20个非常有用的Java代码片段
20个非常有用的Java代码片段
小贱是个程序员 发表于1年前
20个非常有用的Java代码片段
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1. 字符串有整型的相互转换

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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

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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

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

4. 转字符串到日期

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

或者是:

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SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" ); 

Date date = format.parse( myString );

5. 使用JDBC链接Oracle

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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

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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

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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

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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

请先阅读这篇文章 了解一些细节,

并下面这个JAR 文件:json-rpc-1.0.jar (75 kb)

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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

阅读这篇文章 了解更多细节

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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

阅读这篇 文章 了解更多细节。

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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

请先阅读这篇文章 了解更多信息

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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

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public enum SimpleSingleton { 

    INSTANCE; 

    public void doSomething() { 

    

 

//Call the method from Singleton: 

SimpleSingleton.INSTANCE.doSomething();

13. 抓屏程序

阅读这篇文章 获得更多信息。

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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

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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

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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

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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 

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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

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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

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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

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/**

* 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]); 

}

标签: java Java EE
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