20 Very useful Java program Code Snippets _java

Here are 20 very useful pieces of Java program that I hope will be useful to you.

1. Conversion of strings with integral type

String a = string.valueof (2);  Integer to numeric string 
int. i = Integer.parseint (a);//numeric string to an int

2. Add content to end of file

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. Get the name of the current method

String methodname = Thread.CurrentThread (). Getstacktrace () [1].getmethodname ();

4. Turn string to date

Java.util.Date = Java.text.DateFormat.getDateInstance (). Parse (Date String);
Or:
simpledateformat format = new SimpleDateFormat ("DD. Mm.yyyy "); 
Date date = Format.parse (myString);

5. Using JDBC to link Oracle

public class Oraclejdbctest {String Driverclass = "Oracle.jdbc.driver.OracleDriver"; 
 
  Connection con; 
    public void init (FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException { 
    Properties Props = new properties (); 
    Props.load (FS); 
    String url = props.getproperty ("Db.url"); 
    String userName = Props.getproperty ("Db.user"); 
    String Password = props.getproperty ("Db.password"); 
 
    Class.forName (Driverclass); 
  Con=drivermanager.getconnection (URL, userName, password); public void Fetch () throws SQLException, IOException {preparedstatement PS = con.preparestatement ("Select 
    Sysdate from dual "); 
 
    ResultSet rs = Ps.executequery (); 
    while (Rs.next ()) {//Does the thing you do} rs.close (); 
  Ps.close (); 
    public static void Main (string[] args) {oraclejdbctest test = new Oraclejdbctest (); 
    Test.init (); 
  Test.fetch ();
 } 
}

6. Util the Java. Date is converted to SQL. Date

Java.util.Date utildate = new Java.util.Date (); 
Java.sql.Date sqldate = new Java.sql.Date (Utildate.gettime ());

7. Use of NiO for fast file copies

public static void FileCopy (file in, file out) 
      throws IOException 
  { 
    FileChannel inchannel = new Fileinputstr EAM (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) 
      I NT Maxcount = (1024 * 1024)-(* * 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. Create thumbnails of pictures

private void Createthumbnail (string filename, int thumbwidth, int thumbheight, int quality, string outfilename) thro WS Interruptedexception, FileNotFoundException, IOException {//load image from filename image image = Toolk 
    It.getdefaulttoolkit (). getImage (filename); 
    Mediatracker mediatracker = new Mediatracker (new Container ()); 
    Mediatracker.addimage (image, 0); 
    Mediatracker.waitforid (0); 
 
    Use the ' to ' test for errors in 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 imageTo thumbnail image object and//scale it to the new size on-the-fly bufferedimage thumbimage = new Bufferedimag 
    E (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 (Outfi 
    Lename)); 
    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. Create JSON-formatted data

And the following jar file: Json-rpc-1.0.jar (KB)

Import Org.json.JSONObject; 
... 
... 
Jsonobject json = new Jsonobject (); 
Json.put ("City", "Mumbai"); 
Json.put ("Country", "India"); 
... 
String output = json.tostring (); 
...

10. Using Itext jar to generate 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 Fileout 
 
      Putstream (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 (); } 
  } 
}

One. HTTP Proxy settings
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. Single Instance Singleton sample

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; 
  } 
}

Another kind of implementation

public enum Simplesingleton { 
  INSTANCE; 
  public void DoSomething () { 
  } 
} 
 
//call the "method" from Singleton: 
SimpleSingleton.INSTANCE.doSomething ();

13. Screen-catching procedure

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. listing files and directories

 file dir = new File ("DirectoryName"); 
 String[] Children = dir.list (); if (children = = null) {//either Dir does not exist or are not. A directory} else {for (int i=0; i < Childre N.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 of the files that are start with '. ' FilenameFilter filter = new FilenameFilter () {public Boolean accept (File dir, String name) {return!name.start 
   Swith ("."); 
 } 
 }; 
 
 Children = dir.list (filter); 
 
 The list of files can also be retrieved as File objects file[] files = dir.listfiles (); This filter is returns directories FileFilter FileFilter = new FileFilter () {public boolean accept (file file) 
   {return file.isdirectory (); 
 } 
 }; 
Files = dir.listfiles (filefilter); 

15. Create Zip and jar files

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"); 
    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. Parse/Read XML file

XML file

<?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

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.ge 
 
        Tnodename ()); 
 
        NodeList studentlist = docele.getelementsbytagname ("student"); Print total student elements into document System.out. println ("Total Students:" + StudentliSt.getlength ()); if (studentlist!= null && studentlist.getlength () > 0) {for (int i = 0; i < Studentlist.getleng Th (); 
 
            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). Getnodeva 
 
              Lue ()); 
              NodeList = E.getelementsbytagname ("Grade"); System.out.println ("Grade:" + nodelist.item (0). Getchildnodes (). Item (0). Getnodev 
 
              Alue ()); 
              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

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 the Japan is" + countrycapitals.get ("Japan")); 
  System.out.println ("Capital of France are" + countrycapitals.get ("France")); 
 } 

   

18. Send Mail

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 sessions = Session.getdefaultinstance (props, null); 
 
  Session.setdebug (Debug); 
 
  Create a message msg = new MimeMessage (session); 
  Set the "from" 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. HTTP request to send generation data

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. Change the size of the array

/** * 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 (o 
  Ldarray); 
  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)); }

Hopefully this article will help you learn about Java programming.