20个java小程序

Posted 2020-08-21

本文来源：http://mp.weixin.qq.com/s/kZO3_OqRm4sAlPHTDgAHRQ

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下面是20个非常有用的Java程序片段，希望能对你有用。

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

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

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 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

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

File dir = new File("directoryName");
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];
}
}

// 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);

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]);
}