ABezerianos - Remedial Java - Session-3anab/teaching/remedialJava/Session-3.pdf · 9 ABezerianos - Remedial Java - Session-3.key - 6 September 2019 Why packages? Combine similar functionality

(remedial)Java

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1 ABezerianos - Remedial Java - Session-3.key - 6 September 2019

Packages


Packages(1)

package path.to.package.foo;

▪ Eachclassbelongstoapackage

▪ Packagesgroupsclassesthatserveasimilar

purpose.

▪ Packages:

▪ Helpavoidclasseswiththesamename

▪ Classesinotherpackagesneedtobeimported


Packages(2)

package path.to.package.foo;

▪ Arejustdirectories(inthediskorjar)

▪ Forexampleclass3.inheritanceRPGislocatedin

<workspace>\RemedialJava\src\class3\inheritenceRPG

▪ Thecompiledclassesunder…\RemedialJava\bin\...


Packages(3)

▪ Definingpackages

theclassfullnameispackage.ClassName(iepath.to.package.foo.Foo

package path.to.package.foo;class Foo {}


Packages(2)

▪ Definingpackages

▪ Usingpackages

package path.to.package.foo;class Foo {}

import path.to.package.foo.*;import path.to.package.foo.Foo;


Packagesexample

package neighborhood;

public class Car {}

package neighborhood;

public class Road {}


Packagesexample

package citytools;

import neighborhood.Car;import neighborhood.Road;

public class City {public static void main (String[] args){

Car car = new Car();car.move(30);

}}


Packagesexample

package citytools;

// import neighborhood.Car;// import neighborhood.Road;

public class City {public static void main (String[] args){

neighborhood.Car car = new neighborhood.Car();car.move(30);

}}


Whypackages?

▪ Combinesimilarfunctionality

▪ Separatesimilarnames

Convention(domain.society.project.modules)

fr.upsud.libraries.Libraryfr.upsud.libraries.Book

shopping.Listpacking.List

fr.upsud.remedialjava.class3


Whypackages?

▪ Allclasses“see”classesinthesamepackage(no

importneeded)

▪ Allclasses“see”classesinjava.lang

▪ Suchas:java.lang.String; java.lang.System▪ JavahasaLOTofpackages/classes.Reusethemto

avoidextrawork(useyourjavaversion)

▪ http://docs.oracle.com/javase/10/docs/


Collections


RememberourLibrary

▪ Whatwouldhappenifwewantedamorethan10

books?Evenbetter,alibrarythatwecanperiodically

addbookstowithoutworryingaboutitsstorage?

public class Library {

private Book[] _allBooks = new Book[10];

…}


Objectcollections:ArrayLists

▪ Javacomeswithseveralcollections,i.e.,objects

thatrepresentagroupingofotherobjects.

ExamplesincludeArrayLists,Sets,Maps,etc.

(seeJavaAPIoncollectionsformoreinfo)

ArrayList:Modifiablelist(internallyimplementedwithArray)

▪get(index)toaccessitematindex(notarray[i])

▪Add/deleteitems(nosizerestriction)

▪Loopoveritems


ArrayListexample

public class ArrayListExample { public static void main(String[] arguments) {

ArrayList<String> someStrings = new ArrayList<String>(); // creation

// adding as many items as we wantsomeStrings.add("Alice");someStrings.add("Bob");someStrings.add("Steve");

System.out.println( "Size is " + someStrings.size() ); // can get its size

System.out.println(someStrings.get(0)); // can get items in index position 0 System.out.println(someStrings.get(5) ); // error when outside

for (int i = 0; i < dishes.size(); ++i ) // loop using indexSystem.out.println(someStrings.get(i));

dishes.set(2, "Peter");// replace item at index

for (String s: someStrings) // another loop over collectionSystem.out.println( s ); // that gives us references

}}


Exercise1(partone&two)https://www.lri.fr/~anab/teaching/remedialJava/ex-session3.pdf

Reminder:

▪ Importclassesfromotherpackagesusingimportpackage.classname;

▪ ArrayList:

▪ ArrayList<Class> someName = new ArrayList<Class>();

▪ get(index):returnsobjectinindexpos▪ set(index,object):setsobjectinindexpos

▪ add(object):addsanobjectattheendoftheArrayList

▪ remove(?):removesobjectateitherindex,orobjectitself

▪ Loopusingindex,oroverelementsfor(Class c: someCollection)


Objectcollections:Maps

▪Storesa(key,value)pairofobjects▪Lookupthekey,getbackthevalue▪Keyneedstobeunique

Example:

▪Addressbook:mapnametoemailaddress

▪Menu:mapdishnumbertoDishobject

▪TreeMap(sorted),HashMap(pseudorandom)


Mapexamplepublic class MapExample {

public static void main (String[] args){

HashMap<String,String> emails = new HashMap<String, String>(); // creation

emails.put("John", "[email protected]"); // adding entry of type String:Stringemails.put("Cat", "[email protected]");

System.out.println( emails.size()); // gets size

for (String k : emails.keySet()) // gets all keys System.out.println(k);

if ( emails.containsKey("John") ) // checks if key existsemails.replace("John", "[email protected]"); // replaces value

for (String v : emails.values()) // gets all values System.out.println(v);

if (!emails.isEmpty()) // checks if emptySystem.out.println("I have stuff!");

emails.remove("Cat"); // removes key

for (HashMap.Entry<String,String> pairs: emails.entrySet()) System.out.println(pairs); // iterates over all entries

}}


Objectcollections:Maps

▪Mapsaregoodfordictionaries,look-uptasks

whenyouhaveuniqueids,andconvertions

(e.g.,checkMorseCodeexampleincode)


Abitmoreonmethods


Abitmoreonmethods

▪ Methods(includingconstructors)canbe

overloaded(multiplemethods,samename)

(onewayofensuringpolymorphisminJava)

public Car {this.Car("unknown", "white"); // I want a default value

}

public Car(String myname, String mycolor){name = myname; // or this.name = myname;color = mycolor; // or this.color = mycolor;

}


Overloading

▪ Thecompilersearchesforthebestmethod,

basedontheTYPEoftheargumentspassed

public class PrintStream {public void println(String text){...}public void println(double value){...}

public static void main (String args[]){PrintStream out = System.out;out.println(“toto”);out.println(3.0);out.println(2);

}}


Overloading

▪ ThereturnTYPEofthemethodisnotconsidered,

soitisimpossibletodifferentiatemethodsjustby

theirreturnTYPE

public class BadOverloading {public int f(){...}public double f(){...} // f is already defined in BadOverloading

public static void main (String args[]){BadOverloading bo = new BadOverloading();bo.f();

}}


Overloading,whentouse?

▪ Weuseoverloadingwhenmethodshavethe

samesemantic(purpose),butdifferent

arguments

public class Math {public float sqrt(float value){…}public double sqrt (double value){…}

} // this is ok

public class List {public void remove (Object value) {…}public void remove (int index) {…}

} // if we want to be picky, this is NOT ok, why?


Varargs

▪ Itispossibletodefinemethodswithvariable

numberofargumentsusingthenotation“…”

▪ Argumentsareputinatable

public static void main (String[] args) {System.out.printf(“%d\n”,2);

}

public class PrintStream {public void printf (String text, Object... args){

... // args is an ArrayList so we can run a loop

}}


Varargs

▪ Varargsareconsideredastables,sowecannot

overloadvarargsandtables

public class VarargsOverloading {private static int min(int... array){ }

private static int min(double... array){ }

private static int min(int[] array){ } // min(int...) already defined in VarargsOverloading

}


Exercise1(part3)www.lri.fr/~anab/teaching/remedialJava/ex-class3.pdf

Reminder:

▪ Syntexforvarargs

methodTYPE(otherinputparams,TYPE…name)


Inheritance


Abasicinheritanceexample

▪ ImagineaverysimpleRPGgame

public class Person {public String _name;public int _health = 100;public int _mana = 0;

public void sayName (){System.out.println(_name);

}

public void attackPerson(Person target){System.out.println(_name + " attacking " +

target._name); target._health -= 10;

}}



▪ WenowwanttocreateaWizard

public class Wizard {// need to copy all of Person staff// booooring

}

▪ Whygointoallthetrouble?Wizardhasanddoes

everythingthatasimplePersondoes…



▪ WizardisasubclassofPerson

public class Wizard extends Person {

}



▪ WizardisasubclassofPerson

▪ Wizardcanuseeverything*thatPersonhas

e.g.,wizard1._health +=1;

▪ Wizardcandoeverything*thatPersoncando

e.g.,wizard1.attackPerson(person1);

▪ YoucanuseWizardasaPersontoo

e.g.,person1.attackPerson(wizard1);

*exceptforprivatefieldsandmethods(keywordprotectedmaybeused)



▪ Let’simproveWizard

public class Wizard extends Person {

int _mana = 100;ArrayList<String> _spells;

public void cast (String spell){// do cool stuff here_mana -= 10;

}}



▪ Canweinheritfrominheritedclasses?

public class GrandWizard extends Wizard {

public void sayName (){System.out.println("Grand Wizard" + name);

} //overriding (ancestor) parent method}

grandWizard1.name = “Gandalf”;grandWizard1.attackPerson (person1);grandWizard1.sayName();


HowdoesJavadothat?

▪ WhatJavadoeswhenitseesgrandWizard1.sayName();

▪ LookforsayName()inGrandWizard

▪ It’snotthere!DoesGrandWizardhaveaparent?

▪ LookforsayName()inWizard

▪ It’snotthere!DoesWizardhaveaparent?

▪ LookforsayName()inPerson▪ Gotit!CallsayName()andprintthefield_name


Inheritancestructure

Person:

ParentofWizard,Elf,…

SubclassesofPerson

SubclassofWizard

Person

Wizard Fighter Elf

Grand

Wizard


Canonlyinheritfromoneclass

Canonlyinheritfrom

oneparentclass

Multipleinheritance

notpossible…

Why?

Person

Wizard Fighter Elf

Wizard

Elf

IfbothWizardandElfimplement(differently)

sayName(),whichonedoesWizardElfcall?



▪ Adaptingmethodsfrominheritedclasses

public class GrandWizard extends Wizard {

public void sayName (){System.out.println("Grand Wizard" + name);

} //overriding (ancestor) parent method}

grandWizard1.name = “Gandalf”;grandWizard1.attackPerson (person1);grandWizard1.sayName();


Exercise2-partone&twowww.lri.fr/~anab/teaching/remedialJava/ex-class3.pdf

Reminder:

▪ Partone

exampleofasingleclassdiagram

▪ Parttwo

inheritancebetweenclasses

class CHILDCLASS extends PARENTCLASS {}

achildclasscanoverride(i.e.,redefine)amethodfromaparent/

ancestorclassbyredefiningitinitsbody

Wizard

protectedArrayList<String>_spells

publicvoidsayName()

publicStringcastSpell()


Accesstoparentinformation

▪ Aclassinheritsallthemembersoftheparent

(super)class,i.e.,allfieldsandmethodsandcan

usethemwithouthavingtodoanythingmore.

▪ AllexceptConstructors,thatarenotinherited

▪ Unlessthedefaultconstructorisused,constructorsneedtoberedefinedforeachchild(morelater).



▪ Methodsareinherited.

▪ Addingamethodofthesamenameasoneinasuper-classredefines(overrides)themethod.This

waywehaveimplementationsadaptedtothe

semanticsofthemethod.

class Person { public void sayName (){

System.out.println(_name); }}

class GrandWizard extends Wizard { public void sayName (){

System.out.println("GW" + _name); }}



▪ Fieldsareinherited

▪ Whenweaddafieldofthesamenameasoneinaparentclass,thefieldofthesuperclassisno

longervisibletous(wesayitishiddenormasked)

class Person { public int _name; public int sayName (){

System.out.println(_name); }}

class Wizard extends Person { public String _name; public String sayName (){




▪ Whatifwewanttoaccessmethodsorfieldsfrom

thesuperclass,whenwehaveoverriddenthe

methodsormaskedthefields(e.g.,usethe

sayName()ofPersonfortheGrandWizardZed?



▪ Weusethekeywordsuperclass Wizard extends Person {

public String sayName (boolean modest){if (modest)

super.sayName();else




▪ Weusethekeywordsuper

▪ Onlyworksforimmediateparentandnon-static

methods(theybelongtotheirclass)

class Wizard extends Person {

public String sayName (boolean modest){if (modest)

super.sayName();else




▪ Weusethekeywordsuper

▪ Constructorsarenotinherited.

▪ Unlessthedefaultconstructorisused,constructorsneedtoberedefinedforeachchild.

▪ Firstlineintheconstructorneedstobeacalltotheconstructoroftheparentclass,usingsuper

class Person { protected String _name; public Person(String n){ _name = n; }}

class Wizard { public Wizard(String name, int m) { super(name);

_mana = m; }}


Inheritance

Anotherexample:

Inreality,trueinheritanceisrare…

Customer--------------------

name:String

address:Address

-----------------------

validate():boolean

Firm--------------------

name:String

address:Address

firm:String

----------------------

validate():boolean

Customer--------------------

name:String

address:Address

-----------------------

validate():boolean

Firm--------------------

firm:String


Inheritance,reasons

▪ Weuseinheritanceto:

▪ Reusemembers(structural),

e.g.,Customer&Firm

▪ Redefinesomemethods,

e.g.,changethecodeto

validate()

▪ Expresssub-types,e.g.,WizardisaPerson

Customer--------------------

name:String

address:Address

-----------------------

validate():boolean

Firm--------------------

firm:String


Upcasting,downcasting

▪ Upcasting:theabilitytoconsiderareferenceofa

child(sub-class)asifitisareferenceofaparent-

(super-class).

▪ InJavadoneautomatically.





(super-class).


public static main (String[] args){

Wizard wizard = new Wizard();System.out.println(wizard);

Person person = wizard;System.out.println(person);

}

wizard

Wizard

person





(super-class).


▪ Note:theobjectisstillaWizard,butwecantreat

itasaPerson(e.g.,additinanArrayList<Person>)


Wizard wizard = new Wizard();System.out.println(wizard);

Person person = wizard;System.out.println(person);

}

wizard

Wizard

person



▪ Downcasting:theabilitytoconsiderareference

toaparent(super-class)asifitisareferenceofto

achild(sub-class)





achild(sub-class)


// assuming I know Zed is a Wizardfor (Person p: persons){

if ( p.getName().equals( "Zed" ) ){Wizard zed = (Wizard)zed;

}}

}





achild(sub-class)

▪ Ifyouwanttocheckifanobjectbelongstoaclass

canuseinstanceof if(p instanceof Wizard){…}


// assuming I know Zed is a Wizardfor (Person p: persons){

if ( p.getName().equals( "Zed" ) ){Wizard zed = (Wizard)zed;

}}

}


(aside)OOPPolymorphism

▪ Polymorphismistheabilitytotakemanyforms.

InJavaitcanbeseenin:

▪ Objects:becausewecanupcast,anobjectcanbeseenasbeingoftheclassof(oneof)it’sancestors

▪ Methods:becausewecanoverrideamethodthatwe

inherittoadaptitsbehavior

▪ Methods:becausewecanoverloadamethod(i.e.,

haveamethodwithsamenamebutdifferentinput

parameters)toadaptitsimplementationtothe

contextoftheinputparameters


Exercise2–partthreewww.lri.fr/~anab/teaching/remedialJava/ex-class3.pdf

Reminder:

▪ Constructorsarenotinherited

▪ Youcanaccesstheparent’sconstructorwithsuper

▪ Intheconstructor,superneedstobethefirstcommand


Exercise3–Homeworkwww.lri.fr/~anab/teaching/remedialJava/ex-class3.pdf

Reminder:

▪ Constructorsarenotinherited

▪ Youcanaccesstheparent’sconstructorwithsuper

▪ Intheconstructor,superneedstobethefirstcommand


Inheritance

▪ InJavaallclassareinheritingdirectlyorindirectly

fromtheclassjava.lang.Object

▪ Directly:whenwedeclareanewclasswithoutinheritance,thecompileraddsextends Object

▪ Indirectly:whenwedeclareanewclassthatinheritsfromanotherone,theparentclassinherits(directlyor

indirectly)fromtheclassObject


Objectclass

▪ ParentclassofallJavaclasses

▪ Hasbasicmethods(wecanredefine/override)

▪ toString()&hashCode()▪ equals()▪ getClass()▪ clone()▪ finalize()


Equalitytests

▪ Theoperators==and!=test

▪ Valuesforprimitive/basictypes(e.g.,int,double)

▪ ValuesofreferencesforObjects

Point p1;p1=new Point(2,2);Point p2;p2=p1;Point p3;p3=new Point(2,2);p1==p2; //truep1==p3; //falsep2==p3; //false

p1

p2

x:2,y:2

p3 x:2,y:2


equals()

▪ Thereisamethodequals(Object)inObject

▪ Butgenerallyitsimplementationtestsreferences

(whatisanexceptionwehaveused?)

▪ Tostructurallycomparetwoobjectsweneedto

redefine(override)themethodequals().

Point p1=new Point(2,2);Point p3=new Point(2,2);p1==p3; //falsep1.equals(p3); //false

p1

p2

x:2,y:2

p3 x:2,y:2


AdvancedInheritance


Canonlyinheritfromoneclass

Canonlyinheritfrom

oneparentclass

(notentirelytrue…)

Person

Wizard Fighter Elf

Wizard

Elf

IfbothWizardandElfimplement(differently)

sayName(),whichonedoesWizardElfcall?


Interfaces(notUI)

Butwhatifthemethod

wasnot

implemented?

Person

Wizard Fighter Elf

Wizard

Elf


▪ Interfacesdefineatype(asdoclasses)butwithoutcode.Weusethekeywordinterface

▪ Intheinterfaceyoucandeclaremethods

thatsubclasseswillthenhavetoimplement

▪ Wecalltheseabstractmethods

▪ Theyarealwaysdeclaredpublic

Interfaces(notUI)

public interface Elf {public void seeInTheDark(); // simple declaration, no codepublic String otherCoolStuffICanDo ( int coolness );

}


Interfaces

Thenweneedtodefineaclass

thatinheritsthedeclarations

oftheabstractmethodsin

theinterface,andprovides

codeforthem

Weusethekeyword

implements

Person

Wizard Fighter Elf

Wizard

Elf


Interfaceimplementation

public interface Elf {public void seeInTheDark(); // simple declaration, no codepublic String otherCoolStuffICanDo ( int coolness );

}

public class WizardElf extends Wizard implements Elf{

public void seeInTheDark(){System.out.println("I see as well as a cat" );

}

public String otherCoolStuffICanDo ( int coolness ){if ( coolness < 5 )

System.out.println("Not so cool after all" ); }

}


▪ Aninterfacehasnocode(i.e.,isnotimplemented)

andsocannotbeinstantiated(i.e.,wecannot

createobjectsoftheirtypeusingnew)

▪ Thecompilerverifiesthatonceaclassimplements

aninterface,itimplementsallitsmethods

▪ Aclasscanimplementmorethanoneinterfaces,

andthusinheritstheircombineddeclarations

▪ ThisistheclosestJavacomestomultipleinheritance

Interfaces


Interfaces

public interface Elf {public void seeInTheDark(); // simple declaration, no code

}public interface Running {

public int run( int time ); // simple declaration, no code}

public class WizardElf extends Wizard implements Elf, Running{private int _distance = 0;

@Overridepublic void seeInTheDark(){ // inherited from Elf

System.out.println("I see as well as a cat" );}@Overridepublic int run ( int time ){// inherited from Running

_distance += time*10;}

}


▪ Inheritfromaclass:

▪ Thesubclassisatypeoftheparentclass

▪ Implementaninterface:

▪ Enforcetheimplementationofspecificmethods(i.e.,

sharedfunctionality)

▪ Objectsthatsharedifferentgroupsoffunctionalities

(acalltointerfacemethodsisabitslower)

InterfaceorClassInheritance


Exercise5www.lri.fr/~anab/teaching/remedialJava/ex-class3.pdf

Reminder:

▪ Interfacecreation:public interface INTERFACENAME{}

▪ Interfaceonlyhas(abstract)methoddeclarations,allpublic

▪ Aclassimplementsoneormoreinterfaces

class CLASSNAME implements INTERFACENAME{}

▪ Aclassthatimplementsaninterfaceneedstoprovidecodeforalltheabstractmethodsdefinedintheinterface(thekeyword

@Overrideproceedstheircode)


▪ ItispossibletocreateinJavaclasseswithabstract

methods,i.e.,methodsthatchildclassesneedto

implement

▪ Abstractclassescanalsohavefieldsandmethodswith

implementationaswell,soitisapartiallyimplemented

class

▪ Theycaninheritnormallyfromotherclassesor

implementinterfaces

▪ BUTtheycannothaveinstances(i.e.,wecannot

createanobjectfromthemusingnew)

Abstractclasses


Abstractclassespublic interface Moving {

public int run( int time ); public int walk( int time );

}public interface Resting {

public void stand( int time ); public void sleep( int time );

}

public abstract class Person implements Moving,Resting {String _name;int _distance;public int run( int time ){

distance += time*10; // implemented method}public abstract void sayName(); // declaration only

}

// What code does Wizard have to provide?public class Wizard extends Person {}


TypeandClass:reminder

public static void main (String args[]){String s = args[0];Car k = new Car (“peugeot”, “pink”);

}

Asidenote:

▪ VariableshaveaType▪ ReferenceshaveaClass

Type Class


TypeandClass:reminder

public static void main (String args[]){String s = args[0];Car k = new Car (“peugeot”, “pink”);

}

Asidenote:

▪ VariableshaveaType▪ ReferenceshaveaClass

Type Class


Typesvsclasses

▪Whichofthefollowingarecorrect?

public interface Moving { … }public interface Resting {… }public abstract class Person implements Moving,Resting {…}public class Wizard extends Person { … }public class Fighter extends Person { … }

Person p = new Person("John");Wizard z = new Wizard("Zed");Fighter f = new Fighter("Fred");Resting r = new Resting("Resting");Person p2 = z;

ArrayList<Person> people = new ArrayList<Person>();people.add(z);people.add(f);for (Person ip: people){

if (ip instanceof Wizard) {…}}


Objectcollections:ArrayLists

(Aside)

▪ ArrayListsareonetypeofList(thatisan

interface).Oftenitisbesttodeclarethetypeas

Listandinstantiate(i.e.,implement)using

ArrayList.

▪ Thiscanmakeeasierinthefuturetochangethe

implementationofyourList(touseforexample

LinkedList)

(seeJavaAPIoncollectionsformoreinfo)


Documents

ABezerianos - Remedial Java - Session-3anab/teaching/remedialJava/Session-3.pdf · 9 ABezerianos - Remedial Java - Session-3.key - 6 September 2019 Why packages? Combine similar functionality