20个非常有用的Java程序片段

小咪咪 2022-05-26 09:08 160阅读 0赞

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**

请先阅读[这篇文章][Link 1] 了解一些细节，  
并下面这个JAR 文件：[json-rpc-1.0.jar (75 kb)][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**

阅读这篇[文章][Link 2] 了解更多细节

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;  
    
    /** * Java学习交流QQ群：589809992 我们一起学Java！ */
    
    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**

阅读这篇 [文章][Link 3] 了解更多细节。

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**

请先阅读这篇[文章][Link 4] 了解更多信息

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**

阅读这篇[文章][Link 5] 获得更多信息。

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.*;  
    /** * Java学习交流QQ群：589809992 我们一起学Java！ */
    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]);  
    }

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[Link 1]: http://viralpatel.net/blogs/2009/02/creating-parsing-json-data-with-java-servlet-struts-jsp-json.html
[json-rpc-1.0.jar _75 kb]: http://viralpatel.net/blogs/download/json/json-rpc-1.0.jar
[Link 2]: http://viralpatel.net/blogs/2009/04/generate-pdf-file-in-java-using-itext-jar.html
[Link 3]: http://viralpatel.net/blogs/2009/04/http-proxy-setting-java-setting-proxy-java.html
[Link 4]: http://viralpatel.net/blogs/2009/01/java-singleton-design-pattern-tutorial-example-singleton-j2ee-design-pattern.html
[Link 5]: http://viralpatel.net/blogs/2009/01/how-to-take-screen-shots-in-java-taking-screenshots-java.html