public class MainActivity extends Activity {

  private TextView show;
  private EditText et;
  private String filename = "test";
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    show = (TextView) findViewById(R.id.show);
    et = (EditText) findViewById(R.id.et);

    findViewById(R.id.write).setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        try {
          FileOutputStream fos = openFileOutput(filename, Context.MODE_PRIVATE);
          //FileOutputStream是字节流，如果是写文本的话，需要进一步把FileOutputStream包装 UTF-8是编码
          OutputStreamWriter osw = new OutputStreamWriter(fos, "UTF-8");
          //写
          osw.write(et.getText().toString());
          osw.flush();
          fos.flush();
          osw.close();
          fos.close();
        } catch (FileNotFoundException e) {
          e.printStackTrace();
        } catch (UnsupportedEncodingException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        } catch (IOException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      }
    });  

    findViewById(R.id.read).setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        try {
          FileInputStream fis = openFileInput(filename);
          //当输入输出都指定字符集编码的时候，就不会出现乱码的情况
          InputStreamReader isr = new InputStreamReader(fis, "UTF-8");
          //获取文件的可用长度，构建一个字符数组
          char[] input = new char[fis.available()];
          isr.read(input);
          isr.close();
          fis.close();
          String readed = new String(input);
          show.setText(readed);
        } catch (FileNotFoundException e) {
          e.printStackTrace();
        } catch (UnsupportedEncodingException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        } catch (IOException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      }
    });      
  }  
}

// 创建一个URL对象，该对象包含一个IP地址，urlStr指的是网络IP地址 
url = new URL(urlStr); 
// 通过URL对象，来创建一个HttpURLConnection对象
HttpURLConnection urlConn = (HttpURLConnection) url.openConnection();

InputStream input = urlConn.getInputStream();

//在AndroidManifest.xml配置文件中加入权限信息 
<uses-permission android:name="android.permission.INTERNET"/>

Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED);

Environment.getExternalStorageDirectory();

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

public class FileUtils {
  private String SDPATH;
  public String getSDPATH(){
    return SDPATH;
  }
  //构造函数，得到SD卡的目录，这行函数得到的目录名其实是叫"/SDCARD"
  public FileUtils() {
    SDPATH = Environment.getExternalStorageDirectory() +"/";
  }
  //在SD卡上创建文件
  public File createSDFile(String fileName) throws IOException{
    File file = new File(SDPATH + fileName);
    file.createNewFile();
    return file;
  }
  //在SD卡上创建目录
  public File createSDDir(String dirName){
    File dir = new File(SDPATH + dirName);
    dir.mkdir();
    return dir;
  }
  //判断SD卡上的文件夹是否存在
  public boolean isFileExist(String fileName){
    File file = new File(SDPATH + fileName);
    return file.exists();
  }

  //将一个InputStream里面的数据写入到SD卡中
  //将input写到path这个目录中的fileName文件上
  public File write2SDFromInput(String path, String fileName, InputStream input){
    File file = null; 
    OutputStream output = null; 
    try{ 
      createSDDir(path); 
      file = createSDFile(path + fileName); 
      //FileInputStream是读取数据，FileOutputStream是写入数据，写入到file这个文件上去
      output = new FileOutputStream(file); 
      byte buffer [] = new byte[4 * 1024]; 
      while((input.read(buffer)) != -1){ 
        output.write(buffer); 
      } 
      output.flush(); 
    } 
    catch(Exception e){ 
      e.printStackTrace(); 
    } 
    finally{ 
      try{ 
        output.close(); 
      } 
      catch(Exception e){ 
        e.printStackTrace(); 
      } 
    } 
    return file; 
  } 
}