20个常用的java代码片段

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();  

            }  

        }  

    }

8. 创建图片的缩略图

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. 创建 JSON 格式的数据

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 代理设置

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. 单实例Singleton 示例

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

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>

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");  

    }  

}

17. 把 Array 转换成 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. 发送邮件

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. 发送代数据的HTTP 请求

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. 改变数组的大小

/** 

* 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]);  

}