Android 常用代码收集

1、从网上下载文件

Android 2.3 以后的系统可以使用 DownloadManager，但是以前的系统必须自己写下载文件的代码。

这段代码如下：

try {//set the download URL, a url that points to a file on the internet//this is the file to be downloadedURL url = new URL("http://somewhere.com/some/webhosted/file");//create the new connectionHttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();//set up some things on the connectionurlConnection.setRequestMethod("GET");urlConnection.setDoOutput(true);//and connect!urlConnection.connect();//set the path where we want to save the file//in this case, going to save it on the root directory of the//sd card.File SDCardRoot = Environment.getExternalStorageDirectory();//create a new file, specifying the path, and the filename//which we want to save the file as.File file = new File(SDCardRoot,"somefile.ext");//this will be used to write the downloaded data into the file we createdFileOutputStream fileOutput = new FileOutputStream(file);//this will be used in reading the data from the internetInputStream inputStream = urlConnection.getInputStream();//this is the total size of the fileint totalSize = urlConnection.getContentLength();//variable to store total downloaded bytesint downloadedSize = 0;//create a buffer...byte[] buffer = new byte[1024];int bufferLength = 0; //used to store a temporary size of the buffer//now, read through the input buffer and write the contents to the filewhile ( (bufferLength = inputStream.read(buffer)) > 0 ) {//add the data in the buffer to the file in the file output stream (the file on the sd cardfileOutput.write(buffer, 0, bufferLength);//add up the size so we know how much is downloadeddownloadedSize += bufferLength;//this is where you would do something to report the prgress, like this maybeupdateProgress(downloadedSize, totalSize);}//close the output stream when donefileOutput.close();//catch some possible errors...} catch (MalformedURLException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();}// see http://androidsnippets.com/download-an-http-file-to-sdcard-with-progress-notification

注意：下载文件的任务要放到主线程以外去执行，例如可以使用 AsyncTask。上面代码的出处在末尾注释行。

2、对文件进行解压缩

import android.util.Log; import java.io.File; import java.io.FileInputStream; import java.io.FileOutputStream; import java.util.zip.ZipEntry; import java.util.zip.ZipInputStream;  /**  *  * @author jon  */ public class Decompress {   private String _zipFile;   private String _location;    public Decompress(String zipFile, String location) {     _zipFile = zipFile;     _location = location;      _dirChecker("");   }    public void unzip() {     try  {       FileInputStream fin = new FileInputStream(_zipFile);       ZipInputStream zin = new ZipInputStream(fin);       ZipEntry ze = null;       while ((ze = zin.getNextEntry()) != null) {         Log.v("Decompress", "Unzipping " + ze.getName());          if(ze.isDirectory()) {           _dirChecker(ze.getName());         } else {           FileOutputStream fout = new FileOutputStream(_location + ze.getName());           for (int c = zin.read(); c != -1; c = zin.read()) {             fout.write(c);           }            zin.closeEntry();           fout.close();         }                }       zin.close();     } catch(Exception e) {       Log.e("Decompress", "unzip", e);     }    }    private void _dirChecker(String dir) {     File f = new File(_location + dir);      if(!f.isDirectory()) {       f.mkdirs();     }   } } 

有用的资源：

http://www.java-samples.com/showtutorial.php?tutorialid=1521  下载文件、显示进度

http://www.jondev.net/articles/Unzipping_Files_with_Android_%28Programmatically%29 解压文件

3、访问数据库的步骤

1）定义contract class（就是包含一些常量的类，保存表名，列名等），同时implements BaseColumns。

2）subclass SQLiteOpenHelper，实现几个onCreate() onUpgrade() onDowngrade()

3）创建SQLiteOpenHelper子类的对象，调用 getWritableDatabase() / getReadableDatabase()，获取SQLiteDatabase接口

4）使用SQLiteDatabase接口对数据库执行各种操作。