A Java introduction - capsis.cirad.frcapsis.cirad.fr/capsis/_media/javaforcapsis151209.pdf · SDK, FC (UMR AMAP) A Java introduction 15/12/2009 6 / 50. Java application Java programs

A Java introduction

SDK, FC

UMR AMAP

15/12/2009

SDK, FC (UMR AMAP) A Java introduction 15/12/2009 1 / 50

Plan

1 Introduction

2 BasesJava ApplicationVariables & ExpressionsSimple ArraysExceptionsCollectionsControl structureFunctions

3 ObjectsClassesInheritancePolymorphism

4 Java standard library

5 To go further


History

James Gosling and Sun Microsystems

Java, May 20, 1995

Java 1 → Java 6

GPL with classpath exception since 2006


Characteristics

Object Oriented

Interpreted (need a virtual machine)

Portable (Linux, Mac, Windows)

Dynamic (introspection)

Static typing (checks during compilation)

Simplier than C++ (memory management, pointers, headers. . .)


Programming environment

Java Environnment

JRE (Java runtime environment)

JDK (Java Development Kit)

Jave SE (Standard edition)

Java EE (Enterprise edition → Web)

Java ME (Micro edition)

IDE - Editor

Eclipse, NetBeans : Full IDE (completion, compilation...)

Simple Editors : Notepad++, TextPad, Scite (coloration, indentation)


Installation

Windows

Download and install the last JDK (Java SE 6)

Environment variable (Computer → properties)

Add to PATH variable the bin directoryeg : “C :/Program Files/Java/jdk1.6.0 version/bin”

Install editor TextPad or Notepad++

Linuxsudo apt-get install sun-java6-jdk

sudo apt-get remove openjdk-6-jdk

Editor : use scite or gedit


Java application

Java programs written in files with extension “.java”

Applications are .java files with public static void main(...) method

Compile and run Java application

Run the compiler on a .java file : javac package/MyProgram.java⇒ Produce a file of Java byte code : MyProgram.classRun the interperter on a .class file : java package.MyProgram

the tools javac java are part of the JDK


Workflow


Hello world

package h e l l o ;

/∗∗∗ App l i c a t i o n He l l o wor ld∗/

p u b l i c c l a s s Hel loWor ld {

/∗∗ Main Func ion ∗/s t a t i c p u b l i c v o i d main ( S t r i n g [ ] a r g s ) {

// p r i n t to s c r e e nSystem . out . p r i n t l n ( "Hello world !" ) ;

}}

Print to console : System.out.println(“a string”) ;

Commands finish with a ;

Comments : // or /* */

Package : hello = directory

java hello.HelloWorld


Create a new application

Copy directory

Rename directory and file :

class Name and file name should be identicalThe package is a namespace and corresponds to the directoriespackage name and directory should be identical

Exercice : Create a new application “package1.App1”


Variable & Simple type

Variable

A variable has a type and hold a value or an Object

A variable name starts with lowercase, eg : myVariable ;

boolean : true or falseint : signed integer 32 bitslong : signed integer 64 bitsfloat : floating point 32 bitsdouble : floating point 64 bitsObject : a generic object

initialization

by default : 0

int aVariable = 2 ;

float anOtherVariable = 3.56f ;

Use final for constants : final float CONSTANT = 0.1f ;SDK, FC (UMR AMAP) A Java introduction 15/12/2009 11 / 50

Operators

Arithmetic

+, -, *, /, %

myInt++, otherInt−− ;

+=, -=, *=, /= . . .

Precedence : use parenthesis ()

division

2 / 3 different of 2.0 / 3.0

Division par zero → Infinity or NaN

Exercise : arithmetic.DivideByZero


Math

import java.lang.Math

Constante : Math.PI, Math.E

Math.abs(), Math.pow(), Math.sqrt(), Math.exp(), Math.log(),

Math.cos(), Math.tan()...

Math.toDegrees(), Math.toRadians()

Javadoc : Java MathExercice : Calculate triangle hypothenus with pythagore


Simple Arrays

One or 2 dimensions arrays

Dynamic allocation : new keyword

null if not initialized

Not resizable

Access with [] operator

Index begin at 0

S t r i n g [ ] a r r a y1a = new S t r i n g [ 1 2 ] ;S t r i n g [ ] a r r a y1b = {"toto" , "tata" , "titi"} ;

i n t s i z e = 4 ;double [ ] a r r a y 1 c = new double [ s i z e ] ;double [ ] [ ] a r r a y 2 = new double [ 4 ] [ 6 ] ;

a r r a y1a [ 1 1 ] = "a string" ;a r r a y 2 [ 0 ] [ 2 ] = 3d ;a r r a y 2 [ 3 ] [ 5 ] = 1d ;System . out . p r i n t l n ( a r r a y 2 [ 4 ] [ 6 ] ) ; // i ndex e r r o r


Error during execution

Try to use an object not initialized

Try to acces to a non exisiting element in array

double [ ] a r r a y = n u l l ;System . out . p r i n t l n ( a r r a y . l e n g t h ) ;a r r a y = new double [ 1 0 ] ;System . out . p r i n t l n ( a r r a y [ 1 0 ] ) ;

Exceptions are raised and stop the program


Exceptions

Error can be catched anywhere in the program

Specific exceptions can be raised

double [ ] a r r a y = n u l l ;

t r y {System . out . p r i n t l n ( a r r a y . l e n g t h ) ;

} catch ( Excep t i on e ) {

System . e r r . p r i n t l n ( e ) ;}

t r y {double r e s u l t = a r r a y [ 1 0 ] ;

} catch ( Excep t i on e ) {r e s u l t = 0 . ;

}

System . out . p r i n t l n ( "Result : " + r e s u l t ) ;

// i f ( r e s u l t < 0) { throw new Excep t i on (” r e s u l t cannot be n e g a t i v e ” ; }


Complex types

Java manipulates Objects

An object is a concept (e.g a particular data structure)

Objects are instantiated with the keyword new

A variable contains a reference to an object

Affectation is a reference copy (the variable point to the same object)

Objects are destroyed when there is no reference on it

By default an object variable is set to null

Objects have properties and methods accessible with the . operator

MyObject o1 , o2 ; // n u l l ;o1 = new MyObject ( ) ;o1 . v a l u e ;o1 . sum ( ) ;o2 = o1 ;o1 = n u l l ;o2 = n u l l ; // Object w i l l be d e s t r o y ed by the garbage c o l l e c t o r


Collections

Collections are dynamic containers

Associated to a specific type

Specific behaviour : list, set, map..

Simple types (int, float, double...) are not objects ⇒ need a wrapper

int ⇒ Integer

float ⇒ Float

double ⇒ Double

Collection class has method to sort, shuffle, reverse, ... collections

See javadoc : Collection java 1.6

Co l l e c t i o n<St r i ng> c1 ;C o l l e c t i o n<I n t e g e r> c2 ;C o l l e c t i o n<Double> c2 ;


ArrayList

An ordered collection of variable size

Growth automatically

get, add and set method

Insertion is slow

For a stack or a queue, use LinkedList

L i s t<St r i ng> l = new Ar r a yL i s t<St r i ng >();l . add ( "toto" ) ;l . add ( "tata" ) ;l . add ( "titi" , 1 ) ; // i n s e r t a t i nd ex 1l . s e t (1 , "tutu" ) ; // r e p l a c e at i nd ex 1

l . s i z e ( ) ; // 3l . ge t ( 0 ) ; // ” to to ”. . .


HashSet

An unordered collection without duplicated elements

contains method is fast

Set<St r i ng> s = new HashSet<St r i ng >();s . add ( "toto" ) ;s . add ( "tata" ) ;s . add ( "titi" ) ;s . add ( "toto" ) ; // f a l s e

s . s i z e ( ) ; // 3s . c o n t a i n s ( "tutu" ) ; // f a l s e. . .


HashMap

Map associate a key with an object

a key should be unique (like in a Set)

Map<St r i ng , Color> m = new HashMap<St r i ng , Color >();m. put ( "black" , new Co lo r (0 , 0 , 0 ) ) ;m. put ( "red" , new Co lo r (1 , 0 , 0 ) ) ;m. put ( "green" , new Co lo r (0 , 1 , 0 ) ) ;m. put ( "blue" , new Co lo r (0 , 0 , 1 ) ) ;

m. ge t ( "red" ) ; // r e t u r n a c o l o r o b j e c tm. conta i n sKey ( "black" ) ; // t r u em. keySet ( ) ; // s e t o f keys


Boolean arithmetic

boolean v = true ;

! && ||

<= >= < > == !=

== and equals() : reference vs contents

!= and !equals()

use () for precedence


if else

Simple condition

Can be combined

// s imp l e i fi f ( c o n d i t i o n ) {

b l o ck}

// complex i fi f ( c o n d i t i o n ) {

b lock1} e l s e i f ( o t h e r c o n d i t i o n ) {

b lock2} e l s e {

b lock3}

i f ( v1 && ! v2 ) {System . out . p r i n t l n ( "v1 and not v2" ) ;

}


while

Loop with condition

i n t coun t e r = 0 ;w h i l e ( coun t e r < 100 ) {

System . out . p r i n t l n ( "counter : " + coun t e r ) ;c oun t e r++;

}

i n t counte r2 = 0 ;i n t sum = 0 ;w h i l e ( coun te r2 < 100) {

sum += Math . random ( ) ;i f ( sum > 20) { break ; }

coun t e r += 1 ;}


for

Loop with index

// With an a r r a yi n t [ ] a r r a y = new a r r a y [ 1 2 ] ;

i n t sum = 0 ;f o r ( i n t i =0; i<a r r a y . l e n g t h ; i++) {

sum += a r r a y [ i ] ;}

Loop in a collection

// With a l i s tL i s t<I n t e g e r> l = new Ar r a yL i s t<I n t e g e r >();i n t sum = 0 ;f o r ( I n t e g e r v : l ) {

sum += v ;}

// With a MapMap<St r i ng , Object> m = new HashMap<St r i ng , Object >();f o r ( S t r i n g key : m. keySet ( ) ) {

System . out . p r i n t l n (m. ge t ( key ) ) ;}

Exercice : do the sum of a double array


Enumeration

A enumeration is a type with a limited number of value

Type checking

Better than using integer or constante values

// Type d e c l a r a t i o nenum StopL igh t {red , amber , g r een } ;

// Use typeStopL ig th s ;


static Functions

Like a mathematical function

⇒ Reuse a block of code

Inputs Parameters : type + variable name

Output : return type

s t a t i c double aFunct i on ( double param1 , double param2 ) {r e t u r n param1 ∗ param1 + param2 ;

}

param1 and param2 are the parameters

their names have a local scope : they are only available in the function

double a = 2 ;double b = 3 ;double r e s u l t = aFunct i on ( a , b ) ; // aFunct i on (2 , 3 )System . out . p r i n t l n ( r e s u l t ) ;


Functions (2)

a class can contain several functions

if no parameters ⇒ use ()

if no return type ⇒ use void

p u b l i c c l a s s MyClass {

p u b l i c s t a t i c double f u n c t i o n 1 ( double param1 , double param2 ) {. . .

}

p u b l i c s t a t i c v o i d f u n c t i o n 2 ( ) {. . .

}

p u b l i c s t a t i c v o i d main ( S t r i n g [ ] a r g s ) {. . .}

}


Read a file

Scanner tool parses a list of token

import j a v a . u t i l . Scanner ;

. . .// i n a f u n c t i o nScanner sc = new Scanner (new F i l e ( "filename.txt" ) ) ;

w h i l e ( sc . hasNext ( ) ) {

S t r i n g s = sc . nex t ( ) ; // read a s t r i n gi n t i = sc . n e x t I n t ( ) ; // read an i n tf l o a t f = sc . n e x t F l o a t ( ) ; // read a f l o a t

}

// Read i n the s h e l lScanner sc2 = new Scanner ( System . i n ) ;System . out . p r i n t ( "Input a string : " ) ;S t r i n g s = sc2 . nex t ( ) ;


Exercices

Create a file with a list of number : numbers.txt

Create a application which read the file in an ArrayList of Float

ArrayList<Float>

Compute the mean (and the variance) of the data

Put the code in a function

Use args argument in main to pass the filename in the command line

Use the function to compute the mean for multiple files

Try to put strings in the file and recover it

Remove minimal and maximal values in the mean

. . .


Object Oriented programming

Java manipulates Objects

E.g : a list, a number, a car, a species, a plant, a plot. . .

Concepts

Class : a type of object

Instance : a real object

Instance data : the data an object contains

Data are different for each instanciated object

Methods : the functions associated to an object


How to use Object

Object are instanciated with the new keyword

We can instantiate several objects of the same type

Tree t1 = new Tree ( "Spruce" ) ; // i n i t i a l i z a t i o n wi th paramete rTree t2 = new Tree ( "Larch" ) ;Tree t3 = new Tree ( "Fir" ) ;

// each i n s t a n c e has i t s own datat1 . getSpec iesName ( ) ; // ” Spruce ”t2 . getSpec iesName ( ) ; // ” Larch ”t3 . getSpec iesName ( ) ; // ” F i r ”

// P r o p e r t i e st3 . h e i g h t = 2 ;System . out . p r i n t l n ( t1 . h e i g h t ) ;

// Methodst2 . se tSpec ie sName ( "Spruce" ) ;

// t2 != t1 but t2 . e qu a l s ( t1 )

// a f f e c t a t i o nt3 = t1 ; // t3 and t1 po i n t to the same ob j e c t t3 == t1t3 . getSpec iesName ( ) ; // Spruce


Class

A class is a type description

It contains data and methods (members)

p u b l i c c l a s s Tree {

p u b l i c double h e i g h t ;p r i v a t e S t r i n g spec iesName ;

// Con s t r u c t o r sp u b l i c Tree ( S t r i n g n ) {

spec iesName = n ;}

// Acc e s s o r sp u b l i c v o i d se tSpec ie sName ( S t r i n g n ) {

spec iesName = n ;}p u b l i c S t r i n g getSpec iesName ( ) { r e t u r n spec iesName ; }

}


Class...

Access control

Public : no access restriction

Private : only accessible in the class method

Protected : accessible in the class methods and in the subclasses

Constructor

Called when the object is created

Can take parameters or not

Can be declined in different versions

No return type


Exercices

tree/Tree.java

Add a private data : diameter

Add corresponding accessor

Add a method which return the volume of the trunk (we consider acylinder)

Create an application which use this class and this function


This

this keyword represents the current object

Usefull if there is ambiguity with the names

p u b l i c c l a s s Tree {

. . ./∗∗ Volume o f the t runk ∗/p u b l i c double getVolume ( ) {

double volume ;double r a d i u s = t h i s . d i amete r / 2d ;volume = r a d i u s ∗ r a d i u s ∗ 2d ∗ Math . PI ∗ t h i s . h e i g h t ;

r e t u r n volume ;} ;. . .

}


static

static members are associated to the type

static datas are shared by all instance

static methods are not attached to an instance

c l a s s Tree {. . .s t a t i c p u b l i c i n t sha r ed

s t a t i c p u b l i c double g e t Su r f a c e ( double d i amete r ) {double r a d i u s = d iamete r / 2d ;double s u r f a c e = r a d i u s ∗ r a d i u s ∗ 2d ∗ Math . PI ;

r e t u r n s u r f a c e ;}. . .

}

Tree . g e t Su r f a c e ( 2 . )Tree . sha r ed


Inheritance

We can reuse a class to make more specific classes

e.g : a tree with crown, a tree with leaves, etc...

Inheritance corresponds to a “is a” relation

A sub-class has all the data and methods of its parent

All class inherit from the class Object

Multiple inheritance is not allowed


Inheritance

// Tree . j a v ac l a s s Tree { // ex t end s Object

p r o t e c t e d double h e i g h t ;

/∗∗ Con s t r u c t o r ∗/p u b l i c Tree ( double h ) {

h e i g h t = h ;}

p u b l i c double ge tHe i gh t ( ) ;

}

// CrownTree . j a v ac l a s s CrownTree extends Tree {

p r o t e c t e d double crownHeigth ;p r o t e c t e d double crownDiameter ;

/∗∗ Con s t r u c t o r ∗/p u b l i c CrownTree ( double h , double ch , double cd ) {

super ( h ) ; // pa r en t c o n s t r u c t o rcrownHeigth = ch ;crownDiameter = cd ;

}

p u b l i c double getCrownVolume ( ) {. . .

}

}

// TrunkTree . j a v ac l a s s TrunkTree extends Tree {

. . .}


instanceof

All class inherit from Object

instanceof tests the type of an object

Tree t = new Tree (10d ) ;CrownTree c t = new CrownTree (10d , 4d , 4d ) ;

c t i n s t a n c e o f CrownTree ; // t r u ec t i n s t a n c e o f Tree ; // t r u ec t i n s t a n c e o f Object ; // t r u e

t i n s t a n c e o f Tree ; // t r u et i n s t a n c e o f CrownTree ; // f a l s e


Cast

We can use the parent type in the code (“is a” relation)

we use a cast, when we have to change the type to a subtype

a cast can fail

Tree t1 = new Tree (10d ) ;Tree t2 = new CrownTree (10d , 4d , 4d ) ;

t1 . getCrownVolume ( ) ; // f a i l : not a c rownt r e et2 . getCrownVolume ( ) ; // f a i l : j a v a t h i n k i t i s a Tree

CrownTree c t1 = ( CrownTree ) t2 ; //okc t1 . getCrownVolume ( ) ; // ok

CrownTree c t1 = ( CrownTree ) t1 ; // f a i l : t1 i s not a CrownTree


Override

Method can be overriden in sub-class

Use annotation @Override

c l a s s Tree {

/∗∗ Tree growth ∗/p u b l i c v o i d growth ( i n t nbYear ) {

d i amete r += nbYear ∗ dbhInc ;}

}

/∗∗ Tree wi th crown ∗/c l a s s CrownTree extends Tree {

/∗∗ Tree growth ∗/p u b l i c v o i d growth ( i n t nbYear ) {

super . growth ( nbYear ) ;crownDiameter += nbYear ∗ c rownInc ;

}}


Interface

An interface is a kind of contract

A class can implement several interface (multiple inheritance)

An interface do not provide any implementation

An interface is a type

// Growthab le . j a v ai n t e r f a c e Growthab le {

p u b l i c v o i d growth ( i n t nbyear ) ;}

// Tree . j a v ac l a s s Tree implements Growthable , C l on eab l e {

p u b l i c v o i d growth ( i n t nbyear ) {// do growth

}

p u b l i c Object c l o n e ( ) {// do c l o n e . . .

}}

. . .Growthab le g = new Tree ( . . . ) ;g . growth ( 1 0 ) ;g . ge tD iamete r ( ) ; // e r r o r : j a v a don ’ t know i f i t i s a Tree


Abstract class

An abstract class is an incomplete class

It cannot be instantiated

Like an interface but with some implementation

Can be useful to share common methods in an inheritance graph

c l a s s a b s t r a c t Abs t r a c tT re e {

p r i v a t e double h e i g h t ;p r i v a t e double d i amete r ;

p u b l i c double getVolume ( ) {

double volume ;double r a d i u s = t h i s . d i amete r / 2d ;r e t u r n = r a d i u s ∗ r a d i u s ∗ 2d ∗ Math . PI ∗ t h i s . h e i g h t ;

}

a b s t r a c t i n t getNbLeaves ( ) ;

}


Abstract class

Each sub-class implements abstract methods

c l a s s TreeWithNbLeaves extends Abs t r a c tT re e {

p r i v a t e i n t nbLeaves ;

@Over r idei n t getNbLeaves ( ) {

r e t u r n nbLeaves ;}

}

c l a s s TreeWi thL i s tOfLeave s extends Abs t r a c tT re e {

p r i v a t e L i s t<Leaf> l e a v e s ;

@Over r idei n t getNbLeaves ( ) {

r e t u r n l e a v e s . s i z e ( ) ;}

}


Polymorphism

Polymorphism means that we can use an object without knowing itsexact type

The correct function will be called

L i s t<Abst rac tTree> l = new Ar r a yL i s t<Abst rac tTree >();

l . add (new TreeWithNbLeaves ( ) ) ;l . add (new TreeWi thL i s tOfLeave s ( ) ) ;

f o r ( Abs t r a c tT re e t : l ) {System . out . p r i n t l n ( "nb leaves : " + t . getNbLeaves ( ) ) ;

}


Exercices

In the tree package

Create the sub-class CrownTree

Add a method to compule the volume of the CrownTree (use@Override)

Create 2 sub-classes SphericTree and ConicTree

Override the volume function

Create a class Forest containing a list of trees

Add function to build a random forest

Compute the volume of the forest


Java library

See javadoc : http ://java.sun.com/javase/6/docs/api/

GUI

Math

Data Structure (Collections)

Input Output

Networking

MultiThreading

Database

Intropection


Apache commons libraries

See web : http ://commons.apache.org/

Math

IO

Loggin

CLI

Collections

. . .


Some links

Sun’s tutorials : http ://java.sun.com/docs/books/tutorial/

Coding conventions :http ://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html

Resources on Capsis web site : http ://capsis.cirad.fr

Millions of books. . .


Documents

A Java introduction - capsis.cirad.frcapsis.cirad.fr/capsis/_media/javaforcapsis151209.pdf · SDK, FC (UMR AMAP) A Java introduction 15/12/2009 6 / 50. Java application Java programs