java项目小手册

集合了一些常用的小片段

        
1. 字符串有整型的相互转换 
Java代码 
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 
Java代码 
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();    
    }    
}  

Java代码 
[size=medium][b]3. 得到当前方法的名字[/b][/size]  
String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();  

4. 转字符串到日期 
Java代码 
java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);  

或者是： 
Java代码 
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );    
Date date = format.parse( myString );  

5. 使用JDBC链接Oracle 
Java代码 
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代码 
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 
Java代码 
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 
Java代码 
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 了解一些细节， 
并下面这个JAR 文件：json-rpc-1.0.jar (75 kb) 
Java代码 
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 了解更多细节 
Java代码 
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 

阅读这篇 文章 了解更多细节。 
Java代码 
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 了解更多信息 
Java代码 
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 
Java代码 
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 获得更多信息。 
Java代码 
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 
Java代码 
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 
Java代码 
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 
代码 
<?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 
Java代码 
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 
Java代码 
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 
Java代码 
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 
Java代码 
/**  
* 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]);    
}  

    

原文：https://www.cnblogs.com/lovellll/p/10213912.html