20非常有用的Java程序片段(11-15)

11. HTTP 代理设置

阅读这篇 文章 了解更多细节。

System.getProperties().put( "http.proxyHost" , "someProxyURL" );
System.getProperties().put( "http.proxyPort" , "someProxyPort" );
System.getProperties().put( "http.proxyUser" , "someUserName" );
System.getProperties().put( "http.proxyPassword" , "somePassword" );

12. 单实例Singleton 示例

请先阅读这篇文章 了解更多信息

public class SimpleSingleton {
private static SimpleSingleton singleInstance =   new SimpleSingleton();

//Marking default constructor private
//to avoid direct instantiation.
private SimpleSingleton() {
}

//Get instance for class SimpleSingleton
public static SimpleSingleton getInstance() {

return singleInstance;
}
}

另一种实现

public enum SimpleSingleton {
INSTANCE;
public void doSomething() {
}
}

//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();

13. 抓屏程序

阅读这篇文章 获得更多信息。

import java.awt.Dimension;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.Toolkit;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import java.io.File;

...

public void captureScreen(String fileName) throws Exception {

Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
Rectangle screenRectangle = new Rectangle(screenSize);
Robot robot = new Robot();
BufferedImage image = robot.createScreenCapture(screenRectangle);
ImageIO.write(image, "png" , new File(fileName));

}
...

14. 列出文件和目录

File dir = new File( "directoryName" );
String[] children = dir.list();
if (children == null ) {
// Either dir does not exist or is not a directory
} else {
for ( int i= 0 ; i < children.length; i++) {
// Get filename of file or directory
String filename = children[i];
}
}

// It is also possible to filter the list of returned files.
// This example does not return any files that start with `.'.
FilenameFilter filter = new FilenameFilter() {
public boolean accept(File dir, String name) {
return !name.startsWith( "." );
}
};
children = dir.list(filter);

// The list of files can also be retrieved as File objects
File[] files = dir.listFiles();

// This filter only returns directories
FileFilter fileFilter = new FileFilter() {
public boolean accept(File file) {
return file.isDirectory();
}
};
files = dir.listFiles(fileFilter);

15. 创建ZIP和JAR文件

import java.util.zip.*;
import java.io.*;

public class ZipIt {
public static void main(String args[]) throws IOException {
if (args.length < 2 ) {
System.err.println( "usage: java ZipIt Zip.zip file1 file2 file3" );
System.exit(- 1 );
}
File zipFile = new File(args[ 0 ]);
if (zipFile.exists()) {
System.err.println( "Zip file already exists, please try another" );
System.exit(- 2 );
}
FileOutputStream fos = new FileOutputStream(zipFile);
ZipOutputStream zos = new ZipOutputStream(fos);
int bytesRead;
byte [] buffer = new byte [ 1024 ];
CRC32 crc = new CRC32();
for ( int i= 1 , n=args.length; i < n; i++) {
String name = args[i];
File file = new File(name);
if (!file.exists()) {
System.err.println( "Skipping: " + name);
continue ;
}
BufferedInputStream bis = new BufferedInputStream(
new FileInputStream(file));
crc.reset();
while ((bytesRead = bis.read(buffer)) != - 1 ) {
crc.update(buffer, 0 , bytesRead);
}
bis.close();
// Reset to beginning of input stream
bis = new BufferedInputStream(
new FileInputStream(file));
ZipEntry entry = new ZipEntry(name);
entry.setMethod(ZipEntry.STORED);
entry.setCompressedSize(file.length());
entry.setSize(file.length());
entry.setCrc(crc.getValue());
zos.putNextEntry(entry);
while ((bytesRead = bis.read(buffer)) != - 1 ) {
zos.write(buffer, 0 , bytesRead);
}
bis.close();
}
zos.close();
}
}