Java常用基础知识（20个）

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("D:\\asxxsst20170817.log", true));// \需要用\转义   out.write("aString"); //在这编写增加的内容} catch (IOException e) {   // error processing code } finally {   if (out != null) {     try {out.close();} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();}   } }

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

Date date1 = java.text.DateFormat.getDateInstance().parse("2017-08-24");
或者是：

 String myString0 = "24.08.2017";String myString = "2017-08-24 09:47:50";SimpleDateFormat format0 = new SimpleDateFormat( "dd.MM.yyyy" );  SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  try {Date date0 = format0.parse(myString0);Date date = format.parse(myString);System.out.println(date0);System.out.println(date);} catch (ParseException e) {// TODO Auto-generated catch blocke.printStackTrace();}}

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();   } }

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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());System.out.println("今天是："+utilDate+"(java.util.Date)");System.out.println("今天是："+sqlDate+"(java.sql.Date)");

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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();}}}

//在主函数中调用

File in = new File("D:\\asxxsst20170817.log");File out = new File("D:\\456.txt");fileCopy(in, out);

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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();  }

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9.创建 JSON 格式的数据

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

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importorg.json.JSONObject;

...

...

JSONObject json = newJSONObject();

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

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

...

String output = json.toString();

...

10. 使用iText JAR生成PDF

try{      OutputStream file = new FileOutputStream(new File("C:\\Test.pdf"));         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();    }  }

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11. HTTP 代理设置

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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

public class SimpleSingleton {private static SimpleSingleton singleInstance = new SimpleSingleton();// Marking default constructor private// to avoid direct instantiation.private SimpleSingleton() {}// Get instance for class SimpleSingletonpublic static SimpleSingleton getInstance() {return singleInstance;}}

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13. 抓屏程序

public class PrintScreen{public static void main(String[] args) throws Exception {captureScreen("D:\\抓屏.png");}public static 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));   }}

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14. 列出文件和目录 

public class Lists{public static void main(String[] args) {File dir = new File("D:\\zx"); 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];      System.out.println(filename);   } }    // 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);}}

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15. 创建ZIP和JAR文件 

public class ZipAndJar {   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();  }}

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16. 解析/读取XML 文件

XML文件

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<?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 ParseXML;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 documentSystem.out.println("Root element of the document: "+ docEle.getNodeName());NodeList studentList = docEle.getElementsByTagName("student");// Print total student elements in documentSystem.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("1.xml");// parser.getAllUserNames("D:\\1.xml");}}

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17. 把 Array 转换成 Map 

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")); }}

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18. 发送邮件

public class sendMail{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 = newInternetAddress(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);}}

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19. 发送代数据的HTTP 请求

public class Main {   public static void main(String[] args) {     try{//      URL my_url = new URL("http://coolshell.cn/");      URL my_url = new URL("http://baidu.com/");      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();    }  }}

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20. 改变数组的大小

public class ChangeArray {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};  for(int i=0; i<a.length; i++)   System.out.println(a[i]);  a = (int[])resizeArray(a,5);  System.out.println("----------");  a[3] = 4;  a[4] = 5;  for(int i=0; i<a.length; i++)    System.out.println (a[i]); }}

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