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JAVA学习必备——分享20个有用的程序片段

原创|行业资讯|编辑:蒋永|2017-07-25 10:21:52.000|阅读 188 次

概述:今天给大家分享在java学习与开发者可能会用到的20个非常有用的java片段,如果对你有帮助请点个赞哦~也欢迎各路大牛在评论区分享自己的一些经验~ ·

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下面是20个非常有用的Java程序片段,希望能对大家有所帮助。

  1. 字符串有整型的相互转换

    1.String a = String.valueOf(2);   //integer to numeric string  
    2.int i = Integer.parseInt(a); //numeric string to an int
  2. 向文件末尾添加内容

    1.BufferedWriter out = null;  
    2.try {  
    3    out = new BufferedWriter(new FileWriter(”filename”, true));  
    4    out.write(”aString”);  
    5} catch (IOException e) {  
    6   // error processing code  
    7} finally {  
    8   if (out != null) {  
    9        out.close();  
    10   }  
    11}
  3. 得到当前方法的名字

    1.String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
  4. 转字符串到日期

    1.java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);
    或者是:
    1.SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );  
    2.Date date = format.parse( myString );
  5. 使用JDBC链接Oracle

    1.public class OracleJdbcTest  
    2.{  
    3    String driverClass = "oracle.jdbc.driver.OracleDriver";  
    4
    5   Connection con;  
    6
    7   public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException  
    8 {  
    9     Properties props = new Properties();  
    10     props.load(fs);  
    11     String url = props.getProperty("db.url");  
    12     String userName = props.getProperty("db.user");  
    13     String password = props.getProperty("db.password");  
    14     Class.forName(driverClass);  
    15
    16      con=DriverManager.getConnection(url, userName, password);  
    17  }  
    18
    19   public void fetch() throws SQLException, IOException  
    20   {  
    21       PreparedStatement ps = con.prepareStatement("select SYSDATE from dual");  
    22       ResultSet rs = ps.executeQuery();  
    23
    24       while (rs.next())  
    25       {  
    26           // do the thing you do  
    27       }  
    28       rs.close();  
    29       ps.close();  
    30    }  
    31 
    32    public static void main(String[] args)  
    33   {  
    34       OracleJdbcTest test = new OracleJdbcTest();  
    35       test.init();  
    36       test.fetch();  
    37   }  
    38}
  6. 把 Java util.Date 转成 sql.Date

    1.java.util.Date utilDate = new java.util.Date();  
    2.java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());
  7. 使用NIO进行快速的文件拷贝

    1.public static void fileCopy( File in, File out )  
    2.           throws IOException  
    3.   {  
    4.       FileChannel inChannel = new FileInputStream( in ).getChannel();  
    5.      FileChannel outChannel = new FileOutputStream( out ).getChannel();  
    6.      try
    7.      {  
    8.//          inChannel.transferTo(0, inChannel.size(), outChannel);      // original -- apparently has trouble copying large files on Windows  
    9.
    10.           // magic number for Windows, 64Mb - 32Kb)  
    11.          int maxCount = (64 * 1024 * 1024) - (32 * 1024);  
    12.          long size = inChannel.size();  
    13.          long position = 0;  
    14.         while ( position < size )  
    15.         {  
    16.            position += inChannel.transferTo( position, maxCount, outChannel );  
    17.         }  
    18.     }  
    19.     finally
    20.     {  
    21.          if ( inChannel != null )  
    22.          {  
    23.             inChannel.close();  
    24.          }  
    25.          if ( outChannel != null )  
    26.          {  
    27.              outChannel.close();  
    28.          }  
    29.       }  
    30.   }
  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

    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("//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]);  
}

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