Java 程序设计

1. Java 程序设计 Java Programming Fall, 2013

2. Contents for Today • Java Program Structure • How to Compile a Java Program • How to Run a Java Program • Environment Variables • Java Commands

3. Example 2.1: /*****First Java Program: HelloWorld.java *****/ //* Author: Mary public classHelloWorld { public static void main( String[ ] args ) { System.out.println( "Hello, world!" ); } } “main”method

4. Create a Java Source File

5. Create(创建) a Java Source File • Step 1: • Start the Notepadeditor 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

6. Compile the Source Code

7. Compile(编译) the Source Code • Step 2: Compile the Source File into a .class File

8. Run the Program

9. Run(运行) the Program • Step 3: Run the program

10. 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

11. Java Environment Variables • Path: • used by Operating System to search the command to execute; • PATH =…;C:\j2sdk1.4.2_08\bin

12. Java Environment Variables • CLASSPATH: CLASSPATH = .; C:\j2sdk1.4.2_02\lib • ‘.’ — current path; • to tell JVM the path to find the class library.

13. 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

14. 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.

15. Example 2.1: /***** HelloWorld.java *****/ //* Author: Mary public classHelloWorld { public static void main( String[ ] args ) { System.out.println( "Hello, world!" ); } }

16. 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 skipsall 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).

17. 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 startand endof 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 name modifier

18. 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.

19. Example 2.2 /***** Point.java *****/ //* Author: Mary public classPoint { 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

20. 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).

21. Example 2.1: /***** HelloWorld.java *****/ //* Author: Mary public classHelloWorld { public static void main( String[ ] args ) { System.out.println( "Hello, world!" ); } }

22. 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 ofmainmust be defined in the format as follows: • Our main method has only onestatement(语句). public static void main(String[ ] args) {…}

23. 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 (;).

24. 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

25. Java关键字 • 具有特殊含义的字符序列，共48个。 Abstract default if private this Boolean do implements protected throw break double import public throws byte else instanceof return transient case extends int short try catch final interface static void char finally long strictfp volatile class float native super while const for new switch continue goto package synchronized

26. Example 2.2 Class Header /***** Point.java *****/ //* Author: Mary public classPoint { 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 Method

27. Assignment(作业) • Download Eclipse & J2SDK(Java Software Development Tool), and install them on your computers. • To create, compile, and run the HelloWorld.java program on your own computer, and get familiar with the commands and process.