ect! the 'lass is also consist ofa set of attributes and behaiours.



Classes

5erson

3ameAge

Address

+at&leepreath5lay

;atch %2study

5erson ob/ect 8

Joe<8

=ue>on 'ity

+at&leep

reath5lay

;atch %2study

5erson ob/ect <

Anna87

5asay 'ity

+at&leep

reath5lay

;atch %2study

#n this e)ample! twoob/ects are created from

the 5erson class.





Putting Concept into Action

5erson

nameage

address

eatsleep

%his is actually called a 'CA&&,#AG0AM! a kind of 1MC

$unied modelling language(used to design classes



Putting Concept into Action

5erson

- &tring name - &tring age

-&tring address

?oid eat$(D? oid sleep$(D

%his is a formal 'lass,iagram. #t actually containsthe data type and return

types of our attributes andbehaiours.



Let the Coding begin….

public class 5ersonE priate &tring nameD priate &tring ageD priate &tring addressD

public oid eat$(E

&ystem.out.println$A person eats(D public oid sleep$(E &ystem.out.println$A personsleeps(D

5erson


-&tring address

?oid eat$(D? oid sleep$(D

%hereHs still something missingI



Constructor

%his is a special Method! without a returntype! that is used to set the initial alues forour Attributes

#t has the same name as the Class name

A 'lass can have more than one 'onstructor proided that they hae di!erentparameters

"efault Constructor # A constructor without

any parameters Overloaded Constructor # A constructor

with a parameter.



Adding Constructor to our Program

public class 5ersonE priate &tring nameD priate &tring ageD priate &tring addressD

public Person$%&

name ' ()oe*+ age ' (*+ address ' (.C*+

public oid eat$(E

&ystem.out.println$A person eats(D public oid sleep$(E &ystem.out.println$A personsleeps(D

5erson


-&tring address

0Person$% ?oid eat$(D

? oid sleep$(D

3*%+ theconstructor hasthe same name

as the classname



Adding another constructorpublic class 5ersonE

priate &tring nameD priate &tring ageD priate &tring addressD

public Person$%& name ' ()oe*+

age ' (*+ address ' (.C*+

public Person$1tringname%& thisname ' name+ age ' (*+ address ' (.C*+

public oid eat$(E

&ystem.out.println$Aperson eats(D public oid sleep$(E &ystem.out.println$Aperson sleeps(D

Kou may notice that we hae

used the “this” keyword inour program



Thethisreference

%hethis

reference is used to refer to ourglobal variable+ a ariable that can beaccessed by the whole class

%he opposite of the global ariable is the local

variable. Meaning it can be only accessedwithin a method.



Thethisreferencepublic class 5ersonE





c

Global

2ariable

Cocal 2ariable



The private access modifierpublic class 5ersonE





;e want our attributesLelds to bepriate because we donHt want it to

be accessed by other classes.

%his is because of the concept of

**5 called +ncapsulation. ;hich willbe discussed in the ne)t chapter$'oming &oonI(



Using our Class

y now! our class is ready for useI #n order to use our class! we need to create a

new class that will make use of it.

;e can create ne2 Objects by using

3nstantiation 3nstantiation is the process of creating

ob/ects.



Using our Classpublic class %estE

public static oid main$&tringargsN(E 5erson p8 O new 5erson$(D 5erson p< O new5erson$Juan(D

p8.eat$(D p<.sleep$(D

c#nstantiationI



What happens behind the Scenes

5erson

p8 p<



5erson p8 O new 5erson$(D

p8 86868

86868

3ame JoeAge <8

Address ='

eat$(&leep$(





%wo int s and a Scanner walk into a bar. %hey spot an attractie

String on her own.

%he rst int walks up to her and say “ey baby! my VM oryours:”. &he slaps him and he walks back de/ected.

%he second int walks oer. “ey cute-stuP! can we go haesome Java coPee at my place:” After a Quick slapping! he toowalks back.

%hen the Scanner ambles oer casually “;here those two

primitie types bothering you:”! he remarks.

“Kes! #Hm so glad youHre here”! she says. “%hey /ust had noClassI”

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Lecture 1 Classes and Objects