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System development with Java

Instructors: Rina Zviel-Girshin

Interdiciplinary Center HerzliaSchool of the Computer Science

Fall: 2000-2001

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Information Source

• Course site : www.idc.ac.il/

• Course book:

Java Software Solutions /Lewis & Loftus

Thinking in Java / Bruce Eckle

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Course Methodology

• Non-LinearitySpiral model

Hypermedia

• CAI – Computer Assisted Instruction

•  Interactive learning

•  Labs and programming

•  Multi-approach: inductive-deductive trails

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Programming Languages and Java

• Programming languages

• Java language

• The basic program

• Java syntax

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Programming

• The process of writing programs called a programming.

• The process requires:Programming languageA translator of the programming language

into machine language• Programs are written in programming language.

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Syntax

• Syntax describes the grammatical rules of a language.

• Valid words• Valid grammar constructions• Valid punctuation

• Programs must be syntactically correct.

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Semantics

• Semantics gives the meaning of what you write with a language.

• A programming language must precisely define the meaning of every statement that can be written with it.

• Programs must be semantically correct.

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Example

• The following sentence is syntactically correct but semantically incorrect.

“ Java programs are green and yellow.”

– All words are correct words.– Punctuation is correct.– But logically the sentence makes no sense.

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Programming languages

• There are several families of the programming languages:

Procedural ( Pascal, C++, Java)

Logical ( Prolog) Visual ( Visual Basic) Document (HTML, LATEX)

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Programming languages

• Procedural languages can be divided into:Machine languagesLow-level (Assembler )Intermediate-level (C)High-Level (C++, Java)

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Java

• Java is high-level language.• Java is object oriented language.• In this course we will use the terms:

– OOP,

– OOA ,

– OOD.

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OOA and OOD

OOA- Object-Oriented Analysis is a way to analyze a problem.

 OOD- Object-Oriented Design is a way to design a solution.

Both see the world as:• objects and • their relations (behavior, communication).

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OOA and OOD (cont)

• The way to analyze a problem is to define the objects relevant to it and their behavior.

•  The way to solve a problem is to realize the objects relevant and the behavior we need.

•  Both are useful as preliminary stages in OOP- Object Oriented Programming.

 

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OOP

Objects are the building blocks of:

the problemthe solutionthe input sourcethe output targetthe environment

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Objects Objects include both:

• data (fields, variables, state)• processing (behavior, methods, functions)

 

Objects have two kinds of behavior:• outer – I/O, messages, relations with others• inner – processing (usually computing the

messages)

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Objects (cont) 

• Relations between objects are implemented using communication (messages).

• Communication can have many forms.• Some forms are very different from the usual idea

of a message.• Objects acquire structure using their relations.• The structure is usually tree-like.

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High-level languages

• Writing programs in machine language is possible but very difficult and time consuming.

• Programs usually written in a more human readable language - high-level language.

• Java is high-level language.• A program written in any computer language must

be translated into a machine language in order to be executed.

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Translators

• Compiler

• A compiler is a program that translates a source program (usually high level language) into target program (usually machine language program).

• The resulting program can be executed many times.

• Interpreter

• An interpreter is a program that reads, translates and executes the source program statement by statement.

• The translation is done each time the program runs.

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Java

• Java is a high-level, third generation programming language, like C, Fortran, Smalltalk.

• Shares much of C's syntax.• Designed by a group at Sun MicroSystems.• Originally called Oak.

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Java advantages

• Platform independent““Write once, run anywhere”.Write once, run anywhere”.

• Improve robustness, remove unsafe language loopholes.

• According to Sun:“ Java is simple, object-oriented, distributed,

interpreted, robust, secure, architecture-neutral, portable, high-performance, multithread, and dynamic language.”

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Java disadvantages

• The main Java disadvantage:

Efficiency concerns.Efficiency concerns.

Looking to the future:

VM performance will improve.

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Java environments

• Java has many environments. • The number grows as Java evolves. • Among them are:

• Text applications• Beans• Applets• GUI Applications• Packages

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Two main environments

The two main environments: Web Browser Operating System

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Web Browser

• In the browser environment the browser acts as an intermediate between the program and the operating system.

• The JVM resides inside the browser.• The program can be simpler.• The program has to work in graphical mode.• The program is called “Applet” (a small

application).

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Operating System

• In the operating system environment the program is called “application”.

• Application has a more rigid structure.• Application can be textual or graphical.• Application is less secure.

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Learning Java The specific challenge is twofold: Learning the language technical aspect: the large

set of • new features • class libraries • classes • methods

which are provided as part of the language. Learning to maximize the benefits of the object-

oriented paradigm.

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OOA and OOD vs. OOP

OOA is a way of analyzing -

OOD is a way of thinking –

OOP is a way of doing.

OOA is about “why” –

OOD is about “what” –

OOP is about “how”.

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Starting to Program

• The basic idea of the course is to start programming as soon as possible.

• At first you will have to take a lot of things on trust.

• Promise:

we will return to the details later.we will return to the details later.

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Java programs

All Java programs have three parts:Auxiliary prescriptions A class – an envelope around the other

components Code per se –

• Optionally – other components which include the instructions themselves

• Instructions to be executed

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Writing a Java Program

• Use an editor to type the program code.• Save the code file.• Compile the file with Java language compiler.• Fix the bugs!• Run the program.

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A basic Java Program

// Prints “How are you?” to the screen

class My

{

public static void main (String[] args)

{ System.out.println(“How are you?”);

}

}

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A basic Java Program

// Prints “How are you?” to the screen

class My

{

public static void main (String[] args)

{ System.out.println(“How are you?”);

}

}

Our program will becalled My

A comment

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Java program structure

• A Java is made up of one or more classes.• Classes have names.• The ‘My’ program consists of one class named

‘My’.

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Class structure• A Java class contains one or more methods.• Methods have names too.• The program (the class) has to have a main method.• The method is called (surprisingly) – main.

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Main• The main method has a standard syntax:

public static void main(String args[])

{ … // your code

}• There can be only one main per file. • The main method contains the code to be executed

when the program runs.

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Method structure

• A method contains one or more statements.• The main method of My program has a single

statement:

System.out.println(“How are you?”);

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Stages of Writing and Executing

Assumptions:• Using only the basic JDK software• Assuming the JDK was installed properly• In the WIN/DOS environment• Class name is My• File name is My.java• Directory (folder) name is c:\java

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Stages of Writing and Executing

1. Open a text editor that can produce a plain text file (such as Notepad).

2. Create a source code file with the extension on the file name being .java . The file has to have the same name as the outer class.

3. Start a the DOS command environment by opening MSDOS prompt (using the

Start->Programs->MSDOS Prompt series of choices).

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Stages of Writing and Executing

4. Change directory to the directory containing the source file.

5. Compile the file, using the prompt command: javac My.java

6. The result is My.class in the same folder.

7. Run using the prompt command: java My

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AppletsThe program has to import two packages:• applet.Applet• awtThe program (the class) has to extend the Applet

class, using the following syntax:public class My extends Applet{ // your code}

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Basic applet code

The final code looks like this:

import java.awt.*;

import java.applet.*;

public class My extends Applet

{

// your code

}

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Stages of Writing and Executing

• Applet can’t be run directly.• Applet has to be run in Browser and by the

Browser.• Web browser executes web pages.• Web page should call the applet code.

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Call to applet

The call to applet consist of: – the command APPLET– the name of the applet class– the dimensions of the panel in which the applet

will run

 

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Example

• Assuming the name of the applet class is HelloWorld.class the call to applet looks like this:

 

<APPLET

CODE = "HelloWorld.class"

WIDTH = 150

HEIGHT= 50>

</APPLET>

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HTMLThe call to applet has to be in HTML file.The file can look as follows:<HTML><HEAD><TITLE>Java applet test page</TITLE></HEAD><BODY><APPLET

CODE="HelloWorld.class"WIDTH=150

HEIGHT=50></APPLET></BODY></HTML>

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Execution

Assuming the file is called HelloWorld.html

1. The stages common to all, described above.

2. Open the file HelloWorld.html in the Browser.

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Java Compiler

• The Java compiler is called javac.• Java programs are compiled to byte code.

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Byte code

• The Java compiler translates Java program into a special representation called byte code.

• Java byte code is a machine code for Java virtual Machine(JVM).

• VM is a platform-specific tool to interpret the byte code and to translate it to commands for certain processor and OS.

• The use of the byte code makes Java platform independent.

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Compile-interpret-execute cycle

Java soursecode

Javacompiler

Java bytecode

javac My.java

My.java

My.class

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Java soursecode

Javacompiler

Java bytecode

Javainterpreter

javac My.java

java My

My.java

My.class

Compile-interpret-execute cycle

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Errors

• A program can have three types of errors:• Syntax and semantic errors – called compile-

time errors• Run-time errors – occur during program

execution• Logical errors

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Errors

• Compile-time errors occur during program compilation and an executable version of the program is not created.

• Run-time errors occur during program execution and cause abnormal program termination.

• Logical errors occur during program execution and produce incorrect results.

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Java Syntax

• To write without syntax mistakes you have to know Java syntax.

SyntaxSyntax - the study of the patterns of formation of sentences and phrases from words and of the rules for the formation of grammatical sentences in a language.

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Java Syntax

• Case-sensitive• Semi-colon (;) is line terminator• Curly braces ({,}) used for block structure• Several keywords

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Comments

There are two kinds of comments: •  /* text */ A traditional comment: all the text from

the ASCII characters /* to the ASCII characters */ is ignored.

•  // text  An end-of-line comment: all the text from the ASCII characters // to the end of the line is ignored.

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Comments

Comments do not nest. • /* and */ have no special meaning in comments

that begin with //. • // has no special meaning in comments that begin

with /* or /**.

 As a result, the text: /* this comment /* // /** ends here: */is a single complete comment.

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Identifiers

An identifier is:an unlimited-length sequence of Java letters

and Java digits, the first of which must be a Java letter.

 An identifier cannot have the same spelling (Unicode character sequence) as:

• a keyword, • boolean literal, • the null literal

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Identifiers

• Letters and digits may be drawn from the entire Unicode character set (The character set that uses 16 bit per character).

•  Identifier can be written in most writing scripts in use in the world today, including:

Hebrew,Chinese, Japanese, KoreanPractically all languages

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Identifiers

• Uppercase and lowercase are different.

All the following are different identifiers:

MY my My mY my1

 

Examples of identifiers are:

String i3

isLetterOrDigit מונה

MAX_VALUE

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KeywordsThe following are reserved words called keywords

and cannot be used as identifiers:

abstractbooleanbreakbytebyvaluecasecastcatchcharclassconstcontinue

defaultdodoubleelseextendsfalsefinalfinallyfloatforfuturegeneric

gotoifimplementsimportinnerinstanceofintinterfacelongnativenewnull

operatorouterpackageprivateprotectedpublicrestreturnshortstaticsuperswitch

synchronizedthisthrowthrowstransienttruetryvarvoidvolatilewhile

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Keywords

While true and false might appear to be keywords, they are technically Boolean literals.

Similarly, while null might appear to be a keyword, it is technically the null literal.

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Any Questions?