常用的Java代码段有：1、字符串有整型的相互转换；2、向文件末尾添加内容；3、得到当前方法的名字；4、转字符串到日期；5、使用JDBC链接Oracle。

本教程操作环境：windows7系统、java10版，DELL G3电脑。

常用的Java代码段有：

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 了解一些细节，

并下面这个JAR 文件：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 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;   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();          }      }  }