Java-20个非常有用的程序片段

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

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(); 
} 
}

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

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 );

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(); 
} 
}

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

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(); 
} 
} 
}

private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename) 
throws InterruptedException, FileNotFoundException, IOException 
{ 
// load image from filename 
Image image = Toolkit.getDefaultToolkit().getImage(filename); 
MediaTracker mediaTracker = new MediaTracker(new Container()); 
mediaTracker.addImage(image, 0); 
mediaTracker.waitForID(0); 
// use this to test for errors at this point: System.out.println(mediaTracker.isErrorAny()); 

// determine thumbnail size from WIDTH and HEIGHT 
double thumbRatio = (double)thumbWidth / (double)thumbHeight; 
int imageWidth = image.getWidth(null); 
int imageHeight = image.getHeight(null); 
double imageRatio = (double)imageWidth / (double)imageHeight; 
if (thumbRatio < imageRatio) { 
thumbHeight = (int)(thumbWidth / imageRatio); 
} else { 
thumbWidth = (int)(thumbHeight * imageRatio); 
} 

// draw original image to thumbnail image object and 
// scale it to the new size on-the-fly 
BufferedImage thumbImage = new BufferedImage(thumbWidth, thumbHeight, BufferedImage.TYPE_INT_RGB); 
Graphics2D graphics2D = thumbImage.createGraphics(); 
graphics2D.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR); 
graphics2D.drawImage(image, 0, 0, thumbWidth, thumbHeight, null); 

// save thumbnail image to outFilename 
BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(outFilename)); 
JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out); 
JPEGEncodeParam param = encoder.getDefaultJPEGEncodeParam(thumbImage); 
quality = Math.max(0, Math.min(quality, 100)); 
param.setQuality((float)quality / 100.0f, false); 
encoder.setJPEGEncodeParam(param); 
encoder.encode(thumbImage); 
out.close(); 
}

请先阅读这个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(); 
...

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(); 
} 
} 
}

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

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();

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)); 

} 
...

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);

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(); 
} 
}

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>
Java代码package net.viralpatel.java.xmlparser; 

import java.io.File; 
import javax.xml.parsers.DocumentBuilder; 
import javax.xml.parsers.DocumentBuilderFactory; 

import org.w3c.dom.Document; 
import org.w3c.dom.Element; 
import org.w3c.dom.Node; 
import org.w3c.dom.NodeList; 

public class XMLParser { 

public void getAllUserNames(String fileName) { 
try { 
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance(); 
DocumentBuilder db = dbf.newDocumentBuilder(); 
File file = new File(fileName); 
if (file.exists()) { 
Document doc = db.parse(file); 
Element docEle = doc.getDocumentElement(); 

// Print root element of the document 
System.out.println("Root element of the document: " 
+ docEle.getNodeName()); 

NodeList studentList = docEle.getElementsByTagName("student"); 

// Print total student elements in document 
System.out 
.println("Total students: " + studentList.getLength()); 

if (studentList != null && studentList.getLength() > 0) { 
for (int i = 0; i < studentList.getLength(); i++) { 

Node node = studentList.item(i); 

if (node.getNodeType() == Node.ELEMENT_NODE) { 

System.out 
.println("====================="); 

Element e = (Element) node; 
NodeList nodeList = e.getElementsByTagName("name"); 
System.out.println("Name: " 
+ nodeList.item(0).getChildNodes().item(0) 
.getNodeValue()); 

nodeList = e.getElementsByTagName("grade"); 
System.out.println("Grade: " 
+ nodeList.item(0).getChildNodes().item(0) 
.getNodeValue()); 

nodeList = e.getElementsByTagName("age"); 
System.out.println("Age: " 
+ nodeList.item(0).getChildNodes().item(0) 
.getNodeValue()); 
} 
} 
} else { 
System.exit(1); 
} 
} 
} catch (Exception e) { 
System.out.println(e); 
} 
} 
public static void main(String[] args) { 

XMLParser parser = new XMLParser(); 
parser.getAllUserNames("c:\\test.xml"); 
} 
}

import java.util.Map; 
import org.apache.commons.lang.ArrayUtils; 

public class Main { 

public static void main(String[] args) { 
String[][] countries = { { "United States", "New York" }, { "United Kingdom", "London" }, 
{ "Netherland", "Amsterdam" }, { "Japan", "Tokyo" }, { "France", "Paris" } }; 

Map countryCapitals = ArrayUtils.toMap(countries); 

System.out.println("Capital of Japan is " + countryCapitals.get("Japan")); 
System.out.println("Capital of France is " + countryCapitals.get("France")); 
} 
}

import javax.mail.*; 
import javax.mail.internet.*; 
import java.util.*; 

public void postMail( String recipients[ ], String subject, String message , String from) throws MessagingException 
{ 
boolean debug = false; 

//Set the host smtp address 
Properties props = new Properties(); 
props.put("mail.smtp.host", "smtp.example.com"); 

// create some properties and get the default Session 
Session session = Session.getDefaultInstance(props, null); 
session.setDebug(debug); 

// create a message 
Message msg = new MimeMessage(session); 

// set the from and to address 
InternetAddress addressFrom = new InternetAddress(from); 
msg.setFrom(addressFrom); 

InternetAddress[] addressTo = new InternetAddress[recipients.length]; 
for (int i = 0; i < recipients.length; i++) 
{ 
addressTo[i] = new InternetAddress(recipients[i]); 
} 
msg.setRecipients(Message.RecipientType.TO, addressTo); 

// Optional : You can also set your custom headers in the Email if you Want 
msg.addHeader("MyHeaderName", "myHeaderValue"); 

// Setting the Subject and Content Type 
msg.setSubject(subject); 
msg.setContent(message, "text/plain"); 
Transport.send(msg); 
}

import java.io.BufferedReader; 
import java.io.InputStreamReader; 
import java.net.URL; 

public class Main { 
public static void main(String[] args) { 
try { 
URL my_url = new URL("http://coolshell.cn/"); 
BufferedReader br = new BufferedReader(new InputStreamReader(my_url.openStream())); 
String strTemp = ""; 
while(null != (strTemp = br.readLine())){ 
System.out.println(strTemp); 
} 
} catch (Exception ex) { 
ex.printStackTrace(); 
} 
} 
}

/** 
* Reallocates an array with a new size, and copies the contents 
* of the old array to the new array. 
* @param oldArray the old array, to be reallocated. 
* @param newSize the new array size. 
* @return A new array with the same contents. 
*/ 
private static Object resizeArray (Object oldArray, int newSize) { 
int oldSize = java.lang.reflect.Array.getLength(oldArray); 
Class elementType = oldArray.getClass().getComponentType(); 
Object newArray = java.lang.reflect.Array.newInstance( 
elementType,newSize); 
int preserveLength = Math.min(oldSize,newSize); 
if (preserveLength > 0) 
System.arraycopy (oldArray,0,newArray,0,preserveLength); 
return newArray; 
} 

// Test routine for resizeArray(). 
public static void main (String[] args) { 
int[] a = {1,2,3}; 
a = (int[])resizeArray(a,5); 
a[3] = 4; 
a[4] = 5; 
for (int i=0; i<a.length; i++) 
System.out.println (a[i]); 
}