Lect2 Intro Javatechnology

8/10/2019 Lect2 Intro Javatechnology

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JAVSpring 2013

Lecture 1Introduction to Java Technology


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Duc L. M. JAV Spring 2013 2

Lecture outline

Basic terminologies Java programming language Java platform Java virtual machine


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MyProgram.java

Host platforms

Basic terminologies1

Java virtual machine

APIJava platforms

Java language


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Java programming language

Brief history Language features The HelloWorldApp.java program

2


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Brief history (2)

"imilar in synta# to $%$&&: but omits comple#! confusing! unsafe features

"upported by web browsers via e#tensions: Java programs are 'embedded( in HT)L pages

*esign and architecture decisions drew from+iffel! "mallTal ! ,b-ective $! and $edar%)esa


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Language features

"imple! ob-ect oriented! familiar .obust and secure Architecture neutral and portable High performance Interpreted! threaded! and dynamic



he HelloWorldApp.java program

public class HelloWorldApp { public static void main(String[] args {

System.out.println(!Hello World"! #

$$



!eveloping a Java program

An overview of the software development process

javac MyProgram.javajavac MyProgram.java java MyProgramjava MyProgram



Java Application programminginterface (A"#)

A collection of ready/made components thatprovide useful capabilities

0rouped into libraries nown as packages



Java platform (1)

A software/based platform in which Javaprograms run

.uns on top of other hardware/based platforms!e1g1 2indows! Linu#! etc1

3



Java platform (2)

*esigned for classes of host platforms and%orapplications

+#amples of host platform classes:

small devices: restricted configuration P$s: standard hardware configuration servers: high performance configuration

+#amples of application classes: stand alone small scale large scale



Java platform (3)

*iffer in the J3) implementations and%or APIfeatures:

host re4uirements 5 different J3) implementations

application re4uirements 5 different API features Three main platforms:

Java S$ Java ++ Java )+



Java S$

Java "tandard +dition Provides core functionality:

basic types and ob-ects programming abstractions for networ ing! security!

database access! 06I development and 7)Lparsing

$ommon development tools and deploymenttechnologies


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Java S$ platform

threa%


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Java S$ focus in this mo%ule

threa%


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!es&top application

HelloWorldApplication

!$'


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Applet

HelloWorldApplet

!$'


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Java '$

Java )obile +dition *esigned for small devices

e1g1 mobile phones Provides API and a small/footprint J3) API 8 subset of Java "+ API & libraries for small

device applications Java )+ applications often interact with Java

++ platform services


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'o ile application

HelloWorldMidlet

!$'


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Java virtual machine

,verview and features Program e#ecution cycle ,ther selected topics:

"tac memory Heap memory

*


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vervie+ (1)

The J3) is the base for the Java platform )a es Java programs platform independent:

'write once! run anywhere( *ifferent versions e#ist for different hardware/

based platforms


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vervie+ (2)


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,eatures

An abstract computing machine with: instruction set memory management

+mulates the host machines to ensure platform/independent byte codes

*oes not re4uire Java programming language

supports any language that follows the class fileformat


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Virtual machine start up

The method main is invo ed with argumentString[] :

header: public static void main String[] ;

argument is a nullable String array Invocation is typically from the command line:

java HelloWorldApp say %ello &orld"


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HelloWorldApp +ith arguments


'or (int i )# i * args.lengt%# i++ {

System.out.println(args[i]); $

$

$


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Loa%ing

$lass HelloWorldApp is loaded by,lass-oader

The loaded class is an ob-ect of class ,lass cached for subse4uent use

Loading may fail due to: incorrect class file format

incorrect version of the class file format not found


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Lin&ing

$ombines the loaded class into the runtimestate of the J3)

Three steps:

verification : chec the class structure preparation : creating and initialising class fields to

default values resolution : resolve references to other classes


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#nitialisation

+#ecute initialisers: class static; initialisers initialisers for static fields

Also causes any super class es; to be loaded!lin ed and initialised: if not already


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.reation of ne+ class instances

,b-ects are created if re4uired ,b-ect creation involves:

allocating enough memory space for all variables

declared in the class and super class; initialising the variables e#ecuting a constructor method


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HelloWorldApp +ith o /ects


String msg = "Hello orld!";

// or String msg = ne String( Hello orld!#); System.out.println(msg #

$

$


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,inalisation of class instances

.emove ob-ects that are no longer in use Java garbage collector automatically remove

ob-ects these

9inali


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nloa%ing

6nload unused classes to reduce memory use A class is unloaded when its associated

,lass-oader is removed

"ystem classes may never be unloaded


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Virtual machine e-it

2hen one of two things happens: all non/daemon processes threads; finish e#ecution invo es System.e2it or 3un4ime.e2it


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Stac&s

+ach program thread has a stac to: hold local variables and partial results used in method invocation and return

"tac /overflow error is thrown if a stac runs outof memory "tac si


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Specifying threa% stac& si e

To specify a =) stac si


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eap

A run/time memory shared among all J3)threads: created on J3) start up

6sed for storing ob-ects Heap space is reclaimed by a garbage collector ,ut/of/memory error is thrown if heap runs out

of space Heap si


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eap

To specify initial and ma# heap si


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Summary

Java technology includes Java language!platform! virtual machine and API Java language is ob-ect oriented! robust! and

architecture neutral *ifferent types of Java platforms designed for

different classes of hosts and applications Java "+! ++! )+

Java virtual machine is a software abstraction ofthe host! ma ing Java programs platformindependent

Lect2 Intro Javatechnology

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