写在之前
以下是《Java8编程入门官方教程》中的一些知识，如有错误，烦请指正。涉及的程序如需下载请移步http://down1.tupwk.com.cn/qhwkdownpage/978-7-302-38738-1.zip
applet的基础知识
applets是设计用于在Internet间传输，在浏览器上运行的小程序。有两种基本类型：基于抽象窗口工具包的applet和基于Swing的applet。两者都支持创建用户图形界面。
import java.awt.*;
import java.applet.*;
/*
<applet code="SimpleApplet" width=200 height=60>
</applet>
*/
public class SimpleApplet extends Applet {
public void paint(Graphics g) {
g.drawString("Java makes applets easy.", 20, 20);
}
}
- 运行
javac SimpleApplet.java进行编译 - 运行
appletviewer SimpleApplet.java命令，大家注意后面的参数是.java文件而不是类名。
applet架构
applet是基于GUI的程序。applet由事件驱动。由用户发起与applet的交互。
applet框架
// An AWT-based Applet skeleton.
import java.awt.*;
import java.applet.*;
/*
<applet code="AppletSkel" width=300 height=100>
</applet>
*/
public class AppletSkel extends Applet {
// Called first.
public void init() {
// initialization
}
/* Called second, after init(). Also called whenever
the applet is restarted. */
public void start() {
// start or resume execution
}
// Called when the applet is stopped.
public void stop() {
// suspends execution
}
/* Called when applet is terminated. This is the last
method executed. */
public void destroy() {
// perform shutdown activities
}
// Called when an AWT-based applet's window must be restored.
public void paint(Graphics g) {
// redisplay contents of window
}
}
虽然框架没有执行实际的操作，但是仍然可以编译执行。
applet的初始化和终止
开始运行：init--start--paint
终止：stop--destroy\
请求重绘
通过repaint()和update方法实现。
/*
Try This 14-1
A simple banner applet.
This applet creates a thread that scrolls
the message contained in msg right to left
across the applet's window.
*/
import java.awt.*;
import java.applet.*;
/*
<applet code="Banner" width=300 height=50>
</applet>
*/
public class Banner extends Applet implements Runnable {
String msg = " Java Rules the Web ";
Thread t;
boolean stopFlag;
// Initialize t to null.
public void init() {
t = null;
}
// Start thread
public void start() {
t = new Thread(this);
stopFlag = false;
t.start();
}
// Entry point for the thread that runs the banner.
public void run() {
// Display banner
for( ; ; ) {
try {
repaint();
Thread.sleep(250);
if(stopFlag)
break;
} catch(InterruptedException exc) {}
}
}
// Pause the banner.
public void stop() {
stopFlag = true;
t = null;
}
// Display the banner.
public void paint(Graphics g) {
char ch;
ch = msg.charAt(0);
msg = msg.substring(1, msg.length());
msg += ch;
g.drawString(msg, 50, 30);
}
}
使用状态窗口
void showStatus(String msg)
// Using the Status Window.
import java.awt.*;
import java.applet.*;
/*
<applet code="StatusWindow" width=300 height=50>
</applet>
*/
public class StatusWindow extends Applet{
// Display msg in applet window.
public void paint(Graphics g) {
g.drawString("This is in the applet window.", 10, 20);
showStatus("This is shown in the status window.");
}
}
向applet传递形参
String getParameter(String paramName)
// Pass a parameter to an applet.
import java.awt.*;
import java.applet.*;
//html形参
/*
<applet code="Param" width=300 height=80>
<param name=author value="Herb Schildt">
<param name=purpose value="Demonstrate Parameters">
<param name=version value=2>
</applet>
*/
public class Param extends Applet {
String author;
String purpose;
int ver;
public void start() {
String temp;
author = getParameter("author");
if(author == null) author = "not found";
purpose = getParameter("purpose");
if(purpose == null) purpose = "not found"; //检查形参是否存在
temp = getParameter("version");
try {
if(temp != null)
ver = Integer.parseInt(temp);
else
ver = 0;
} catch(NumberFormatException exc) { //确保数值转换成功
ver = -1; // error code
}
}
public void paint(Graphics g) {
g.drawString("Purpose: " + purpose, 10, 20);
g.drawString("By: " + author, 10, 40);
g.drawString("Version: " + ver, 10, 60);
}
}
Applet类
所有的基于AWT的applet都是Applet类的子类。
委派事件模型
概念：一个源产生一个事件，然后将它送到一个或多个监听器那里。
事件
委派模型中，描述源中状态变化的对象。
事件源：产生事件的对象。一个源必须注册监听器，以求可以接受关于特定时间类型的通知。
public void addTypeListener(TypeListener el)
源必须提供一个方法，以允许侦听器取消对某种特定类型事件的注册。
public void removeTypeListener(TypeListener el)
事件侦听器：这是在事件发生时被告知的那个对象。需要两个条件：必须向一个或多个源注册；必须实现一些方法，接受并处理这些通知。
使用模型：
- 在侦听器中国实现相应接口，使其接收所需类型的事件
- 实现代码将侦听器作为事件通知的接收者来注册或者取消注册
// Demonstrate the mouse event handlers.
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
/*
<applet code="MouseEvents" width=300 height=100>
</applet>
*/
public class MouseEvents extends Applet
implements MouseListener, MouseMotionListener {
String msg = "";
int mouseX = 0, mouseY = 0; // coordinates of mouse
public void init() {
addMouseListener(this); //注册该类作为鼠标事件的监听者
addMouseMotionListener(this);
}
// Handle mouse clicked.
public void mouseClicked(MouseEvent me) {
mouseX = 0;
mouseY = 10;
msg = "Mouse clicked.";
repaint();
}
// Handle mouse entered.
public void mouseEntered(MouseEvent me) {
mouseX = 0;
mouseY = 10;
msg = "Mouse entered.";
repaint();
}
// Handle mouse exited.
public void mouseExited(MouseEvent me) {
mouseX = 0;
mouseY = 10;
msg = "Mouse exited.";
repaint();
}
// Handle button pressed.
public void mousePressed(MouseEvent me) {
// save coordinates
mouseX = me.getX();
mouseY = me.getY();
msg = "Down";
repaint();
}
// Handle button released.
public void mouseReleased(MouseEvent me) {
// save coordinates
mouseX = me.getX();
mouseY = me.getY();
msg = "Up";
repaint();
}
// Handle mouse dragged.
public void mouseDragged(MouseEvent me) {
// save coordinates
mouseX = me.getX();
mouseY = me.getY();
msg = "*";
showStatus("Dragging mouse at " + mouseX + ", " + mouseY);
repaint();
}
// Handle mouse moved.
public void mouseMoved(MouseEvent me) {
// show status
showStatus("Moving mouse at " + me.getX() + ", " + me.getY());
}
// Display msg in applet window at current X,Y location.
public void paint(Graphics g) {
g.drawString(msg, mouseX, mouseY);
}
}
其他Java关键字
transient：在存储对象时，它的值不需要存储。
volatile：告知编译器，程序的其他部分可以任意改动该变量。
instanceof：如果是指定的类型，或者可以强制转换成指定类型，运算符返回ture
assert：程序开发期间创建断言。
assert condition;
assert n>0;//如果断言失败抛出AssertionError
assert condition: expr;//如果断言失败，该值被替换成它的字符串格式，并在断言失败时显示、
native：申明一个本机方法，同时必须编写好，并按照步骤来链接Java代码。
