阿里P7整理20個非常有用的Java程式片段，你知道嗎？

Java · 發表 2018-09-14 19:38:51

摘要： 1、字串有整型的相互轉換 String a = String.valueOf(2); //integer to numeric string int i = Integer.parseInt(a); //numeric string to an int 2、...

1、字串有整型的相互轉換

String a = String.valueOf(2);  //integer to numeric string  
int i = Integer.parseInt(a); //numeric string to an int

2、向檔案末尾新增內容

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、得到當前方法的名字

String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();

4、轉字串到日期

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連結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 轉成 sql.Date

java.util.Date utilDate = new java.util.Date();  
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());

7、使用NIO進行快速的檔案拷貝

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、建立圖片的縮圖

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、建立 JSON 格式的資料

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 代理設定

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、單例項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;  
  }  
}

另一種實現

public enum SimpleSingleton {  
  INSTANCE;  
  public void doSomething() {  
  }  
}  
//Call the method from Singleton:  
SimpleSingleton.INSTANCE.doSomething();

13、抓屏程式

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、列出檔案和目錄

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、建立ZIP和JAR檔案

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/讀取XML 檔案

XML檔案

    John 
    B 
    12 
 
    Mary 
    A 
    11 
 
    Simon 
    A 
    18

Java程式碼

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 轉換成 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. 傳送郵件

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. 傳送代資料的HTTP 請求

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. 改變陣列的大小

/**
* 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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