Chap 1 Declarations and Acces Control Update

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Chapitre 1

Declarationsand Access Control

MohamedRomdhani

INSAT


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Plan

Identifiers and JavaBeans Legal Idenfiers Sun's Java Code Conventions JavaBeans Naming Standards

Declare Classes

Source File Declaration Rules Class Declarations and Modifiers Class Access Oter !Nonaccess" Class Modifiers

Declare Interfaces

Declaring an Interface Declaring Interface Constants

Declare Class Members


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Identifiers and JavaBeans


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Legal Idenfiers

Identifiers must start with a letter, a currency character ($), or aconnectin character such as the underscore ( ! )"

Identifiers cannot start #it a num$er %

After te first caracter& identifiers can contain an com$ination of letters&currenc caracters& connecting caracters& or num$ers(

In )ractice& tere is no limit to te num$er of caracters an identifier can contain(

Identifiers in Java are case#sensitive foo and %oo are two differentidentifiers"

&ou can't use a Java eyword as an identifier" Java * +e#ords


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Legal Identifiers (!a"ples#

*am+les of some leal identifiers

int ,a-

int .c-

int ,,,,,,/,#-

int ,.-

int tis,is,a,ver,detailed,name,for,an,identifier-

he followin are illeal (it's your -ob to reconi.e why)

int $0-

int 1d-

int e2-

int (f-

int 3g-


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%&n's Java Code Conventions

Classes and interfaces

he first letter should be ca+itali.ed& and if several #ords are lin4ed togeter toform te name& te first letter of te inner #ords sould $e u))ercase !a format tat'ssometimes called 56ascalCase5"( For classes& te names sould t)icall $e nouns(

For e7am)le0 Dog & Account & 6rint8riter

For interfaces& te names sould t)icall $e ad9ectives li4e 0Runna$le Seriali:a$le

Methods

he first letter should be lowercase& and ten normal camelCase rules sould $eused( In addition& te names sould t)icall $e ver$1noun )airs(

For e7am)le0 getBalance!" doCalculation!"& setCustomerName!"

/ariables

Li4e metods& te camelCase format sould $e used& startin with a lowercaseletter( Sun recommends sort& meaningful names& #ic sounds good to us( Some e7am)les0 $utton8idt accountBalance mString

Constants

Java constants are created $ mar4ing varia$les static and final( ;e sould $enamed using u++ercase letters#it underscore caracters as se)arators0 MIN,,


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JavaBeans %tandards

%irst, JavaBeans are Java classes that have properties" For our )ur)oses& tin4 of )ro)erties as )rivate instance varia$les(

;e metods tat cange a )ro)ert's value are called settermetods& and te metods tatretrieve a )ro)ert's value are called gettermetods(

JavaBean 0ro+erty 1amin Rules

If te )ro)ert is not a $oolean& the etter method's +refi* must be get(

If te )ro)ert is a $oolean& te getter metod's )refi7 is eiter get or is( For e7am)le&getSto))ed!" or isSto))ed!" are $ot valid JavaBeans names for a $oolean )ro)ert(

he setter method's +refi* must be set(

;o com)lete te name of a getter or setter metod& cange te first letter of te)ro)ert name to u))ercase& and ten a))end it to te a))ro)riate )refi7 !get& is&or set"(

Setter metod signatures must $e mar4ed )u$lic& #it a void return t)e and anargument tat re)resents te )ro)ert t)e(

>etter metod signatures must $e mar4ed )u$lic& ta4e no arguments& and ave areturn t)e tat matces te argument t)e of te setter metod for tat )ro)ert(


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JavaBeans %tandards

the JavaBean s+ec su++orts events, which allow com+onents to notify

each other when somethin ha++ens" ;e event model is often used in >?I a))lications #en an event li4e a mouse clic4 is

multicast to man oter o$9ects tat ma ave tings to do #en te mouse clic4 occurs(

JavaBean 2istener 1amin Rules

Listener metod names used to 5register5 a listener #it an event source must

use the +refi* add& follo#ed $ te listener t)e( For e7am)le& addActionListener!" is a valid name for a metod tat an

event source #ill ave to allo# oters to register for Action events(

Listener metod names used to remove !5unregister5" a listener must use the+refi* remove& follo#ed $ te listener t)e !using te same rules as teregistration add metod"(

;e t)e of listener to $e added or removed must $e )assed as te argument tote metod(

=7am)les of valid JavaBean metod signatures are

)u$lic void addMListener!MListener m"

)u$lic void removeMListener!MListener m" =7am)les of invalid JavaBean metod signatures are

)u$lic void add@Listener!MListener m" listener t)e mismatc


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Declare Classes


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%o&rce ,ile Declaration -&les

here can be only one +ublic class +er source code file"

Comments can a++ear at the beinnin or end of any line in the source code file3they are inde+endent of any of the +ositionin rules discussed here"

If there is a +ublic class in a file, the name of the file must match the name of the+ublic class"

If the class is +art of a +acae, the +acae statement must be the first line inthe source code file, before any im+ort statements that may be +resent"

If there are im+ort statements, they must o between the +acae statement (ifthere is one) and the class declaration"

im+ort and +acae statements a++ly to all classes within a source code file" In

other words, there's no way to declare multi+le classes in a file and have them indifferent +acaes, or use different im+orts"

4 file can have more than one non +ublic class"

%iles with no +ublic classes can have a name that does not match any of theclasses in the file"


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Class Declarations and .odifiers

4ccess control in Java is a little tricy because there are four access

controls (levels of access) but only three access modifiers(+rivate, +rotected, +ublic)"

;e fourt access control level !called default or )ac4age access" is#at ou get #en ou don't use an of te tree access modifiers(

In oter #ords& ever class& metod& and instance varia$le ou declare asan access control& #eter ou e7)licitl t)e one or not( Altoug all four

access controls !#ic means all tree modifiers" #or4 for most metodand varia$le declarations& a class can be declared with only+ublic ordefault access- te oter t#o access control levels don't ma4e sense for aclass& as ou'll see(


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Class Access

Default 4ccess

A class #it default access as no modifier )receding it in tedeclaration% A class #it default access can be seen only by classeswithin the same +acae(

0ublic 4ccess

A class declaration #it te )u$lic 4e#ord gives all classes from all+acaes access to the +ublic class(

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/ther (0onaccess# Class .odifiers

&ou can modify a class declaration usin the eyword final, abstract, or

strictf+" Mar4ing a class as strictf+means tat an metod code in te class #ill

conform to te I=== 3* standard rules for floating )oints(

%inal Classes

8en used in a class declaration& te final 4e#ord means te class can't$e su$classed( In oter #ords& no oter class can ever e7tend !inerit from"

a final class& and an attem)ts to do so #ill give ou a com)iler error( 4bstract Classes

An a$stract class can never $e instantiated& Its sole )ur)ose is to $ee7tended !su$classed"(

&ou can't mar a class as both abstractand final" hey have nearly

o++osite meanins"

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Declare Interfaces

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Declaring an Interface

hin of an interface as a 566#+ercentabstract class" hese rules are strict 4ll interface methods are im+licitly +ublic

and abstract"ou do not need to actuallt)e te )u$lic or a$stract modifiers in temetod declaration& $ut te metod is stillal#as )u$lic and a$stract(

4ll variables defined in an interface mustbe +ublic, static, and final7in oter #ords&interfaces can declare onl constants& notinstance varia$les(

Interface methods must not be static(

Because interface metods are a$stract& tecannot $e mar4ed final& strictf)& or native(

An interface can e7tend one or more otherinterfaces"

An interface cannot e7tend anting $utanoter interface(

4n interface cannot im+lement anotherinterface or class"

Interface t)es can $e used )olmor)icall(

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Declaring Interface Constants

&ou need to remember one ey rule for interface constants" hey must always be+ublic static final

Because interface constants are defined in an interface& te don't ave to $edeclared as )u$lic& static& or final( ;e must $e )u$lic& static& and final& but youdon't have to actually declare them that way (

=7am)le 0 interface Foo int BAR E /-void go!"-

%or e*am+le, the followin interface definitions that define constants are correctand arre all identical"

)u$lic int 7 E G- Loo4s non1static and non1final&

$ut isn't%

int 7 E G- Loo4s default& non1final&

non1static& $ut isn't%

static int 7 E G- Doesn't so# final or )u$lic

final int 7 E G- Doesn't so# static or )u$lic

)u$lic static int 7 E G- Doesn't so# final

)u$lic final int 7 E G- Doesn't so# static

static final int 7 E G Doesn't so# )u$lic

)u$lic static final int 7 E G- #at ou get im)licitl

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Declare Class .e"ers

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Access .odifiers

Members can use all four

)u$lic & )rotected & default& )rivate

6u$lic Mem$ers

8en a metod or varia$le mem$er is declared )u$lic& it means all oterclasses& regardless of te )ac4age te $elong to& can access te mem$er!assuming te class itself is visi$le"(

6rivate Mem$ers

Mem$ers mar4ed )rivate can't $e accessed $ code in an class oter tante class in #ic te )rivate mem$er #as declared(

6rotected and Default Mem$ers

;e )rotected and default access control levels are almost identical& $ut#it one critical difference( 4 default member may be accessed only ifthe class accessin the member belons to the same +acae,whereas a +rotected member can be accessed (throuh inheritance)

by a subclass even if the subclass is in a different +acae(

2ocal /ariables and 4ccess Modifiers

Can access modifiers $e a))lied to local varia$lesH NO%

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0onaccess .e"er .odifiers

%inal Methods

;e final 4e#ord )revents a metod from $eing overridden in a su$class& and isoften used to enforce te A6I functionalit of a metod( Final Arguments 0 A final argument must 4ee) te same value tat te )arameter

ad #en it #as )assed into te metod(

4bstract Methods ou mar4 a metod a$stract #en ou #ant to force su$classes to )rovide te

im)lementation(

8ynchroni.ed Methods

;e sncroni:ed 4e#ord indicates tat a metod can $e accessed $ onl onetread at a time(

1ative Methods

;e native modifier indicates tat a metod is im)lemented in )latform1de)en dentcode& often in C(

8trictf+ Methods

strictf) forces floating )oints to adere to te I=== 3* standard

Methods with /ariable 4rument 2ists (var#ars)

Java * allo#s ou to create metods tat can ta4e a varia$le num$er of arguments(

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Constr&ctor Declarations

very class has a constructor, althouh if you don't create one

e*+licitly, the com+iler will build one for you"

hey must have the same name as the class in which they are declared

Constructor declarations can however have all of the normal accessmodifiers, and they can tae aruments (includin var#ars), -ust lie

methods"

Constructors can't be mared static (they are after all associated withob-ect instantiation), they can't be mared final or abstract (becausethey can't be overridden)"

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ariale Declarations

here are two ty+es of variables in Java

0rimitives A )rimitive can $e one of eigt t)es0 char, boolean, byte, short, int, lon,

double, or float(

Reference variables A reference varia$le is used to refer to !or access" an o$9ect( A reference varia$le

is declared to $e of a s)ecific t)e and tat t)e can never $e canged( Instancearia$les

Instance variables are the fields that belon to each uni9ue ob-ect" %or thee*am, you need to now that instance variables

Can use an of te four access levels !#ic means te can $e mar4ed #it anof te tree access modifiers"

Can $e mar4ed final Can $e mar4ed transient Cannot $e mar4ed a$stract Cannot $e mar4ed sncroni:ed Cannot $e mar4ed strictf) Cannot $e mar4ed native Cannot $e mar4ed static& $ecause ten te'd $ecome class varia$les(

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2ocal (4utomatic:8tac:Method) /ariables

Local varia$les are varia$les declared #itin a metod(

4rray Declarations

In Java& arras are o$9ects tat store multi)le varia$les of te same t)e& orvaria$les tat are all su$classes of te same t)e(

Declarin an 4rray of 0rimitives intK 4e- Suare $rac4ets $efore name !recommended"

int 4e K- Suare $rac4ets after name !legal $ut less reada$le"

Declaring an Arra of O$9ect References ;readK treads- Recommended ;read treads K- Legal $ut less reada$le

%inal /ariables

Declaring a varia$le #it te final 4e#ord ma4es it im)ossi$le to reinitiali:e tatvaria$le once it as $een initiali:ed #it an e7)licit value( Burn tis in0 tere are no final o$9ects& onl final references

ransient /ariables

If ou mar4 an instance varia$le as transient& ou're telling te JM to s4i) !ignore"tis varia$le #en ou attem)t to seriali:e te o$9ect containing it(

ariale Declarations

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ariale Declarations

/olatile /ariables

;e volatile modifier tells te JM tat a tread accessing te varia$lemust al#as reconcile its o#n )rivate co) of te varia$le #it temaster co) in memor(

8tatic /ariables and Methods

;e static modifier is used to create varia$les and metods tat #ill e7istinde)endentl of an instances created for te class( ;ings ou canmar4 as static0

Metods

aria$les

A class nested #itin anoter class& $ut not #itin a metod

Initiali:ation $loc4s

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Declaring n&"s

he basic com+onents of an enum are its constants

enum Seasons Automn& 8inter& S)ring& Summer-

nums can be declared as their own se+arate class, or as a classmember, however they must not be declared within a method ;

8o what ets created when you mae an enum3

Coffee drin E new Coffee()3 drin"si.e E Coffee8i.e" /R@2MI1>3

8ystem"out"+rintln(drinl"si.e"etunces())3 :: +rints ? 8ystem"out"+rintln(drin"si.e" etunces())3 :: +rints 5 FF

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n&"s Constr&ctors

he ey +oints to remember about enum constructors are

ou can N==R invo4e an enum constructor directl( ;e enumconstructor is invo4ed automaticall& #it te arguments ou define afterte constant value(

For e7am)le& BI>!" invo4es te CoffeeSi:e constructor tat ta4es an int&)assing te int literal to te constructor( !Beind te scenes& of course&ou can imagine tat BI> is also )assed to te constructor& $ut #e don'tave to 4no#or carea$out te details("

ou can define more tan one argument to te constructor& and ou canoverload te enum constructors& 9ust as ou can overload a normal classconstructor(

8e discuss constructors in muc more detail in Ca)ter /( ;o initiali:e a

Coffee;)e #it $ot te num$er of ounces and& sa& a lid t)e& ou'd )asst#o arguments to te constructor as BI>!& 5A5"& #ic means ou ave aconstructor in CoffeeSi:e tat ta4es $ot an int and a string(

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Chap 1 Declarations and Acces Control Update