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常用的Java代码汇总

2024-07-06 08:58:10   231人阅读

1. 字符串有整型的相互转换

 

 
 
 
 
 
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<strong>Stringa=String.valueOf(2);   //integer to numeric string  
inti=Integer.parseInt(a);//numeric string to an int </strong>

2. 向文件末尾添加内容

 

 

 
 
 
 
 
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<strong>BufferedWriter out=null;  
try{  
    out=newBufferedWriter(newFileWriter(”filename”,true));  
    out.write(”aString”);  
}catch(IOExceptione){  
    // error processing code  
}finally{  
    if(out!=null){  
        out.close();  
    }  
}</strong>

3. 得到当前方法的名字

 

 

 
 
 
 
 
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<strong>StringmethodName=Thread.currentThread().getStackTrace()[1].getMethodName(); </strong>

4. 转字符串到日期

 

 

 
 
 
 
 
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<strong>java.util.Date=java.text.DateFormat.getDateInstance().parse(date String); </strong>

或者是：

 

 

 
 
 
 
 
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<strong>SimpleDateFormat format=newSimpleDateFormat("dd.MM.yyyy");  
Date date=format.parse(myString); </strong>

5. 使用JDBC链接Oracle

 

 

 
 
 
 
 
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<strong>publicclassOracleJdbcTest  
{  
    StringdriverClass="oracle.jdbc.driver.OracleDriver";  
  
    Connection con;  
  
    publicvoidinit(FileInputStream fs)throwsClassNotFoundException,SQLException,FileNotFoundException,IOException  
    {  
        Properties props=newProperties();  
        props.load(fs);  
        Stringurl=props.getProperty("db.url");  
        StringuserName=props.getProperty("db.user");  
        Stringpassword=props.getProperty("db.password");  
        Class.forName(driverClass);  
  
        con=DriverManager.getConnection(url,userName,password);  
    }  
  
    publicvoidfetch()throwsSQLException,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();  
    }  
  
    publicstaticvoidmain(String[]args)  
    {  
        OracleJdbcTest test=newOracleJdbcTest();  
        test.init();  
        test.fetch();  
    }  
} </strong>

6. 把 Java util.Date 转成 sql.Date

 

 

 
 
 
 
 
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<strong>java.util.Date utilDate=newjava.util.Date();  
java.sql.Date sqlDate=newjava.sql.Date(utilDate.getTime());</strong>

7. 使用NIO进行快速的文件拷贝

 

 

 
 
 
 
 
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<strong>publicstaticvoidfileCopy(File in,File out)  
            throwsIOException  
    {  
        FileChannel inChannel=newFileInputStream(in).getChannel();  
        FileChannel outChannel=newFileOutputStream(out).getChannel();  
        try 
        {  
//          inChannel.transferTo(0, inChannel.size(), outChannel);      // original -- apparently has trouble copying large files on Windows  
  
            // magic number for Windows, 64Mb - 32Kb)  
            intmaxCount=(64*1024*1024)-(32*1024);  
            longsize=inChannel.size();  
            longposition=0;  
            while(position<size)  
            {  
               position+=inChannel.transferTo(position,maxCount,outChannel);  
            }  
        }  
        finally 
        {  
            if(inChannel!=null)  
            {  
               inChannel.close();  
            }  
            if(outChannel!=null)  
            {  
                outChannel.close();  
            }  
        }  
    } </strong>

8. 创建图片的缩略图

 

 

 
 
 
 
 
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<strong>privatevoidcreateThumbnail(Stringfilename,intthumbWidth,intthumbHeight,intquality,StringoutFilename)  
        throwsInterruptedException,FileNotFoundException,IOException  
    {  
        // load image from filename  
        Image image=Toolkit.getDefaultToolkit().getImage(filename);  
        MediaTracker mediaTracker=newMediaTracker(newContainer());  
        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  
        doublethumbRatio=(double)thumbWidth/(double)thumbHeight;  
        intimageWidth=image.getWidth(null);  
        intimageHeight=image.getHeight(null);  
        doubleimageRatio=(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=newBufferedImage(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=newBufferedOutputStream(newFileOutputStream(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();  
    } </strong>

9.创建 JSON 格式的数据

并下面这个JAR 文件：json-rpc-1.0.jar (75 kb)

 

 
 
 
 
 
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<strong>importorg.json.JSONObject;  
...  
...  
JSONObject json=newJSONObject();  
json.put("city","Mumbai");  
json.put("country","India");  
...  
Stringoutput=json.toString();  
...</strong>

10. 使用iText JAR生成PDF

 

 
 
 
 
 
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<strong>importjava.io.File;  
importjava.io.FileOutputStream;  
importjava.io.OutputStream;  
importjava.util.Date;  
  
importcom.lowagie.text.Document;  
importcom.lowagie.text.Paragraph;  
importcom.lowagie.text.pdf.PdfWriter;  
  
publicclassGeneratePDF{  
  
    publicstaticvoidmain(String[]args){  
        try{  
            OutputStream file=newFileOutputStream(newFile("C:\\Test.pdf"));  
  
            Document document=newDocument();  
            PdfWriter.getInstance(document,file);  
            document.open();  
            document.add(newParagraph("Hello Kiran"));  
            document.add(newParagraph(newDate().toString()));  
  
            document.close();  
            file.close();  
  
        }catch(Exceptione){  
  
            e.printStackTrace();  
        }  
    }  
} </strong>

 

11. HTTP 代理设置

 

 
 
 
 
 
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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

 
 
 
 
 
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<strong>  
publicclassSimpleSingleton{  
    privatestaticSimpleSingleton singleInstance= newSimpleSingleton();  
  
    //Marking default constructor private  
    //to avoid direct instantiation.  
    privateSimpleSingleton(){  
    }  
  
    //Get instance for class SimpleSingleton  
    publicstaticSimpleSingleton getInstance(){  
  
        returnsingleInstance;  
    }  
} </strong>

另一种实现

 

 
 
 
 
 
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<strong>publicenumSimpleSingleton{  
    INSTANCE;  
    publicvoiddoSomething(){  
    }  
}  
  
//Call the method from Singleton:  
SimpleSingleton.INSTANCE.doSomething(); </strong>

13. 抓屏程序

 

 

 
 
 
 
 
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<strong>importjava.awt.Dimension;  
importjava.awt.Rectangle;  
importjava.awt.Robot;  
importjava.awt.Toolkit;  
importjava.awt.image.BufferedImage;  
importjavax.imageio.ImageIO;  
importjava.io.File;  
  
...  
  
publicvoidcaptureScreen(StringfileName)throwsException{  
  
   Dimension screenSize=Toolkit.getDefaultToolkit().getScreenSize();  
   Rectangle screenRectangle=newRectangle(screenSize);  
   Robot robot=newRobot();  
   BufferedImage image=robot.createScreenCapture(screenRectangle);  
   ImageIO.write(image,"png",newFile(fileName));  
  
}  
... </strong>

14. 列出文件和目录

 

 
 
 
 
 
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<strong>File dir=newFile("directoryName");  
  String[]children=dir.list();  
  if(children==null){  
      // Either dir does not exist or is not a directory  
  }else{  
      for(inti=0;i<children.length;i++){  
          // Get filename of file or directory  
          Stringfilename=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=newFilenameFilter(){  
      publicbooleanaccept(File dir,Stringname){  
          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=newFileFilter(){  
      publicbooleanaccept(File file){  
          returnfile.isDirectory();  
      }  
  };  
  files=dir.listFiles(fileFilter); </strong>

15. 创建ZIP和JAR文件

 

 

 
 
 
 
 
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<strong>  
importjava.util.zip.*;  
importjava.io.*;  
  
publicclassZipIt{  
    publicstaticvoidmain(Stringargs[])throwsIOException{  
        if(args.length<2){  
            System.err.println("usage: java ZipIt Zip.zip file1 file2 file3");  
            System.exit(-1);  
        }  
        File zipFile=newFile(args[0]);  
        if(zipFile.exists()){  
            System.err.println("Zip file already exists, please try another");  
            System.exit(-2);  
        }  
        FileOutputStream fos=newFileOutputStream(zipFile);  
        ZipOutputStream zos=newZipOutputStream(fos);  
        intbytesRead;  
        byte[]buffer=newbyte[1024];  
        CRC32 crc=newCRC32();  
        for(inti=1,n=args.length;i<n;i++){  
            Stringname=args[i];  
            File file=newFile(name);  
            if(!file.exists()){  
                System.err.println("Skipping: "+name);  
                continue;  
            }  
            BufferedInputStream bis=newBufferedInputStream(  
                newFileInputStream(file));  
            crc.reset();  
            while((bytesRead=bis.read(buffer))!=-1){  
                crc.update(buffer,0,bytesRead);  
            }  
            bis.close();  
            // Reset to beginning of input stream  
            bis=newBufferedInputStream(  
                newFileInputStream(file));  
            ZipEntry entry=newZipEntry(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();  
    }  
} </strong>

 

 

 

16. 解析/读取XML 文件

XML文件

 

 
 
 
 
 
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<?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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packagenet.viralpatel.java.xmlparser;  
  
importjava.io.File;  
importjavax.xml.parsers.DocumentBuilder;  
importjavax.xml.parsers.DocumentBuilderFactory;  
  
importorg.w3c.dom.Document;  
importorg.w3c.dom.Element;  
importorg.w3c.dom.Node;  
importorg.w3c.dom.NodeList;  
  
publicclassXMLParser{  
  
    publicvoidgetAllUserNames(StringfileName){  
        try{  
            DocumentBuilderFactory dbf=DocumentBuilderFactory.newInstance();  
            DocumentBuilder db=dbf.newDocumentBuilder();  
            File file=newFile(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(inti=0;i<studentList.getLength();i++){  
  
                        Node node=studentList.item(i);  
  
                        if(node.getNodeType()==Node.ELEMENT_NODE){  
  
                            System.out  
                                    .println("=====================");  
  
                            Elemente=(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(Exceptione){  
            System.out.println(e);  
        }  
    }  
    publicstaticvoidmain(String[]args){  
  
        XMLParser parser=newXMLParser();  
        parser.getAllUserNames("c:\\test.xml");  
    }  
}

17. 把 Array 转换成 Map 

 

 
 
 
 
 
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importjava.util.Map;  
importorg.apache.commons.lang.ArrayUtils;  
  
publicclassMain{  
  
  publicstaticvoidmain(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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importjavax.mail.*;  
importjavax.mail.internet.*;  
importjava.util.*;  
  
publicvoidpostMail(Stringrecipients[],Stringsubject,Stringmessage,Stringfrom)throwsMessagingException  
{  
    booleandebug=false;  
  
     //Set the host smtp address  
     Properties props=newProperties();  
     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=newMimeMessage(session);  
  
    // set the from and to address  
    InternetAddress addressFrom=newInternetAddress(from);  
    msg.setFrom(addressFrom);  
  
    InternetAddress[]addressTo=newInternetAddress[recipients.length];  
    for(inti=0;i<recipients.length;i++)  
    {  
        addressTo[i]=newInternetAddress(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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importjava.io.BufferedReader;  
importjava.io.InputStreamReader;  
importjava.net.URL;  
  
publicclassMain{  
    publicstaticvoidmain(String[]args) {  
        try{  
            URL my_url=newURL("http://coolshell.cn/");  
            BufferedReader br=newBufferedReader(newInputStreamReader(my_url.openStream()));  
            StringstrTemp="";  
            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. 
*/ 
privatestaticObjectresizeArray(ObjectoldArray,intnewSize){  
   intoldSize=java.lang.reflect.Array.getLength(oldArray);  
   ClasselementType=oldArray.getClass().getComponentType();  
   ObjectnewArray=java.lang.reflect.Array.newInstance(  
         elementType,newSize);  
   intpreserveLength=Math.min(oldSize,newSize);  
   if(preserveLength>0)  
      System.arraycopy(oldArray,0,newArray,0,preserveLength);  
   returnnewArray;  
}  
  
// Test routine for resizeArray().  
publicstaticvoidmain(String[]args){  
   int[]a={1,2,3};  
   a=(int[])resizeArray(a,5);  
   a[3]=4;  
   a[4]=5;  
   for(inti=0;i<a.length;i++)  
      System.out.println(a[i]);  
}

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