[JAVA技术] Java中常用的代码汇总

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. 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 );

4. 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();

  }

}

5. 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());

6. 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();

  }

7.创建 JSON 格式的数据

导入下面这个JAR 文件：json-rpc-1.0.jar (75 kb)/http://t.cn/Rz0bhUA

import org.json.JSONObject;

...

...

JSONObject json = new JSONObject();

json.put("city", "Mumbai");

json.put("country", "India");

...

String output = json.toString();

...

8. 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();

    }

  }

}

9. 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));

   

}

...

10. 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());

      ent
4000
ry.setCrc(crc.getValue());

      zos.putNextEntry(entry);

      while ((bytesRead = bis.read(buffer)) != -1) {

        zos.write(buffer, 0, bytesRead);

      }

      bis.close();

    }

    zos.close();

  }

}

11. 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"));

}

}

12. 发送带数据的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();

    }

  }

}

13.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]);

}