Lecture 2Software Concepts

Instructors: Fu-Chiung Cheng

( 鄭福炯 )Associate Professor

Computer Science & EngineeringTatung Institute of Technology

Outline

• Structure of Java standalone applications• Basic program elements• Executing a program• Helpful support for writing software• Java applets

Java Program Structure

• A program is made up of one or more classes• A class contains one or more methods• A method contains program statements• A Java application always executes the main method

class Lincoln { public static void main (String[] args) { System.out.println ("Whatever you are, be a good one."); } // method main} // class Lincoln

Java Program StructureWhite Space

• white space: Spaces, blank lines, and tabs. • White space is used to separate words and symbols (tokens) in a program• Extra white space is ignored.• A valid Java program can be formatted many different ways:class Lincoln2 { public static void main (String[] args){ System.out.println ("Whatever you are, be a good one."); } }

• Readability: consistent indentation

Java Program Structure:Comments

• Comments: same syntax as C/C++.

• One-line comment: //

// comment runs to the end of the line

• Multiple-line comment:

/* comment runs to terminating * symbol, even across line breaks */

Java Program Structure:Indentifiers

• Identifiers: class name, method name, variables, key words.• Most identifiers have no predefined meaning except as specified by the programmer• An identifier can be made up of letters, digits, the underscore character (_), and the dollar sign• They cannot begin with a digit• Java is case sensitive, therefore Total and total are different identifiers

Java Program Structure:Indentifiers

Identifiers in Lincoln.java:• class name: Lincoln• method name: main, System.out.println• variables: args• key words: String

Good Programming Practice

Naming Conventions:• Classes: first word capitalized• Methods and variables: first word lower case• Intermediate words capitalized• Words run together, no underscores

class IntegerList (X Integer_List)int listItem (X list_Item)

• Constant: all caps with underscores to separate words MAX_INTEGER_ARRAY

Java Program Structure:Reserved words

• reserved words have specific meanings in Java and cannot be used in other ways

abstractbooleanbreakbytebyvaluecasecastcatchcharclassconstcontinue

defaultdodoubleelseextendsfalsefinalfinallyfloatforfuturegeneric

gotoifimplementsimportinnerinstanceofintinterfacelongnativenewnull

operatorouterpackageprivateprotectedpublicrestreturnshortstaticsuperswitch

synchronizedthisthrowthrowstransienttruetryvarvoidvolatilewhile

Java Program Structure:Literals

• Integer literals:25 69 -4288

• Floating point literals:3.14159 42.075 -0.5

• String literals:"The result is: "

"To thine own self be true."

Java Program StructureJava API

• The Java Application Programmer Interface (API) is a collection of classes that can be used as needed• Java API: print and println; • Java APIs are not part of the Java language itself.

java.appletjava.awtjava.beansjava.iojava.langjava.math

java.netjava.rmijava.securityjava.sqljava.textjava.util

Java Program Structure

Operator overloading (+ )• String concatenation: String + String • String concatenation: String + numeric data• addition: numeric data + numeric data

class Sum { public static void main (String[] args) { System.out.println ("6 + 9 = " + (6+9)); System.out.print (" Java " + " Programming"); System.out.println ("for Antarctica is " + 672); } // method main} // class Sum

Program Languages

• Traditional programming languages:A. high-level languages ==> machine languagesB. Each CPU has its own specific machine language

• The Java compiler translates Java source code into a special representation called bytecode• Java bytecode is not the machine language for any traditional CPU• Java interpreter translates bytecode into machine language and executes it.

Java Translation and Execution

Standalone applications:

Java sourcecode

Machinecode

Javabytecode

Javainterpreter

Bytecodecompiler

Javacompiler

Java Translation and Execution

Applet

Java sourcecode

Javabytecode

Javacompiler

Javainterpreter

Web browser

local computer

remotecomputer

Java Translation and Execution

• compiling Java programs into bytecodes:> javac Lincoln.java

• The bytecode of Lincoln.java is Lincoln.class• Java interpreter (Java Virtual Machines):

> java Lincoln

Errors:

• A program can have three types of errors:

A. compile-time errors: syntax errors.

B. run-time errors:divide by zero

C. logical errors: incorrect results.

Object-Oriented Programming:

• Everything is an object. • Programs are made from objects• Each object has its own memory made up of other objects.• Objects communication: send messages (requests).• An object contains data and methods• An object is defined by a class (type)• Multiple objects can be created from the same class

Object-Oriented Programming:

• A class (type) represents a concept and • An object (instance) represents the realization of that concept.

Car

My first car

John's car

Dad's car

Class

Objects

Object-Oriented Programming:

• An object contains data and methods.• Composition: (reuse objects): Ex, A car has a engine.

Car

drive();start();

etc...

enginewheel[4]door[2]

etc...

Class

Methods

Data

Object-Oriented Programming:

• Objects communication: send messages.

Object1Object2

Object3

messagemessage

message

Object-Oriented Programming:

• Inheritance: reuse the interface.

Shape

draw();erase();

Line

draw();erase();

Circle

draw();erase();

Square

draw();erase();

Object-Oriented Programming:

• Polymorphism (dynamic binding).• shape.draw() will call the right draw function of circle line or square.

Shape Line

Circledraw();erase();

Square

Dynamic Binding

void doStuff(Shape s) {s.erase();

// ... s.draw();}

// ...Circle c = new Circle();Triangle t = new Triangle();Line l = new Line();doStuff(c);doStuff(t);doStuff(l);

Importing Packages

• Using a class from the Java API: fully qualified name: java.lang.System.out.println();• import statement:: import java.applet.*; import java.util.Random; …. Random coin = new Random();• The java.lang package: automatically imported into every Java program

Conclusions

• Simple Structure of Java: classes• Object-oriented concepts• Import statement

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