20個非常有用的Java程序片段

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下面是20個非常有用的Java程序片段，希望能對你有用。

1. 字符串有整型的相互轉換

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

2. 向文件末尾添加內容

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. 得到當前方法的名字

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

4. 轉字符串到日期

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鏈接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 轉成 sql.Date

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

7. 使用NIO進行快速的文件拷貝

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(); 
 } 
 } 
 }

8. 創建圖片的縮略圖

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(); 
 }

9. 創建 JSON 格式的數據

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

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 代理設置

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. 單實例Singleton 示例

請先閱讀這篇文章 瞭解更多信息

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; 
 } 
}

另一種實現

public enum SimpleSingleton { 
 INSTANCE; 
 public void doSomething() { 
 } 
} 
//Call the method from Singleton: 
SimpleSingleton.INSTANCE.doSomething();

13. 抓屏程序

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. 列出文件和目錄

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. 創建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/讀取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>

Java代碼

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"); 
 } 
}

17. 把 Array 轉換成 Map

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")); 
 } 
}

18. 發送郵件

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); 
}

19. 發送代數據的HTTP 請求

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(); 
 } 
 } 
}

20. 改變數組的大小

/** 
* 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]); 
}

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