Context inside non activity class

Question

I need to use a database handler inside a non activity class but for that I need the context and I can't get it. I have tried using this but no luck,

protected Context context;

    public Example(Context context){
        this.context = context.getApplicationContext();
    }

It always throws this error:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference

EDIT:

public class Telnet extends AsyncTask<Void, Void, Void> {
   wifiHistoryDatabaseHandler wifihist;

    @Override
    protected Void doInBackground(Void... params) {


        wifihist = new wifiHistoryDatabaseHandler(context);
 }

WifiHistoryDatabaseHandler:

public class wifiHistoryDatabaseHandler  extends SQLiteOpenHelper {
    private static final int DATABASE_VERSION = 1;
    // Database Name
    private static final String DATABASE_NAME = "information_db_232";
    // Contacts table name
    private static final String TABLE_CONTACTS = "wifi_history";

    public static final String ID = "id";
    public static final String message = "message";
    public static final String message_source = "message_source";
    public static final String time = "time";


    public wifiHistoryDatabaseHandler(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    // Creating Tables
    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_CONTACTS_TABLE =  "CREATE TABLE "
                + TABLE_CONTACTS + "( " + ID
                + " INTEGER PRIMARY KEY,"+message+","+message_source+","+time+" );";
        db.execSQL(CREATE_CONTACTS_TABLE);
    }

    // Upgrading database
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // Drop older table if existed
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_CONTACTS);

        // Create tables again
        onCreate(db);
    }


    //add line
    public void addLine(wifiHisotryClass info) {
        SQLiteDatabase db = this.getWritableDatabase();

        ContentValues values = new ContentValues();
        values.put(message, info.get_message());
        values.put(message_source, info.get_message_source());
        values.put(time, info.get_time());



        // Inserting Row
        db.insert(TABLE_CONTACTS, null, values);
        db.close(); // Closing database connection
    }


    // Getting single contact
    public wifiHisotryClass getContact(int id) {
        SQLiteDatabase db = this.getReadableDatabase();

        Cursor cursor = db.query(TABLE_CONTACTS, new String[]{ID
                        ,message,message_source, time}, ID + "=?",
                new String[]{String.valueOf(id)}, null, null, null, null);
        if (cursor != null)
            cursor.moveToFirst();

        wifiHisotryClass info = new wifiHisotryClass(Integer.parseInt(cursor.getString(0)),cursor.getString(1), cursor.getString(2), cursor.getString(3));

        // return info
        return info;
    }

    // Getting contacts Count
    public int getContactsCount() {
        String countQuery = "SELECT  * FROM " + TABLE_CONTACTS;
        SQLiteDatabase db = this.getReadableDatabase();

        Cursor cursor = db.rawQuery(countQuery, null);
        int num = cursor.getCount();
        cursor.close();

        // return count
        return num;
    }


    // Updating single contact
    public int updateContact(wifiHisotryClass info) {
        SQLiteDatabase db = this.getWritableDatabase();

        ContentValues values = new ContentValues();
        values.put(message, info.get_message());
        values.put(message_source, info.get_message_source());
        values.put(time, info.get_time());

        // updating row
        return db.update(TABLE_CONTACTS, values, ID + " = ?",
                new String[] { String.valueOf(info.getID()) });
    }


    // Deleting single contact
    public void deleteContact(Info info) {
        SQLiteDatabase db = this.getWritableDatabase();
        db.delete(TABLE_CONTACTS, ID + " = ?",
                new String[]{String.valueOf(info.getID())});
        db.close();
    }

    void deleteAll()
    {
        SQLiteDatabase db= this.getWritableDatabase();
        db.delete(TABLE_CONTACTS, null, null);

    }


}

The error is not in code that you are showing. Please, show the code where you are opening DB. I haven't downvoted BTW — Héctor, Jan 19 '16 at 16:08
Use Singleton class check this link http://stackoverflow.com/a/21819009/4848308. — Gueorgui Obregon, Jan 19 '16 at 16:17
@g2o Can I extend AsyncTask and Application at the same time? — scottbear, Jan 19 '16 at 16:19

Alessandro Verona · Accepted Answer · 2016-01-19T16:35:25.840

0

public Example(Context context){
        this.context = context.getApplicationContext();
    }

why? use

this.context = context;

instead.

or

 wifihist = new wifiHistoryDatabaseHandler(getActivity());

The context variable in telnet class is null.

You can consider to pass context directly in the constructor of Telnet async task

public Telnet(Context c)
{ 
   mContext = c;
}

edited Jan 19 '16 at 16:35

answered Jan 19 '16 at 16:22

Alessandro Verona

1,157
9
23

context in telnet class is null. try wifihist = new wifiHistoryDatabaseHandler(getActivity()); – Alessandro Verona Jan 19 '16 at 16:31
"cannot resolve methos getActivity" That's what I am using in activity classes. – scottbear Jan 19 '16 at 16:33
try YourAcitivity.getActivity() – Alessandro Verona Jan 19 '16 at 16:34
The same. It is a non activity class – scottbear Jan 19 '16 at 16:35

score 0 · Answer 2 · answered Jan 19 '16 at 16:35

0

Move wifiHistoryDatabaseHandler class's object declaration inside the function where you're trying to access it rather than declaring it globally.

answered Jan 19 '16 at 16:35

Ramkishore V S

124
1
8

score 0 · Answer 3 · edited May 23 '17 at 12:23

You should use Application as a singleton class

public class MyApplication extends Application {

    private static Context mContext;

    @Override
    public void onCreate() {
        super.onCreate();
        mContext = getApplicationContext();
    }

    public static Context getContext() {
        return mContext;
    }
}

Don't forget to declare the application in your manifest file:

<application
    android:name=".MyApplication"
    android:icon="@drawable/icon"
    android:label="@string/app_name" >

With this way you can get application context from any non activity class of your app using MyApplication.getContext()

Answer from: https://stackoverflow.com/a/21819009/4848308

Context inside non activity class

3 Answers3