Figure 5.2 Two separate applications getting and setting SharedPreferences in Java

Drawer QR in Java Figure 5.2 Two separate applications getting and setting SharedPreferences

Android s filesystem is based on Linux and supports mode-based permissions. You can access this filesystem in several ways. You can create and read files from within applications, you can access raw resource files, and you can work with specially compiled custom XML files. In this section, we ll explore each approach.

Android s stream-based system of manipulating files will feel familiar to anyone who s written I/O code in Java SE or Java ME. You can easily create files in Android and store them in your application s data path. The following listing demonstrates how to open a FileOutputStream and use it to create a file.

public class CreateFile extends Activity { private EditText createInput; private Button createButton; @Override public void onCreate(Bundle icicle) { super.onCreate(icicle); setContentView(R.layout.create_file); createInput = (EditText) findViewById(R.id.create_input); createButton = (Button) findViewById(R.id.create_button); createButton.setOnClickListener(new OnClickListener() { public void onClick(final View v) { FileOutputStream fos = null; try { Use fos = openFileOutput("filename.txt", openFileOutput Context.MODE_PRIVATE); fos.write(createInput.getText(). Write data toString().getBytes()); to stream } catch (FileNotFoundException e) { Log.e("CreateFile", e.getLocalizedMessage()); } catch (IOException e) { Log.e("CreateFile", e.getLocalizedMessage()); } finally { if (fos != null) { try { fos.flush(); Flush and fos.close(); close stream } catch (IOException e) { // swallow } } } startActivity( new Intent(CreateFile.this, ReadFile.class)); } }); } }

Android provides a convenience method on Context to get a FileOutputStream namely openFileOutput(String name, int mode) B. Using this method, you can create a stream to a file. That file will ultimately be stored at the data/data/ [PACKAGE_NAME]/files/file.name path on the platform. After you have the stream, you can write to it as you would with typical Java C. After you re finished with a stream, you should flush and close it to clean up D. Reading from a file within an application context (within the package path of the application) is also simple; in the next section we ll show you how.

Similarly to openFileOutput, the Context also has a convenience openFileInput method. You can use this method to access a file on the filesystem and read it in, as shown in the following listing.

public class ReadFile extends Activity { private TextView readOutput; private Button gotoReadResource; @Override public void onCreate(Bundle icicle) { super.onCreate(icicle); setContentView(R.layout.read_file); readOutput = (TextView) findViewById(R.id.read_output); FileInputStream fis = null; Use try { openFileInput fis = openFileInput("filename.txt"); for stream byte[] reader = new byte[fis.available()]; while (fis.read(reader) != -1) {} Read data readOutput.setText(new String(reader)); from stream } catch (IOException e) { Log.e("ReadFile", e.getMessage(), e); } finally { if (fis != null) { try { fis.close(); } catch (IOException e) { // swallow } } } . . . goto next Activity via startActivity omitted for brevity } }

For input, you use openFileInput(String name, int mode) to get the stream B, and then read the file into a byte array as with standard Java C. Afterward, close the stream properly to avoid hanging on to resources. With openFileOutput and openFileInput, you can write to and read from any file within the files directory of the application package you re working in. Also, as we

Occasionally, setting the user ID of your application can be extremely useful. For instance, if you have multiple applications that need to share data with one another, but you also don t want that data to be accessible outside that group of applications, you might want to make the permissions private and share the UID to allow access. You can allow a shared UID by using the sharedUserId attribute in your manifest: android:sharedUserId="YourID".

discussed in the previous section, you can access files across different applications if the permissions allow it and if you know the package used to obtain the full path to the file. In addition to creating files from within your application, you can push and pull files to the platform using the adb tool, described in section 2.2.3. The File Explorer window in Eclipse provides a UI for moving files on and off the device or simulator. You can optionally put such files in the directory for your application; when they re there, you can read these files just like you would any other file. Keep in mind that outside of development-related use, you won t usually push and pull files. Rather, you ll create and read files from within the application or work with files included with an application as a raw resource, as you ll see next.