Caching Objects in Android Internal Storage

Android provides several options for persisting application data, such as SQLite Databases, SharedPreferences, internal and external storage.

In this post we’ll take a look how we can use the internal storage to persist application data. By default, files saved to internal storage are private to the application and they cannot be accessed by other applications. When the application is uninstalled, the files are removed.

To create and write a file to internal storage, openFileInput(); should be used. This opens a private file associated with the Context’s application package for writing. If the file does not already exits, then it is first created.

openFileInput() returns a FileInputStream, and we could use its write() method, which accepts a byte array as argument, to write the data. However, in the below example we will wrap the FileInputStream into an ObjectOutputStream which provides 2 convenient methods  for writing and reading objects: writeObject() and readObject().

So, lets  pretend that we need to save a List of some objects. The model class of our object looks like this:

public class Entry implements Serializable{
   private String name;

   public Entry(String name) {
      this.name = name;
   }

   public String getName() {
      return name;
   }
}

Make sure that the model class implements Serializable, otherwise you may get a java.io.NotSerializableException when attempting to write the object on internal storage.

Below is the utility class that provides 2 methods, one for storing objects to internal storage, and another for retrieving objects from internal storage.

public final class InternalStorage{

   private InternalStorage() {}

   public static void writeObject(Context context, String key, Object object) throws IOException {
      FileOutputStream fos = context.openFileOutput(key, Context.MODE_PRIVATE);
      ObjectOutputStream oos = new ObjectOutputStream(fos);
      oos.writeObject(object);
      oos.close();
      fos.close();
   }

   public static Object readObject(Context context, String key) throws IOException,
         ClassNotFoundException {
      FileInputStream fis = context.openFileInput(key);
      ObjectInputStream ois = new ObjectInputStream(fis);
      Object object = ois.readObject();
      return object;
   }
}

An this is how InternalStorage class can be used to persist and retrieve data from internal storage.

// The list that should be saved to internal storage.
List<Entry> entries = new ArrayList<Entry>();
entries.add(new Entry("House"));
entries.add(new Entry("Car"));
entries.add(new Entry("Job"));

try {
   // Save the list of entries to internal storage
   InternalStorage.writeObject(this, KEY, entries);

   // Retrieve the list from internal storage
   List<Entry> cachedEntries = (List<Entry>) InternalStorage.readObject(this, KEY);

   // Display the items from the list retrieved.
   for (Entry entry : cachedEntries) {
     Log.d(TAG, entry.getName());
   }
} catch (IOException e) {
   Log.e(TAG, e.getMessage());
} catch (ClassNotFoundException e) {
   Log.e(TAG, e.getMessage());
}