java technologies 1

8/6/2019 Java Technologies 1

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Java Technologies

Prof. Nitin ZadpeMIT School of Telecom Management

1 MIT School of Telecom Management


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What is JAVA Technology?



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ELEPHANT AND THE BLIND MEN



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What is Java Technology?

The Java technology is:

A programming language

A development environment

An application environment

A deployment environment



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Programming Language

As a programming language, Java cancreate all kinds of applications that you

could create using any conventionalprogramming language.



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A Development Environment

As a development environment, Javatechnology provides you with a large suite of

tools: A compiler (javac)

An interpreter (java)

A documentation generator (javadoc) A class file packaging tool

and so on...



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An Application and Runtime

Environment

There are two main deployment environments:

1. The JRE supplied by the Java 2 Software Development Kit (SDK)

contains the complete set of class files for all the Java technology

packages, which includes basic language classes, GUI component

classes, and so on.

2. The other main deployment environment is on your web browser.

Most commercial browsers supply a Java technology interpreter and

runtime environment.

Java technology applications are typically general-purposeprograms that run on any machine where the Java runtimeenvironment (JRE) is installed.



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Java Features

Some features of Java:

The Java Virtual Machine

Garbage Collection

Code Security



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The Java Virtual Machine Java Virtual Machine (JVM)

an imaginary machine that is implemented by emulatingsoftware on a real machine

provides the hardware platform specifications to whichyou compile all Java technology code

Bytecode

a special machine language that can be understood by

the Java Virtual Machine (JVM) independent of any particular computer hardware, so any

computer with a Java interpreter can execute thecompiled Java program, no matter what type of

computer the program was compiled on 9 MIT School of Telecom Management


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Garbage Collection

Garbage collection thread

responsible for freeing any memory that can be

freed. This happens automatically during thelifetime of the Java program.

programmer is freed from the burden of having

to deallocate that memory themselves



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Code Security

Code security is attained in Java through theimplementation of its Java Runtime Environment

(JRE). JRE

runs code compiled for a JVM and performs

class loading (through the class loader), codeverification (through the bytecode verifier) and

finally code execution



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Code Security

Class Loader responsible for loading all classes needed for the Java

program adds security by separating the namespaces for the

classes of the local file system from those that areimported from network sources

After loading all the classes, the memory layout of theexecutable is then determined. This adds protectionagainst unauthorized access to restricted areas of thecode since the memory layout is determined duringruntime



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Code Security

Bytecode verifier

tests the format of the code fragments and

checks the code fragments for illegal code thatcan violate access rights to objects



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Phases of a Java Program

The following figure describes the process ofcompiling and executing a Java program



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Phases of a Java Program



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If you have any doubt. You should ask..



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History of Java In the early 1990's, putting intelligence into home appliances was

thought to be the next "hot" technology. Examples of intelligent home appliances:

Coffee pots and lights that can be controlled by a computer'sprograms.

Televisions that can be controlled by an interactive television

device's programs. Anticipating a strong market for such things, Sun Microsystems

in 1991 funded a research project (code named Green) whosegoal was to develop software for intelligent home appliances.

An intelligent home appliance's intelligence comes from itsembedded processor chips and the software that runs on the

processor chips. Appliance processor chips change often because engineers

continually find ways to make them smaller, less expensive, andmore powerful.

To handle the frequent turnover of new chips, appliance softwaremust be extremely portable.

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History of Java Originally, Sun planned to use C++ for its home appliance

software, but they soon realized that C++ was less than idealbecause it wasn't portable enough and it relied too heavily onhard-to-maintain things called pointers.

Thus, rather than write C++ software and fight C++'s inherent

deficiencies, Sun decided to develop a whole new programminglanguage to handle its home appliance software needs.

Their new language was originally named Oak (for the tree thatwas outside project leader James Gosling's window), but it wassoon changed to Java.

When the home appliance software work dried up, Java almostdied before being released.

Fortunately for Java, the World Wide Web exploded in popularityand Sun realized it could capitalize on that.

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History of Java Web pages have to be very portable because they can be

downloaded onto any type of computer.

What's the standard language used for Web pages?

Java programs are very portable and they're better than HTML interms of providing user interaction capabilities.

Java programs that are embedded in Web pages are called

applets.

Although applets still play a significant role in Java's currentsuccess, some of the other types of Java programs have

surpassed applets in terms of popularity. In this course, we cover Standard Edition (SE) Java applications.

They are Java programs that run on a standard computer adesktop or a laptop, without the need of the Internet.2

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MIT School of Telecom Management 20

History of Java

Year Development1990 Sun decided to developed special software that could be

used for electronic devices. A project called Green Projectcreated and head by James Gosling.

1991 Explored possibility of using C++, with some updates

announced a new language named Oak

1992 The team demonstrated the application of their newlanguage to control a list of home appliances using ahand held device.

1993 The World Wide Web appeared on the Internet andtransformed the text-based interface to a graphical richenvironment. The team developed Web applets (timeprograms) that could run on all types of computersconnected to the Internet.


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Year Development

1994 The team developed a new Web browsed called HotJava to locate and run Applets. HotJava gained instancesuccess.

1995 Oak was renamed to Java, as it did not survive legalregistration. Many companies such as Netscape andMicrosoft announced their support for Java

1996 Java established itself it self as both 1. the language forInternet programming 2. a general purpose OO

language.1997 A class libraries, Community effort and standardization,

Enterprise Java, Clustering, etc..

History of Java


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Year Development

1998 JDK 1.1 Realese

1998 Java 2 platform source code is released

2004 Java 2 Platform, Standard Edition 5 (Project Tiger) is released

2006 Java SE and ME initial components are released as opensource

2007 Sun released the source code of the Class library under GPL

2010 Oracle acquires Sun Microsystems.

History of Java


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Java Features (1)

Simple fixes some clumsy features of C++

no pointers

automatic garbage collection

rich pre-defined class library

Object oriented

focus on the data (objects) and methods manipulating the data

all functions are associated with objects

almost all datatypes are objects (files, strings, etc.) potentially better code organization and reuse


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Interpreted java compiler generate byte-codes, not native machine code

the compiled byte-codes are platform-independent

java bytecodes are translated on the fly to machine readable

instructions in runtime (Java Virtual Machine)

Portable(Platform Independence )

same application runs on all platforms

the sizes of the primitive data types are always the same

the libraries define portable interfaces The Write-Once-Run-Anywhere

Java Features (2)


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Java Features (3)

Reliable

extensive compile-time and runtime error checking

no pointers but real arrays. Memory corruptions or unauthorizedmemory accesses are impossible

automatic garbage collection tracks objects usage over time

Secure

usage in networked environments requires more security

memory allocation model is a major defense

access restrictions are forced (private, public)


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Java Features (4)

Multithreaded

multiple concurrent threads of executions can run simultaneously

utilizes a sophisticated set of synchronization primitives (basedon monitors and condition variables paradigm) to achieve this

Dynamic

java is designed to adapt to evolving environment

libraries can freely add new methods and instance variableswithout any effect on their clients

interfaces promote flexibility and reusability in code by specifyinga set of methods an object can perform, but leaves open howthese methods should be implemented

can check the class type in runtime


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Java Features (5) Robust

Exception handling built-in, strong type checking (that is, all datamust be declared an explicit type), local variables must beinitialized.

Architecture Neutral

The growing popularity of networks makes developers thinkdistributed. In the world of network it is essential that theapplications must be able to migrate easily to different computersystems. Not only to computer systems but to a wide variety ofhardware architecture and Operating system architectures aswell.


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Total Platform Independence

JAVA COMPILER

JAVA BYTE CODE

JAVA INTERPRETER

Windows XP Macintosh Solaris Linux

(translator)

(same for all platforms)

(one for each different system)


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Java

Syrup

Java

Capsules

Injectable

JavaJava

HairOil

Dont wait for these

products..


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Quiz Questions

1. Pseudocodea) uses precise computer syntax.b) compiles into object code.c) is often used when writing an algorithm.

2. Java programs can be embedded in Web pages, and suchprograms are called

a) appletsb) applicationsc) operating systems


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Primitive Data types

Java has 8 (or so) primitive types: float

double

booleanchar

byte

short

int

long

Also the void type

real numbers

integer numbers

two values: true and false

a single character



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Primitive real (floating-point)

types A float takes up 4 bytes of space

Has 6 decimal places of accuracy: 3.14159

A double takes up 8 bytes of space

Has 15 decimal places of accuracy:3.14159265358979

Always use doubles

It will save you quite a headache!



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Primitive integer types

Consider a byte:

0 1 0 0 0 1 0 1

1 byte = 8 bits

Each bit has two possibilities: 0 or 1

28 = 256

Thus, a byte can have any one of 256 values

A Java byte can have values from -128 to 127

From -27 to 27-1

C/C++ has unsigned versions; Java does not



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Primitive integer types

Type Bytes Minimum value Maximum value

byte 1 -27=-128 27-1=127

short 2 -215

=-32,768

215

-1=32,767

int 4 -231=-2,147,483,648 231-1=2,147,483,647

long 8 -263=-9,223,372,036,

854,775,808

263-1=9,223,372,036,

854,775,807



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Primitive character type

All characters have a integer equivalent

0 = 48

1 = 49

A = 65

a = 97

Thus, you can refer to B as A+1



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Primitive boolean type

The boolean type has only two values:

true

false

There are boolean-specific operators

&& is and || is or

! is not



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Identifiers Identifiers are names for variables, classes, etc.

Good ones are compact, but inidicate what they stand for

radius, width, height, length

Bad ones are either too long

theRadiusOfTheCircle

theWidthOfTheBoxThatIsBeingUsed

the_width_of_the_box_that_is_being_used

Or too short

a, b, c, d, e

Good identifiers will help the graders understandyour program!



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// Authors: J. P. Cohoon and J. W. Davidson// Purpose: display a quotation in a console window

public class DisplayForecast {

// method main(): application entry point

public static void main(String[] args) {System.out.print("I think there is a world market for");System.out.println(" maybe five computers.");System.out.println(" Thomas Watson, IBM, 1943.");

}}





}}

Keywords





}}

Some words are reserved, and cant be used

as identifiers



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Capitalization

Case matters!

public Public PUBLICThis is different

You can use Public as a identifier

Not recommended, though!



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Defining variables

Weve seen variables before in math y= mx+ b

Here y, m, x, and bcan hold any value

To store things in a computer program, wealso use variables

Example: int x = 5;

This defines an integer variable with value 5

The variable is x

The type is int40 MIT School of Telecom Management


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More on variables

An integer variable can only hold integers In other words, it cant hold 4.3

To hold floating point values, we use thedouble type

double d = 4.3;

The variable is d

The type is double



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Assignment operator =Allows the memory location for a variable to be

updated

Considerint j = 11;

j = 1985;

Primitive variable assignment



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Variables must be declared

before use The following code will not work:

x = 5;System.out.println (x);

Java requires you to declare x before youuse it



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Variable initialization

Consider the following code:int x;

System.out.println(x);

What happens?

Error message:

variable x might not have been initialized

Java also requires you to give x a valuebefore you use it



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Constants

Consider the following:

final int x = 5;

The value of x can NEVER be changed!

The value assigned to it is final

This is how Java defines constants



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Expressions What is the value used to initialize expression

int expression = 4 + 2 * 5;

What value is displayedSystem.out.println(5 / 2.0);

Java rules in a nutshell Each operator has a precedence level and an

associativity

Operators with higher precedence are done first

* and / have higher precedence than + and -

Associativity indicates how to handle ties

When floating-point is used the result is floating point



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Question on expressions

Does the following statement compute the

average of double variables a, b, and c?Why or why not?

double average = a + b + c / 3.0;



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Java operators

The following are the common operators for ints: + - / * % Division is integer division

6 / 2 yields 3 7 / 2 yields 3, not 3.5 Because everything is an int, the answer is an int

Modulus is % Returns the remainder 7 % 2 yields 1 6 % 2 yields 0

Floats and doubles use the same first four operators + - / * 7.0 / 2.0 yields 3.5 7.0 / 2 yields 3.5 7 / 2.0 yields 3.5 7 / 2 yields 3



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Java operators

Booleans have their own operators && is AND

Only true when both operands are true

true && true yields true

false && true yields false || is OR

True when either of the operands (or both) are true

true || false yields true

false || false yields false ! is NOT

Changes the value

!true yields false

!false yields true49 MIT School of Telecom Management


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Functions In Java, functions are called methods

Think of mathematical functions: sin()

cos()

tan()

They take input (the angle) And produce output (the result)

In Java, they are called Math.sin(),Math.cos(), etc.Meaning, from the Math library, call the sin() method



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Hello World!

Lets consider how to write this program.

What would be the input? Nothing

What would be the output? Hello World!

What would thealgorithm look like?

Display the textHello World!

on the screen.


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Sample Java Program

Here is the complete Java code for the HelloWorld! program.

Lets look at it line by line.


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Sample Java Program

Lines 1-5 are called comment lines. shown in green by most IDEs ignored by the Java compiler

not executed

used for internal documentation


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Sample Java Program

A multi-line comment is enclosed with/*...*/Middle lines of code usually begin with an asterisks

(*) to improve readability, but it is not a

requirement.

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Sample Java Program

The text beginning with//at the right side of somelines are single line comments.

They are also ignored by the Java compiler and not

executed.

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Sample Java Program

Line 7 is called the class header.

uses the Java keywords public and class

keywords are displayed in blueby most IDEs

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Sample Java Program


identifies the name of this class or program

HelloWorld is the class name in this program

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Sample Java Program


The Java filename should match the class name. Therefore, the Java filename should be HelloWorld.java on your disk.

Created by doing a File/Save As from the text editor.

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Sample Java Program


Note: Java is case sensitive.

HelloWorld does not equal Helloworld.

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Sample Java Program

Lines 8 through 13 are called the class body.

The class body begins with a left curly brace ( { )on the line after the class header.

The class body ends with a right curly brace ( } )on the last line of the program.

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Sample Java Program

Lines 8 through 13 are called the class body.

A class body is divided into one or more methods.

This class has only one methodthe main method.

The main method is a required method in every

Java application (program).

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Sample Java Program

Line 9 is called the method header.

It uses the keywords public static void

(meanings will be explained later in the semester).

It also uses the name of the method (main).

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Sample Java Program

Line 9 is called the method header.

The rest of the method header will also be explained in a laterlesson.

For now, use the entire main method header as is without being

concerned about understanding it.

Sample Java Program


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Sample Java Program

Lines 10 through 12 are called a method body.

Like a class body, a method body is enclosed bycurly braces.

These are indented to improve readability.

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Sample Java Program

Line 11 is an executable statement.

Java executable statements end with a semicolon.

Notice that class headers and method headers are NOT

terminated with a semicolon.

Sample Java Program


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Sample Java Program


System.out.println displays to the screen the contents thatare enclosed in parenthesis.

In this case, Hello World! will be displayed on the screen.

Sample Java Program


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Sample Java Program


The content inside the parentheses is called a string literal.

It must be surrounded by double quotes.


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