20个非常有用的java程序片段-亚博电竞手机版
java学习
2020年03月26日 23:26
0
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
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
john b 12 mary a 11 simon a 18
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 ; 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
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