20个有用的java代码分别是什么

20个有用的java代码分别是什么，很多新手对此不是很清楚，为了帮助大家解决这个难题，下面小编将为大家详细讲解，有这方面需求的人可以来学习下，希望你能有所收获。

20个有用的java代码

1. Strings转int和int转String

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

2.在文件内追加文字

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.获取当前的方法名

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

4.String转换为Date

java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);//orSimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );Date date = format.parse( myString );

5.JDBC连接Oracle数据库

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util.Date转换为sql.Date

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

7.Java Fast File Copy using NIO

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.创建一个图片的缩略图

private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename)throws InterruptedException, FileNotFoundException, IOException    {// load image from filenameImage 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 HEIGHTdouble 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-flyBufferedImage 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 outFilenameBufferedOutputStream 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.生成JSON数据

// Download JAR file 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.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.单例

public class SimpleSingleton {private static SimpleSingleton singleInstance =  new SimpleSingleton(); //Marking default constructor private//to avoid direct instantiation.private SimpleSingleton() {    } //Get instance for class SimpleSingletonpublic static SimpleSingleton getInstance() { return singleInstance;    }}// One more implementation of Singleton class. Thanks to Ralph and Lukasz Zielinski for pointing this out.public enum SimpleSingleton {    INSTANCE;public void doSomething() {    }} //Call the method from Singleton:SimpleSingleton.INSTANCE.doSomething();

13.截屏

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.文件监控

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.生成.jar,.zip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 streambis = 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.操作XML

JohnB12MaryA11SimonA18

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 documentSystem.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.Array转化为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 SessionSession session = Session.getDefaultInstance(props, null);    session.setDebug(debug); // create a messageMessage msg = new MimeMessage(session); // set the from and to addressInternetAddress 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 Wantmsg.addHeader("MyHeaderName", "myHeaderValue"); // Setting the Subject and Content Type    msg.setSubject(subject);    msg.setContent(message, "text/plain");    Transport.send(msg);}

19.远程抓取数据

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://www.viralpatel.net/blogs/");            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.重新定义数组大小

/*** 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
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