Java编程语言基础：动手指南第一部分

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"Essentials of the Java Programming Language: A Hands-On Guide, Part 1" 这篇教程是针对初学者的，特别是那些已经对其他编程语言有一定了解，并熟悉基础的文本显示、图形处理或简单计算的读者。它由Monica Pawlan撰写，主要介绍了如何使用Java 2 Platform来创建和运行三种常见的Java平台程序：应用程序、小程序（Applets）和Servlets。首先，文章会阐述应用程序、小程序和Servlet之间的相似性和差异。应用程序通常是在本地系统上独立运行的，而小程序则设计用于在网络浏览器中运行，Servlet则专门用于服务器端，处理HTTP请求并生成动态内容。接下来，教程将引导读者构建一个基本的用户界面，这个界面能处理简单的用户输入。在Java中，这通常涉及到Swing或JavaFX库，用于创建窗口、按钮、文本框等组件。读者将学习如何布局管理器来组织这些组件，并响应用户的交互事件。在文件和数据库操作方面，教程会讲解如何读取和写入数据。Java提供了丰富的I/O流API，包括FileInputStream和FileOutputStream用于文件操作，以及JDBC（Java Database Connectivity）用于连接和操作数据库。读者将学习如何使用这些工具来实现数据的持久化存储。网络通信是Java编程的一个重要方面，特别是在开发Servlet时。教程将涵盖如何使用Socket和ServerSocket类进行网络数据的发送和接收。此外，HTTP协议的使用，如通过HttpURLConnection或者HttpClient库发送HTTP请求和接收响应，也将被讲解。最后，虽然这个教程不会全面覆盖所有Java编程的细节，但它会提供一个坚实的基础，帮助读者理解Java编程的核心概念和技术。随着学习的深入，读者将能够扩展这些基础知识，进一步探索Java的高级特性，如多线程、集合框架、异常处理和面向对象设计原则。 “Essentials of the Java Programming Language: A Hands-On Guide, Part 1”是入门Java编程的理想资源，它不仅提供了理论知识，还强调了实践操作，使读者能够快速掌握Java编程技能。

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Java

Programming Language Basics,

Part 1

Lesson 3: Building Applets

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Like applications, applets are created from classes. However,

applets do not have a

main method as an entry point, but instead,

have several methods to control specific aspects of applet

execution.

This lesson converts an application from Lesson 2 to an applet and

describes the structure and elements of an applet.

Application to Applet●

Run the Applet●

Applet Structure and Elements●

Packages●

More Information●

Application to Applet

The following code is the applet equivalent to the LessonTwoB

application from Lesson 2. The figure below shows how the running

applet looks. The structure and elements of the applet code are

discussed after the section on how to run the applet just below.

import java.applet.Applet;

import java.awt.Graphics;

import java.awt.Color;

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Applet Structure and Elements

The Java API Applet class provides what you need to design the

appearance and manage the behavior of an applet. This class

provides a graphical user interface (GUI) component called a

Panel

and a number of methods. To create an applet, you extend (or

subclass) the

Applet class and implement the appearance and

behavior you want.

The applet's appearance is created by drawing onto the Panel or by

attaching other GUI components such as push buttons, scrollbars,

or text areas to the

Panel. The applet's behavior is defined by

implementing the methods.

Extending a Class

Most classes of any complexity extend other classes.

To extend another class means to write a new class

that can use the fields and methods defined in the

class being extended. The class being extended is the

parent class, and the class doing the extending is the

child class. Another way to say this is the child class

inherits the fields and methods of its parent or chain

of parents. Child classes either call or override

inherited methods. This is called single inheritance.

The SimpleApplet class extends Applet class, which

extends the

Panel class, which extends the Container

class. The Container class extends Object, which is the

parent of all Java API classes.

The Applet class provides the init, start, stop, destroy, and

paint methods you saw in the example applet. The

SimpleApplet class overrides these methods to do what the

SimpleApplet class needs them to do. The Applet class provides no

functionality for these methods.

However, the Applet class does provide functionality for the

setBackground method,which is called in the init method. The call to

setBackground is an example of calling a method inherited from a

parent class in contrast to overriding a method inherited from a

parent class.

You might wonder why the Java language provides methods

without implementations. It is to provide conventions for everyone

to use for consistency across Java APIs. If everyone wrote their

own method to start an applet, for example, but gave it a different

name such as

begin or go, the applet code would not be

interoperable with other programs and browsers, or portable across

multiple platforms. For example, Netscape and Internet Explorer

know how to look for the

init and start methods.

Behavior

An applet is controlled by the software that runs it. Usually, the

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underlying software is a browser, but it can also be appletviewer as

you saw in the example. The underlying software controls the

applet by calling the methods the applet inherits from the

Applet

class.

The init Method: The init method is called when the applet is first

created and loaded by the underlying software. This method

performs one-time operations the applet needs for its operation

such as creating the user interface or setting the font. In the

example, the

init method initializes the text string and sets the

background color.

The start Method: The start method is called when the applet is

visited such as when the end user goes to a web page with an

applet on it. The example prints a string to the console to tell you

the applet is starting. In a more complex applet, the

start method

would do things required at the start of the applet such as begin

animation or play sounds.

After the start method executes, the event thread calls the paint

method to draw to the applet's Panel. A thread is a single sequential

flow of control within the applet, and every applet can run in

multiple threads.

Applet drawing methods are always called from a

dedicated drawing and event-handling thread.

The stop and destroy Methods: The stop method stops the applet

when the applet is no longer on the screen such as when the end

user goes to another web page. The example prints a string to the

console to tell you the applet is stopping. In a more complex

applet, this method should do things like stop animation or sounds.

The destroy method is called when the browser exits. Your applet

should implement this method to do final cleanup such as stop live

threads.

Appearance

The Panel provided in the Applet class inherits a paint method from its

parent

Container class. To draw something onto the Applet's Panel,

you implement the

paint method to do the drawing.

The Graphics object passed to the paint method defines a graphics

context for drawing on the

Panel. The Graphics object has methods

for graphical operations such as setting drawing colors, and

drawing graphics, images, and text.

The paint method for the SimpleApplet draws the I'm a simple applet

string in red inside a blue rectangle.

public void paint(Graphics g){

System.out.println("Paint");

//Set drawing color to blue

g.setColor(Color.blue);

//Specify the x, y, width and height for a rectangle

g.drawRect(0, 0,

getSize().width -1,

getSize().height -1);

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