Android

So this week I was working with getting some data from the sqlite database in android and someone who was a beginner in android also asked me to help him with the same. I asked him what he knew and he said that even after reading up a lot he wasn’t able to figure out what exactly to do with the data he wants to save and use in his app. I have seen that this is a problem with a lot of people starting to develop android apps. So, I decided to write a blog on how can you handle your data in your android apps.

Most of the android apps need to save data even if only to save some user preferences. So primarily there are 3 ways to save your data :

1. In form of key values (SharedPreferences)
2. Reading/Writing to files
3. Writing to a database

So let’s go step by step. When we need to store just some preferences of the users like if they want notifications or what kind of theme they want : light or dark etc. So basically if we want to store a key value in the persitant storage of the app we can do that using SharedPreferences. To use sharedpreferences, we initialise the sharedpreference object like

SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(this);

in oncreate and cache it. Then we just need to add or retrieve what we want using this cached SharedPreferences object. To Add a key value pair :

sharedPreferences.edit().putString("someKey", "someValue").apply();

Also you can put all kinds of stuff here. For example right now we added a string with key “someKey” and Value “someValue”. We can also add Booleans, Floats, Ints, Longs, StringSets etc.

To retrieve the same value we do something like this

sharedPreferences.getString("someKey", "DefaultValue");

Now if you want some logs or some values that can be exported and sent to your server, you should write them to files and maybe read some json inputs etc. as well.

Basically android has a File system similar to other platforms. All android devices have two file storage areas : “Internal” and “external” storage. The difference between them is as follows :

Internal :

• Always available
• Files saved here are accesible by only your app
• When user uninstalls the app, system removes all your app’s files from internal storage

Best to use this when you want to be sure that neither the user nor the other app’s can access your files

External :

• It’s not always available because user can mount external storage as USB storage and remove it as well
• It’s readable by anything(Other apps, users etc.)
• When you uninstall, system removes your app’s files from here only if you save them in the directory from getExternalFilesDir()

Now to read and write files, you need extra permissions

• android.permission.WRITE_EXTERNAL_STORAGE
• android.permission.READ_EXTERNAL_STORAGE

So now let’s get down to it. How do I save and read files in my app?

You first initialise the File object

File file = new File(context.getFilesDir(), filename);

This will create a file with filename in the internal storage. For external storage

first check if the storage is available, then just create a file using getExternalStoragePublicDirectory

File file = new File(Environment.getExternalStoragePublicDirectory(
            Environment.DIRECTORY_PICTURES), albumName);
    if (!file.mkdirs()) {
        Log.e(LOG_TAG, "Directory not created");
    }
    return file;

This is for writing public files.

Now we move onto the most used part in an android app which is a database.Android has a built in SQLite database package which helps us in writing databases in files with syntax similar to SQL.

You need to add 2 classes which are mandatory and another class which basically helps you get organised. So the first is a Contract. This is where you actually write statements that will be executed later on to initialise or create the tables we want. For this make an a static abstract inner class that implements BaseColums like

public static abstract class Microlocation implements BaseColumns {
    public static final String TABLE_NAME = "microlocation";

    public static final String ID = "id";

    public static final String NAME = "name";

    public static final String LATITUDE = "latitude";

    public static final String LONGITUDE = "longitude";

    public static final String FLOOR = "floor";

    public static final String[] FULL_PROJECTION = {
            ID,
            NAME,
            LATITUDE,
            LONGITUDE,
            FLOOR

    };

    public static final String CREATE_TABLE =
            "CREATE TABLE " + TABLE_NAME
                    + " ("
                    + ID + INT_TYPE + PRIMARY_KEY + COMMA_SEP
                    + NAME + TEXT_TYPE + COMMA_SEP
                    + LATITUDE + REAL_TYPE + COMMA_SEP
                    + LONGITUDE + REAL_TYPE + COMMA_SEP
                    + FLOOR + INT_TYPE
                    + " );";

    public static final String DELETE_TABLE = "DROP TABLE IF EXISTS " + TABLE_NAME;


}

Here we are making static final Strings for column names and then creating a static final String CREATE_TABLE which is basically a statement that creates the table Microlocation with the specified key, columns, data types etc.

After adding this structure for all the tables we want to have in our database, we move on to adding a DbHelper class that extends SQLiteOpenHelper which basically has two Abstract methods called onCreate(SQLiteDatabase db) and onUpgrade(SQLiteDatabase db) which are called when the database is created and database version is changed respectively. We call all our CREATE_TABLE static Strings in onCreate which in turn creates all the tables. Something like this :

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL(DbContract.Speakers.CREATE_TABLE);
    db.execSQL(DbContract.Sponsors.CREATE_TABLE);
    db.execSQL(DbContract.Sessions.CREATE_TABLE);
    db.execSQL(DbContract.Tracks.CREATE_TABLE);
    db.execSQL(DbContract.Sessionsspeakers.CREATE_TABLE);
    db.execSQL(DbContract.Event.CREATE_TABLE);
    db.execSQL(DbContract.Microlocation.CREATE_TABLE);
    db.execSQL(DbContract.Versions.CREATE_TABLE);
    db.execSQL(DbContract.Bookmarks.CREATE_TABLE);
    db.execSQL(DbContract.EventDates.CREATE_TABLE);
}

You can also call DELETE_TABLE Strings in onUpgrade and the call onCreate again if you like but it’s not compulsory.

Now that you’re database is initialised, let’s add some records into it. For example I have to add a new Micrlocation I’d create a method in my data model where I’ll add a basic structure for the query and then format it with the values for a particular object of the model. Something, like this

public String generateSql() {
    String insertQuery = "INSERT INTO %s VALUES ('%d', %s, '%f', '%f', '%d');";
    return String.format(Locale.ENGLISH,
            insertQuery,
            DbContract.Microlocation.TABLE_NAME,
            id,
            DatabaseUtils.sqlEscapeString(StringUtils.optionalString(name)),
            latitude,
            longitude,
            floor);
}

and then I’d execute the string returned by the call

String query = model.generateSql();

by this

public void insertQuery(String query, DbHelper mDbHelper) {
    SQLiteDatabase db = mDbHelper.getWritableDatabase();
    db.beginTransaction();

    db.execSQL(query);
  
    db.setTransactionSuccessful();
    db.endTransaction();
}

Where db is just a SQLiteDatabase instance.

Now that we have records we want to retrieve them according to usage and for that we create helper methods. This is an example of the retrieving all the microlocations added to the database in ASCENDING order of NAME

public ArrayList<org.fossasia.openevent.data.Microlocation> getMicrolocationsList(SQLiteDatabase mDb) {
    String sortOrder = DbContract.Microlocation.NAME + ASCENDING;
    Cursor cursor = mDb.query(
            DbContract.Microlocation.TABLE_NAME,
            DbContract.Microlocation.FULL_PROJECTION,
            null,
            null,
            null,
            null,
            sortOrder
    );

    ArrayList<org.fossasia.openevent.data.Microlocation> microlocations = new ArrayList<>();
    org.fossasia.openevent.data.Microlocation microlocation;

    cursor.moveToFirst();
    while (!cursor.isAfterLast()) {
        microlocation = new org.fossasia.openevent.data.Microlocation(
                cursor.getInt(cursor.getColumnIndex(DbContract.Microlocation.ID)),
                cursor.getString(cursor.getColumnIndex(DbContract.Microlocation.NAME)),
                cursor.getFloat(cursor.getColumnIndex(DbContract.Microlocation.LATITUDE)),
                cursor.getFloat(cursor.getColumnIndex(DbContract.Microlocation.LONGITUDE)),
                cursor.getInt(cursor.getColumnIndex(DbContract.Microlocation.FLOOR))
        );
        microlocations.add(microlocation);
        cursor.moveToNext();
    }
    cursor.close();
    return microlocations;
}

First we create a cursor and then just iterate of the cursor to get microlocation objects and add them to an Arralist and return the Arraylist to the calling method.

So This are most of the things that are there to handling data in Android. Should be sufficient to get you started.

Sorry for the long post but the content couldn’t be made any smaller but I hope you gain something from this post. You can checkout implementations I have followed for the Open event project in the github repo https://github.com/fossasia/open-event-android. You can also write to me anytime on FB, Twitter, Email etc. and I’ll be happy to answer any queries. Adios!

References : 1) developers.android.com

2) https://github.com/fossasia/open-event-android