Java Java 程序设计程序设计Java Programming

Fall, 2013

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Contents for Today

Java Program Structure How to Compile a Java Program How to Run a Java Program

Environment Variables Java Commands

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Example 2.1:

/*****First Java Program: HelloWorld.java *****///* Author: Mary

public class HelloWorld{ public static void main( String[ ] args ) {

System.out.println( "Hello, world!" ); }}

“main” method

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Create a Java Source File

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Create(创建 ) a Java Source File

Step 1: Start the Notepad editor and type in the source

code; Save the source code in a directory the name of

HelloWorld.java

Java源程序后缀名

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Compile the Source Code

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Compile(编译 ) the Source Code

Step 2: Compile the Source File into a .class File

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Run the Program

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Run(运行 ) the Program

Step 3: Run the program

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Java Environment Variables(环境变量）

JAVA_HOME: The path in which JDK is installed provide general information to applications which

need to use Java commands and JVM; JAVA_HOME = C:\j2sdk1.4.2_08

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Java Environment Variables

Path: used by Operating System to search the

command to execute; PATH =…;C:\j2sdk1.4.2_08\bin

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Java Environment Variables

CLASSPATH:

CLASSPATH = .; C:\j2sdk1.4.2_02\lib

‘.’ — current path; to tell JVM the path to find the class library.

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Java Commands

javac - the Java compiler java - the Java bytecode interpreter (JVM) jdb - the Java debugger javadoc – a documentation generator that

lets you generate documentation from your Java source code and the Javadoc comments you place in your source code

jar - Java archive utility

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Java Program Structure

What is a Class( 类 )? A class is a prototype that defines the variables and the

methods common to all objects of a certain kind. What is an Object(对象 )?

Software objects are often used to model real-world objects in everyday life.

An object is a software bundle of related variables and methods.

An object is an instance(实例 ) of some class.

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Example 2.1:

/***** HelloWorld.java *****///* Author: Mary

public class HelloWorld{ public static void main( String[ ] args ) {

System.out.println( "Hello, world!" ); }}

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Java Program Structure

Comments（注释） Anything between /* and */

or following // on a single line Programmers use comments in their code to provide

information to the reader on what the code is doing. When a program is compiled, the compiler skips all

comments. It is common (and good) practice to use a comment at

the top of the code containing general information about the file (known as a header).

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Java Program Structure Class

All Java code must be part of a “class”. Every class has a name:

public class HelloWorld{……}

Braces { and } are used to mark the start and end of the class.

Other class information may also appear with the name: A “public” class is one that can be referred to by

other classes.

class namemodifie

r

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Components of Classes

Inside a class, there can be fields( 域 ): store values of some information methods(方法 ): a collection of statements

that performs a sequence of operations on the variables.

In our first program, we have zero attributes( 域 /属性 ) and one method.

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Example 2.2/***** Point.java *****///* Author: Mary

public class Point { public int x;

public int y;

public Point(int a; int b) { //constructor(构造函数 ) x = a; y = b; } public void print() { //method System.out.print("The Point's coordinate is"); System.out.println("("+x+", "+y+")"); }} // end of the class ‘Point’

域

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Method（方法） Each method has 2 parts:

a header The header contains the name and other information

about the method.

a body The body describes what the method should do.

Braces { and } are used to mark the start and end of the method body.

A method body consists of a set of statements(语句 ).

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Example 2.1:

/***** HelloWorld.java *****///* Author: Mary

public class HelloWorld{ public static void main( String[ ] args ) {

System.out.println( "Hello, world!" );

}}

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Method(方法 )

In our first program Example 2.1, we have one method. The name of the method is main main is a special name. It means:

“when you run the program, start here.”

The header of main must be defined in the format as follows:

Our main method has only one statement(语句 ).

public static void main(String[ ] args) {…}

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Statements(语句 )

A statement ( 语句 ) represents an action or a sequence of actions.

System.out.println("Welcome to Java!");

a statement to display the greeting "Welcome to Java!" ;

Every statement in Java ends with a semicolon (;).

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Keywords（关键字）

Reserved Words/keywords(保留字 / 关键字 ) have a specific meaning to the compiler; cannot be used for other purposes in the program.

eg.: class, public, static, and void

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Java关键字具有特殊含义的字符序列，共 48个。

Abstract defaultif private thisBoolean do implements protected throwbreak double import public

throwsbyte else instanceof return transientcase extends int short trycatch final interface static voidchar finally long strictfp volatileclass float native super whileconst for new switchcontinue goto package synchronized

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Example 2.2/***** Point.java *****///* Author: Mary

public class Point { public int x;

public int y;

public Point(int a; int b) { x = a; y = b; } public void print(){ System.out.print("The Point's coordinate is"); System.out.println("("+x+", "+y+")"); }

} // end of the class ‘Point’

Attributes

Constructor

Class Header

Method

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Assignment(作业 )

1. Download Eclipse & J2SDK(Java Software Development Tool), and install them on your computers.

2. To create, compile, and run the HelloWorld.java program on your own computer, and get familiar with the commands and process.