Java basics

Java$Basics$
Dr. Shahid Raza! 1!

Three Java Editions
• Java Platform, Standard Edition (Java SE)
• Java Platform, Enterprise Edition (Java EE)
• Java Platform, Micro Edition (Java ME)
Lecture 2: Overview: Java Peculiarities 3

The Java Platform, Standard Edition
(Java SE)

Java Platform, Enterprise Edition
(Java EE)
• Enterprise Application
Technologies
– Enterprise JavaBeans (EJB)
– J2EE Connector Architecture
– Java Message Service (JMS)
• Web Application
Technologies
– Java Servlet
– JavaServer Pages (JSP)
– JavaServer Faces (JSF)
– Java Persistence API (JSR)
• Provides a POJO persistence
model for object-relational
mapping. Developed and use
for EJB, but can be used
directly
– Java Transaction API (JTA)
– JavaMail
• Management and Security
Technologies
– J2EE Application
Deployment
– J2EE Management
– Java Authorization Contract
for Containers

Object.Oriented$Nomenclature$
• Class$means$a$category$of$things$
– A$class$name$can$be$used$in$Java$as$the$type$of$a$field$or$
local$variable$or$as$the$return$type$of$a$func?on$(method)$
• Object$means$a$par?cular$item$that$belongs$to$a$
class$
– Also$called$an$instance$
• For$example,$consider$the$following$line:$
$ $$ $$$$String s1 = Hello;$
– Here,$String$is$the$class,$s1$is$an$instance$variable$of$class$
String,$whose$value$is$the$String$object$Hello$$
Dr. Shahid Raza! 2!

Example$1:$Instance$Variables$
(Fields$or$Data$Members)$
class Ship1 {
public double x, y, speed, direction;
public String name;
}
public class Test1 {
public static void main(String[] args) {
Ship1 s1 = new Ship1();
s1.x = 0.0;
s1.y = 0.0;
s1.speed = 1.0;
s1.direction = 0.0; // East
s1.name = Ship1;
System.out.println(s1.name + is initially at (
+ s1.x + , + s1.y + ).);
s1.x = s1.x + s1.speed
* Math.cos(s1.direction * Math.PI / 180.0);
s1.y = s1.y + s1.speed
* Math.sin(s1.direction * Math.PI / 180.0);
System.out.println(s1.name + has moved to (
+ s1.x + , + s1.y + ).);
}
}
Dr. Shahid Raza! 3!

Instance$Variables:$Results$
• Compiling$and$Running:$
javac Test1.java
java Test1
Output:$
Ship1 is initially at (1,0).
Ship2 has moved to (-1.41421,1.41421).
Dr. Shahid Raza! 4!

Primitive Data Types
• Much like in C
– byte
– short
– char
– int
– long
– float
– double
• See Examples in Exercise 0.

Widening Primitive Conversions
• Do not lose information about the overall
magnitude of a numeric value.
From To
byte short, int, long, float, double
short int, long, float, double
char int, long, float, double
int long, float, double
long float, double
float double

Narrowing Primitive Conversions
• May lose information about the overall magnitude of
a numeric value and may also lose precision.
From To
byte char
short byte, char
char byte, short
int byte, short, char
long byte, short, char, int
float byte, short, char, int, long
double byte, short, char, int, long, float

Example$1:$Major$Points$
• Java$naming$conven?on$
• Format$of$class$defini?ons$
• Crea?ng$classes$with$new$
• Accessing$fields$with$
variableName.fieldName$
Dr. Shahid Raza! 5!

Java$Naming$Conven?ons$
• Leading$uppercase$leTer$in$class$name$
public class MyClass {
...
}
• Leading$lowercase$leTer$in$field,$local$
variable,$and$method$(func?on)$names$
– myField,myVar,myMethod
Dr. Shahid Raza! 6!

First$Look$at$Java$Classes$
• The$general$form$of$a$simple$class$is$
modifier class Classname {
modifier data-type field1;
modifier data-type field2;
...
modifier data-type fieldN;
modifier Return-Type methodName1(parameters) {
//statements
}
...
modifier Return-Type methodName2(parameters) {
//statements
}
}
Dr. Shahid Raza! 7!

Objects$and$References$
• Once$a$class$is$defined,$you$can$easily$declare$a$
variable$(object$reference)$of$the$class$
Ship s1, s2;
Point start;
Color blue;
• Object$references$are$ini?ally$null$$$
– The$null$value$is$a$dis?nct$type$in$Java$and$should$
not$be$considered$equal$to$zero$$
– A$primi?ve$data$type$cannot$be$cast$to$an$object$(use$
wrapper$classes)$
• The$new$operator$is$required$to$explicitly$create$
the$object$that$is$referenced$
ClassName variableName = new ClassName();$
Dr. Shahid Raza! 8!

Accessing$Instance$Variables$
• Use$a$dot$between$the$variable$name$and$the$field$
name,$as$follows:$
variableName.fieldName$$
• For$example,$Java$has$a$built.in$class$called$Point$that$
has$x$and$y$fields$
Point p = new Point(2, 3); // Build a Point object
int xSquared = p.x * p.x; // xSquared is 4
p.x = 7;
• One$major$excep?on$applies$to$the$access$fields$
through$varName.fieldName$rule$
– Methods$can$access$fields$of$current$object$without$varName$
– This$will$be$explained$when$methods$(func?ons)$are$discussed$
Dr. Shahid Raza! 9!

Example$2:$Methods$
class Ship2 {
public double x=0.0, y=0.0, speed=1.0, direction=0.0;
public String name = UnnamedShip;
private double degreesToRadians(double degrees) {
return(degrees * Math.PI / 180.0);
}
public void move() {
double angle = degreesToRadians(direction);
x = x + speed * Math.cos(angle);
y = y + speed * Math.sin(angle);
}
public void printLocation() {
System.out.println(name + is at (
+ x + , + y + ).);
}
}
Dr. Shahid Raza! 10!

Methods$(Con?nued)$
s1.name = Ship1;
s2.direction = 135.0; // Northwest
s2.speed = 2.0;
s2.name = Ship2;
s1.move();
s2.move();
s1.printLocation();
s2.printLocation();
}
}
• Compiling$and$Running:$
javac Test2.java
java Test2
• Output:$
Ship1 is at (1,0).
Ship2 is at (-1.41421,1.41421).

• Format$of$method$defini?ons$
• Methods$that$access$local$fields$
• Calling$methods$
• Sta?c$methods$
• Default$values$for$fields$
• public/private$dis?nc?on$

Defining$Methods$
(Func?ons$Inside$Classes)$
• Basic$method$declara?on:$
p u b l i c R e t u r n T y p e m e t h o d N a m e ( ttyyppee12 aarrgg12,, ...) {
...
return(something of ReturnType);
}
• Excep?on$to$this$format:$if$you$declare$the$return$
type$as$void
– This$special$syntax$that$means$this$method$isnt$
going$to$return$a$value$–$it$is$just$going$to$do$some$
side$effect$like$prin?ng$on$the$screen$$
– In$such$a$case$you$do$not$need$(in$fact,$are$not$
permiTed),$a$return$statement$that$includes$a$
value$to$be$returned$

Examples$of$Defining$Methods$
• Here$are$two$examples:$
– The$first$squares$an$integer$$
– The$second$returns$the$faster$of$two$Ship$objects,$assuming$that$a$
class$called$Ship$has$been$defined$that$has$a$field$named$speed$$
// Example function call:
// int val = square(7);
public int square(int x) {
return(x*x);
}
// Example function call:
// Ship faster = fasterShip(someShip, someOtherShip);
public Ship fasterShip(Ship ship1, Ship ship2) {
if (ship1.speed ship2.speed) {
return(ship1);
} else {
return(ship2);
}
}$

Excep?on$to$the$Field$Access$with$
Dots$Rule$
• Normally$access$a$field$via: variableName.fieldName $
$
but$an$excep?on$is$when$a$method$of$a$class$wants$to$access$
fields$of$that$same$class$$
– In$that$case,$omit$the$variable$name$and$the$dot$
– For$example,$a$move$method$within$the$Ship$class$might$do:$
x = x + speed * Math.cos(direction);
...
}
• Here,$x,$speed,$and$direction$are$all$fields$within$the$class$that$
the$move$method$belongs$to,$so$move$can$refer$to$the$fields$directly$
$
– As$well$see$later,$you$s?ll$can$use$the$variableName.fieldName$
approach,$and$Java$invents$a$variable$called$this$that$can$be$used$for$
that$purpose$

Sta?c$Methods$
• Sta?c$func?ons$are$like$global$func?ons$in$other$
languages$
$
• You$can$call$a$sta?c$method$through$the$class$name$
ClassName.functionName(arguments);
– For$example,$the$Math$class$has$a$sta?c$method$called$cos$
that$expects$a$double$precision$number$as$an$argument$
• So$you$can$call$Math.cos(3.5)$without$ever$having$any$object$
(instance)$of$the$Math$class$

Method$Visibility$
• public/private$dis?nc?on$
– A$declara?on$of$private$means$that$outside$methods$
cant$call$it$..$only$methods$within$the$same$class$can$
• Thus,$for$example,$the$main$method$of$the$Test2$class$could$
not$have$done$$
double x = s1.degreesToRadians(2.2);
– ATemp?ng$to$do$so$would$have$resulted$in$an$error$at$compile$?me$$
– Only$say$public$for$methods$that$you$want%to%guarantee%
your%class%will%make%available%to%users$$
– You$are$free$to$change$or$eliminate$private$methods$
without$telling$users$of$your$class$about$
$$$

Example$3:$Constructors$
class Ship3 {
public double x, y, speed, direction;
public String name;
public Ship3(double x, double y,
double speed, double direction,
String name) {
this.x = x; // this differentiates instance vars
this.y = y; // from local vars.
this.speed = speed;
this.direction = direction;
this.name = name;
}
private double degreesToRadians(double degrees) {
return(degrees * Math.PI / 180.0);
}
...

Constructors$(Con?nued)$
double angle = degreesToRadians(direction);
x = x + speed * Math.cos(angle);
y = y + speed * Math.sin(angle);
}
public void printLocation() {
System.out.println(name + is at (
+ x + , + y + ).);
}
}
Ship3 s1 = new Ship3(0.0, 0.0, 1.0, 0.0, Ship1);
Ship3 s2 = new Ship3(0.0, 0.0, 2.0, 135.0, Ship2);
s1.move();
s2.move();
s1.printLocation();
s2.printLocation();
}
}$

Constructor$Example:$Results$
• Compiling$and$Running:
javac Test3.java
java Test3
• Output:$
Ship1 is at (1,0).
Ship2 is at (-1.41421,1.41421).

• Format$of$constructor$defini?ons$
• The$this$reference$
• Destructors$(not!)$

Constructors$
• Constructors$are$special$func?ons$called$when$a$class$is$
created$with$new$
– Constructors$are$especially$useful$for$supplying$values$of$fields$
– Constructors$are$declared$through:$
public ClassName(args) {
...
}
– No?ce$that$the$constructor$name$must$exactly$match$the$class$
name$
– Constructors$have$no$return$type$(not$even$void),$unlike$a$
regular$method$
– Java$automa?cally$provides$a$zero.argument$only$if$the$class$doesnt$define$its$own$constcrouncstotrru$$ctor$if$and$
• Thats$why$you$could$say$
$$$in$the$first$example,$even$though$a$constructor$was$never$defined$

The$this$Variable$
• The$this$object$reference$can$be$used$inside$any$non.sta?c$
method$to$refer$to$the$current$object$
• The$common$uses$of$the$this$reference$are:$
1. To$pass$a$reference$to$the$current$object$as$a$parameter$to$other$
methods$
someMethod(this);
2. To$resolve$name$conflicts$
• Using$this$permits$the$use$of$instance$variables$in$methods$that$
have$local$variables$with$the$same$name$
– Note$that$it$is$only$necessary$to$say$this.fieldName$when$you$
have$a$local$variable$and$a$class$field$with$the$same$name;$otherwise$
just$use$fieldName$with$no$this

Destructors$
This Page Intentionally Left Blank

Summary$
• Class$names$should$start$with$uppercase;$method$names$
with$lowercase$
• Methods$must$define$a$return$type$or$void$if$no$result$
is$returned$
• If$a$method$accepts$no$arguments,$the$arg.list$in$the$
method$declara?on$is$empty$instead$of$void$as$in$C$
• Sta?c$methods$do$not$require$an$instance$of$the$class;$
sta?c$methods$can$be$accessed$through$the$class$name$
• The$this$reference$in$a$class$refers$to$the$current$
object$$
• Class$constructors$do$not$declare$a$return$type$
• Java$performs$its$own$memory$management$and$
requires$no$destructors$
$$$

Java basics

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Java basics