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 downloaded
	URL url = new URL("http://somewhere.com/some/webhosted/file");

	//create the new connection
	HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();

	//set up some things on the connection
	urlConnection.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 created
	FileOutputStream fileOutput = new FileOutputStream(file);

	//this will be used in reading the data from the internet
	InputStream inputStream = urlConnection.getInputStream();

	//this is the total size of the file
	int totalSize = urlConnection.getContentLength();
	//variable to store total downloaded bytes
	int 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 file
	while ( (bufferLength = inputStream.read(buffer)) > 0 ) {
		//add the data in the buffer to the file in the file output stream (the file on the sd card
		fileOutput.write(buffer, 0, bufferLength);
		//add up the size so we know how much is downloaded
		downloadedSize += bufferLength;
		//this is where you would do something to report the prgress, like this maybe
		updateProgress(downloadedSize, totalSize);

	}
	//close the output stream when done
	fileOutput.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。