[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i Text 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());
      entry.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.Buffered Reader;
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]);
}