20个开发人员非常有用的Java功能代码

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开发 后端
本文将为大家介绍20人员非常有用的Java功能代码。这20段代码,可以成为大家在今后的开发过程中,Java编程手册的重要部分。


1. 把Strings转换成int和把int转换成String

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

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

2. 向Java文件中添加文本

Updated: Thanks Simone for pointing to exception. I have 

changed the code.  
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();  
    }  


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Java现在正调用的方法名

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

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

4. 在Java中将String型转换成Date型

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

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

5. 通过Java JDBC链接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 ();  
    }  


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.将Java中的util.Date转换成sql.Date

这一片段显示如何将一个java util Date转换成sql Date用于数据库

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

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

7. 使用NIO快速复制Java文件

 
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 )  
            {  
             & nbsp; position += inChannel.transferTo( position, maxCount, outChannel ); 

 
            }   
        }  
        finally 
        {  
            if ( inChannel != null )  
            {  
             & nbsp; inChannel.close();  
            }   
            if ( outChannel != null )  
            {  
             & nbsp;  outChannel.close();  
            }   
        }  
    } 


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. 在Java中创建缩略图

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 ();  
    } 


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.drawImag e(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. 在Java中创建JSON数据

Read this article for more details.
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 ();  
... 


import org.json.JSONObject;
...
...
JSONObject json = new JSONObject();
json.put("city", "Mumbai");
json.put("country", "India");
...
String output = json.toString();
...

10. 在Java中使用iText JAR打开PDF


Read this article for more details.


 
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();  
        }   
    }  


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();
        }
    }
}

#p#

11. 在Java上的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"); 


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. Java Singleton 例子

Read this article for more 

details.
Update: Thanks Markus for the comment. I have updated the code and changed it to

more robust implementation.


 
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;  
    }  


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;
 }
}
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(); 


public enum SimpleSingleton {
 INSTANCE;
 public void doSomething() {
 }
}

//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();

13. 在Java上做屏幕截图

Read this article for more details.


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));  
 
}   
... 


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. 在Java中的文件,目录列表

>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); 


  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. 在Java中创建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()) {  
             & nbsp;  System.err.println("Skipping: " + name);  
             & nbsp;  continue;  
            }   
            BufferedInputStream bis = new BufferedInputStream(  
             & nbsp;  new FileInputStream(file));  
            crc.reset ();  
            while ((bytesRead = bis.read(buffer)) != -1) {  
             & nbsp;  crc.update(buffer, 0, bytesRead);  
            }   
            bis.close();  
            // Reset to beginning of input stream  
            bis = new BufferedInputStream(  
             & nbsp;  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) {  
             & nbsp;  zos.write(buffer, 0, bytesRead);  
            }   
            bis.close();  
        }   
        zos.close ();  
    }  


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);
            & nbsp;   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);
          & nbsp; 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. Parsing / Reading XML file in Java
Sample XML file.


 
 
 
     
        John 
        B 
        12 
   
 
     
        Mary 
        A 
        11 
   
 
     
        Simon 
        A 
        18 
   
 
 




 
  John
   B
  12
 

 
  Mary
  A
  < AGE>11
 

 
  Simon
  A
  18
 


Java code to parse above XML.


 
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()) {  
             & nbsp;  Document doc = db.parse(file);  
             & nbsp;  Element docEle = doc.getDocumentElement();  
 
             & nbsp;  // Print root element of the document  
             & nbsp;  System.out.println("Root element of the document: " 
             & nbsp;          + docEle.getNodeName ());  
 
             & nbsp;  NodeList studentList = docEle.getElementsByTagName ("student");  
 
             & nbsp;  // Print total student elements in document  
             & nbsp;  System.out  
             & nbsp;          .println("Total students: " + studentList.getLength());  
 
             & nbsp;  if (studentList != null && studentList.getLength() > 0) {  
             & nbsp;      for (int i = 0; i < studentList.getLength(); i++) {  
 
             & nbsp;          Node node = studentList.item(i);  
 
             & nbsp;          if (node.getNodeType() == Node.ELEMENT_NODE) {  
 
             & nbsp;              System.out  
             & nbsp;                       .println ("=====================");  
 
             & nbsp;              Element e = (Element) node;  
             & nbsp;              NodeList nodeList = e.getElementsByTagName("name");  
             & nbsp;              System.out.println("Name: " 
             & nbsp;                       + nodeList.item(0).getChildNodes ().item(0)  
             & nbsp;                             ;   .getNodeValue());  
 
             & nbsp;              nodeList = e.getElementsByTagName("grade");  
             & nbsp;              System.out.println("Grade: " 
             & nbsp;                       + nodeList.item(0).getChildNodes ().item(0)  
             & nbsp;                             ;   .getNodeValue());  
 
             & nbsp;              nodeList = e.getElementsByTagName("age");  
             & nbsp;              System.out.println("Age: " 
             & nbsp;                       + nodeList.item(0).getChildNodes ().item(0)  
             & nbsp;                             ;   .getNodeValue());  
             & nbsp;          }  
             & nbsp;      }  
             & nbsp;  } else {  
             & nbsp;      System.exit(1);  
             & nbsp;  }  
            }   
        } catch (Exception e) {  
            System.out.println(e);  
        }  
    }  
    public static void main(String[] args) {  
 
        XMLParser parser = new XMLParser ();  
        parser.getAllUserNames("c:\\test.xml");  
    }   


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
     &nb sp;.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. Convert Array to Map in Java

 
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"));  
  }  


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. Send Email using Java

 
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);  


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. Send HTTP request & fetching data using Java

 
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/");&nbs p; 
            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();  
        }   
    }  


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. Resize an Array in Java

 
/** 
* 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      System.out.println (a[i]);  

 

16. 在Java中解析/读取XML文件

view plaincopy to clipboardprint?
 
<?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> 


<?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 code to parse above XML.

view plaincopy to clipboardprint?
 
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");  
    }  


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. 在Java中将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"));  
  }  


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. 在Java中发送电子邮件

view plaincopy to clipboardprint?
 
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);  


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. 使用Java发送HTTP请求和提取数据

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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://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();  
        }  
    }  


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. 在Java中调整数组

view plaincopy to clipboardprint?
 
/** 
* 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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