JAVA学习必备——分享20个有用的程序片段

今天给大家分享在java学习与开发者可能会用到的20个非常有用的java片段，如果对你有帮助请点个赞哦~也欢迎各路大牛在评论区分享自己的一些经验~
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下面是20个非常有用的Java程序片段，希望能对大家有所帮助。

1. 字符串有整型的相互转换

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

2. 向文件末尾添加内容

   1.BufferedWriter out = null; 2.try { 3    out = new BufferedWriter(new FileWriter(”filename”, true)); 4    out.write(”aString”); 5} catch (IOException e) { 6   // error processing code 7} finally { 8   if (out != null) { 9        out.close(); 10   } 11}

3. 得到当前方法的名字

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

4. 转字符串到日期

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

   或者是：

   1.SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" ); 2.Date date = format.parse( myString );

5. 使用JDBC链接Oracle

   1.public class OracleJdbcTest 2.{ 3    String driverClass = "oracle.jdbc.driver.OracleDriver"; 45   Connection con; 67   public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException 8 { 9     Properties props = new Properties(); 10     props.load(fs); 11     String url = props.getProperty("db.url"); 12     String userName = props.getProperty("db.user"); 13     String password = props.getProperty("db.password"); 14     Class.forName(driverClass); 1516      con=DriverManager.getConnection(url, userName, password); 17  } 1819   public void fetch() throws SQLException, IOException 20   { 21       PreparedStatement ps = con.prepareStatement("select SYSDATE from dual"); 22       ResultSet rs = ps.executeQuery(); 2324       while (rs.next()) 25       { 26           // do the thing you do 27       } 28       rs.close(); 29       ps.close(); 30    } 3132    public static void main(String[] args) 33   { 34       OracleJdbcTest test = new OracleJdbcTest(); 35       test.init(); 36       test.fetch(); 37   } 38}

6. 把 Java util.Date 转成 sql.Date

   1.java.util.Date utilDate = new java.util.Date(); 2.java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());

7. 使用NIO进行快速的文件拷贝

   1.public static void fileCopy( File in, File out ) 2.           throws IOException 3.   { 4.       FileChannel inChannel = new FileInputStream( in ).getChannel(); 5.      FileChannel outChannel = new FileOutputStream( out ).getChannel(); 6.      try7.      { 8.//          inChannel.transferTo(0, inChannel.size(), outChannel);      // original -- apparently has trouble copying large files on Windows 9.10.           // magic number for Windows, 64Mb - 32Kb) 11.          int maxCount = (64 * 1024 * 1024) - (32 * 1024); 12.          long size = inChannel.size(); 13.          long position = 0; 14.         while ( position < size ) 15.         { 16.            position += inChannel.transferTo( position, maxCount, outChannel ); 17.         } 18.     } 19.     finally20.     { 21.          if ( inChannel != null ) 22.          { 23.             inChannel.close(); 24.          } 25.          if ( outChannel != null ) 26.          { 27.              outChannel.close(); 28.          } 29.       } 30.   }

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

<ml version="1.0"gt;<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 contentsof 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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