20个非常有用的Java程序片段

转载至知乎
原文地址：https://zhuanlan.zhihu.com/p/28140439

1. 字符串语整型的相互转换

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

public void test2() throws Exception{    BufferedWriter out = null;    try {        FileWriter fileWriter = new FileWriter("/home/administrator/practice_java/idea/jdk_start/hello.txt", true);        out = new BufferedWriter(fileWriter);        out.write("AAAAAAAAAAAAA");    } catch (IOException e) {        e.printStackTrace();    } 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

Date date = new Date();java.sql.Date sqlDate = new java.sql.Date(date.getTime());System.out.println(date);System.out.println(sqlDate);

7. 使用NIO进行快速的文件拷贝

public static void fileCopy(File in, File out) throws FileNotFoundException{        FileChannel inChannel = new FileInputStream(in).getChannel();        FileChannel outChannel = new FileOutputStream(out).getChannel();        try {//            inChannel.transferFrom(0, inChannel.size(), outChannel);            int maxCount = (64 * 1024 * 1024) - (32 * 1024);            long position = 0;            long size = inChannel.size();            while (position < size) {                position += inChannel.transferTo(position, maxCount, outChannel);            }        } catch (Exception e) {            e.printStackTrace();        }finally {            try {                if (inChannel != null) {                    inChannel.close();                }                if (outChannel != null) {                    outChannel.close();                }            } catch (IOException e) {                e.printStackTrace();            }        }    }

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();          }      }  }

11. HTTP 代理设置

阅读这篇 文章 了解更多细节。

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

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

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。

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

public void test6() {    File dir = new File("/home/administrator/practice_java/idea/jdk_start");    String children[] = dir.list();    if (children == null) {        System.out.println("there is haven't child");    }else {        for(int i = 0 ; i < children.length; i++) {            String childName = children[i];            System.out.println(childName);        }    }}

15. 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();      }  }

16. 解析/读取XML 文件

XML文件

<?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

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

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

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

/** * 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]);  }

21.通过序列化实现深度clone

public static <T extends Serializable> T clone(T obj) throws Exception {        ByteArrayOutputStream bout = new ByteArrayOutputStream();        ObjectOutputStream oos = new ObjectOutputStream(bout);        oos.writeObject(obj);        ByteArrayInputStream bin = new ByteArrayInputStream(.toByteArray());        ObjectInputStream ois = new ObjectInputStream(bin);        return (T) ois.readObject();        // 说明：调用ByteArrayInputStream或ByteArrayOutputStream对象的close方法没有任何意义        // 这两个基于内存的流只要垃圾回收器清理对象就能够释放资源，这一点不同于对外部资源（如文件流）的释放    }