20个非常实用的Java程序代码片段

下面是20个非常有用的java程序片段，希望能对你有用。

1. 字符串有整型的相互转换

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

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

string methodname = thread.currentthread().getstacktrace()[1].getmethodname();

4. 转字符串到日期

java.util.date = java.text.dateformat.getdateinstance().parse(date string);
或者是：
simpledateformat format = new simpledateformat( "dd.mm.yyyy" ); 
date date = format.parse( mystring );

5. 使用jdbc链接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()) 
    { 
      // 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

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

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

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

并下面这个jar 文件：json-rpc-1.0.jar (75 kb)

import org.json.jsonobject; 
... 
... 
jsonobject json = new jsonobject(); 
json.put("city", "mumbai"); 
json.put("country", "india"); 
... 
string output = json.tostring(); 
...

10. 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

<?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

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 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]); 
}

希望本文所述对大家学习java程序设计有所帮助。