20个非常有用的Java程序片段

1. 字符串有整型的相互转换

String a = String.valueOf(2); //integer to numeric string
int i = Integer.parseInt(a); //numeric string to an int

2. 向文件末尾添加内容

BufferedWriter out = null;
try {
    out = new BufferedWriter(new FileWriter(”filename”, true));
    out.write(”aString”);
} catch (IOException e) {
    // error processing code
} finally {
    if (out != null) {
        out.close();
    }
}

3. 得到当前方法的名字

String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();

4. 转字符串到日期

java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);

或者是：

SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
Date date = format.parse( myString );

5. 使用JDBC链接Oracle

public class OracleJdbcTest
{
String driverClass = "oracle.jdbc.driver.OracleDriver";

Connection con;

public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException
{
Properties props = new Properties();
props.load(fs);
String url = props.getProperty("db.url");
String userName = props.getProperty("db.user");
String password = props.getProperty("db.password");
Class.forName(driverClass);

con=DriverManager.getConnection(url, userName, password);
}

public void fetch() throws SQLException, IOException
{
PreparedStatement ps = con.prepareStatement("select SYSDATE from dual");
ResultSet rs = ps.executeQuery();

while (rs.next())
{
// do the thing you do
}
rs.close();
ps.close();
}

public static void main(String[] args)
{
OracleJdbcTest test = new OracleJdbcTest();
test.init();
test.fetch();
}
}

6. 把 Java util.Date 转成 sql.Date

java.util.Date utilDate = new java.util.Date();
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());

7. 使用NIO进行快速的文件拷贝

public static void fileCopy( File in, File out )
throws IOException
{
FileChannel inChannel = new FileInputStream( in ).getChannel();
FileChannel outChannel = new FileOutputStream( out ).getChannel();
try
{
// inChannel.transferTo(0, inChannel.size(), outChannel); // original -- apparently has trouble copying large files on Windows

// magic number for Windows, 64Mb - 32Kb)
int maxCount = (64 * 1024 * 1024) - (32 * 1024);
long size = inChannel.size();
long position = 0;
while ( position < size )
{
position += inChannel.transferTo( position, maxCount, outChannel );
}
}
finally
{
if ( inChannel != null )
{
inChannel.close();
}
if ( outChannel != null )
{
outChannel.close();
}
}
}

9. 创建 JSON 格式的数据

请先阅读这篇文章了解一些细节，
并下面这个JAR 文件：json-rpc-1.0.jar (75 kb)

import org.json.JSONObject;
...
...
JSONObject json = new JSONObject();
json.put("city", "Mumbai");
json.put("country", "India");
...
String output = json.toString();
...

10. 使用iText JAR生成PDF

阅读这篇文章了解更多细节

import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.util.Date;

import com.lowagie.text.Document;
import com.lowagie.text.Paragraph;
import com.lowagie.text.pdf.PdfWriter;

public class GeneratePDF {

public static void main(String[] args) {
try {
OutputStream file = new FileOutputStream(new File("C:\Test.pdf"));

Document document = new Document();
PdfWriter.getInstance(document, file);
document.open();
document.add(new Paragraph("Hello Kiran"));
document.add(new Paragraph(new Date().toString()));

document.close();
file.close();

} catch (Exception e) {

e.printStackTrace();
}
}
}

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;
    }
}

另外一种实现

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();
 }
}

16. 解析/读取XML 文件

XML文件

<?xml version="1.0"?>
<students>
 <student>
 <name>John</name>
 <grade>B</grade>
 <age>12</age>
 </student>
 <student>
 <name>Mary</name>
 <grade>A</grade>
 <age>11</age>
 </student>
 <student>
 <name>Simon</name>
 <grade>A</grade>
 <age>18</age>
 </student>
</students>

转载自：http://blog.csdn.net/tw19811220/article/details/12199469