NullpointerException using sqlite database on android

Question

I'am trying to insert playlist object into sqlite database with android but I get a NullPointerException when opening the database. Here is my PlaylistDbAdapter:

    import java.util.ArrayList;
    import java.util.List;

    import android.content.ContentValues;
    import android.content.Context;
    import android.database.Cursor;
    import android.database.SQLException;
    import android.database.sqlite.SQLiteDatabase;
    import android.database.sqlite.SQLiteOpenHelper;

    public class PlaylistDbAdapter{

// All Static variables
// Database Version
private static final int DATABASE_VERSION = 2;

// Database Name
private static final String DATABASE_NAME = "playlistdb";

// Contacts table name
private static final String TABLE_NAME = "playlists";

// Contacts Table Columns names
private static final String KEY_ID = "id";
private static final String KEY_NAME = "name";
private DatabaseHelper dh;
 private SQLiteDatabase mDb;
private final Context mCtx;

private static class DatabaseHelper extends SQLiteOpenHelper{

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

// Creating Tables
@Override
public void onCreate(SQLiteDatabase db) {
    String CREATE_PLAYLISTS_TABLE =  "create table playlists (id integer primary key autoincrement, name text not null);";
    db.execSQL(CREATE_PLAYLISTS_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_NAME);

    // Create tables again
    onCreate(db);
}
}


 public  PlaylistDbAdapter(Context ctx) {
        this.mCtx = ctx;
    }
 public PlaylistDbAdapter open() throws SQLException {
        dh = new DatabaseHelper(mCtx);
        mDb = dh.getWritableDatabase();
        return this;
    }

 public void close() {
        dh.close();
    }

/**
 * All CRUD(Create, Read, Update, Delete) Operations
 * @return 
 */

void addPlaylist(Playlist playlist) {

    ContentValues values = new ContentValues();
    values.put(KEY_NAME, playlist.getName()); // Playlist Name

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

// Getting single playlist
Playlist getPlaylist(int id) {
    SQLiteDatabase db = dh.getReadableDatabase();

    Cursor cursor = db.query(TABLE_NAME, new String[] { KEY_ID,
            KEY_NAME}, KEY_ID + "=?",
            new String[] { String.valueOf(id) }, null, null, null);
    if (cursor != null)
        cursor.moveToFirst();

    Playlist playlist = new Playlist (Integer.parseInt(cursor.getString(0)),
            cursor.getString(1));
    // return playlist
    System.out.println(playlist._name);
    return playlist;
}

// Getting All Playlists
public List<Playlist> getAllPlaylists() {
    List<Playlist> contactList = new ArrayList<Playlist>();
    // Select All Query
    String selectQuery = "SELECT  * FROM " + TABLE_NAME;

    SQLiteDatabase db = dh.getWritableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);

    // looping through all rows and adding to list
    if (cursor.moveToFirst()) {
        do {
            Playlist playlist = new Playlist();
            playlist.setID(Integer.parseInt(cursor.getString(0)));
            playlist.setName(cursor.getString(1));
            // Adding playlist to list
            contactList.add(playlist);
        } while (cursor.moveToNext());
    }

    // return playlist list
    return contactList;
}

// Updating single playlist
public int updatePlaylist(Playlist playlist) {
    SQLiteDatabase db = dh.getWritableDatabase();

    ContentValues values = new ContentValues();
    values.put(KEY_NAME, playlist.getName());

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

// Deleting single playlist
public void deletePlaylist(Playlist playlist) {
    SQLiteDatabase db = dh.getWritableDatabase();
    db.delete(TABLE_NAME, KEY_ID + " = ?",
            new String[] { String.valueOf(playlist.getID()) });
    db.close();
}


// Getting playlists Count
public int getPlaylistCount() {
    String countQuery = "SELECT  * FROM " + TABLE_NAME;
    SQLiteDatabase db = dh.getReadableDatabase();
    Cursor cursor = db.rawQuery(countQuery, null);
    cursor.close();
    // return count
    return cursor.getCount();
}

    }

this is my playlist object

    public class Playlist {


//private variables
int _id;
String _name;

// Empty constructor
public Playlist(){

}
// constructor
public Playlist(int id, String name){
    this._id = id;
    this._name = name;
}

// constructor
public Playlist(String name){
    this._name = name;
}
// getting ID
public int getID(){
    return this._id;
}

// setting id
public void setID(int id){
    this._id = id;
}

// getting name
public String getName(){
    return this._name;
}

// setting name
public void setName(String name){
    this._name = name;
}

    }

and this is AddPlaylist class in which I add the new playlist (using an an alert dialog in which the user chooses the name of the playlist).

    import android.os.Bundle;

    import android.text.InputType;
    import android.view.View;
    import android.widget.EditText;
    import android.widget.ImageButton;
    import android.widget.Toast;
    import android.app.Activity;
    import android.app.AlertDialog;
    import android.content.Context;
    import android.content.DialogInterface;
    import android.database.sqlite.SQLiteDatabase;


    public class AddPlaylist extends Activity{
private ImageButton btnAddPlaylist;
private String playlistName = "";
public PlaylistDbAdapter dh;
private SQLiteDatabase db;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);     
    setContentView(R.layout.add_playlist);
    btnAddPlaylist = (ImageButton) findViewById(R.id.btnAddPlaylist);
    btnAddPlaylist.setOnClickListener(new View.OnClickListener(){

        @Override
        public void onClick(View arg0) {
            popUpNewPlaylist();
        }

    });
}
public void popUpNewPlaylist()
{   

    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setTitle("Playlist name");

    // Set up the input
    final EditText input = new EditText(this);
    // Specify the type of input expected; this, for example, sets the input as a password, and will mask the text
    input.setInputType(InputType.TYPE_CLASS_TEXT | InputType.TYPE_CLASS_TEXT);
    builder.setView(input);

    // Set up the buttons
    builder.setPositiveButton("Add", new DialogInterface.OnClickListener() { 
        @Override
        public void onClick(DialogInterface dialog, int which) {
            playlistName = input.getText().toString();
            Playlist tmpPLST = new Playlist();
            tmpPLST.setName(playlistName);
            dh.open();
            //dh.addPlaylist(tmpPLST);
            popupSuccessToast();
        }
    });
    builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            dialog.cancel();
            popupCancelToast();
        }
    });

    builder.show();
}

public void popupSuccessToast()
{
    Context context = getApplicationContext();
    CharSequence text = "Playlist "+playlistName+" Added";
    int duration = Toast.LENGTH_SHORT;

    Toast toast = Toast.makeText(context, text, duration);
    toast.show();
}

public void popupCancelToast()
{
    Context context = getApplicationContext();
    CharSequence text = "Operation canceled";
    int duration = Toast.LENGTH_SHORT;

    Toast toast = Toast.makeText(context, text, duration);
    toast.show();
}


    } 

my crash log

    04-27 18:43:34.151: E/AndroidRuntime(4338): FATAL EXCEPTION: main
    04-27 18:43:34.151: E/AndroidRuntime(4338): java.lang.NullPointerException
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at                               ca.uqac.powerplayer.AddPlaylist$2.onClick(AddPlaylist.java:58)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at             com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java)
     04-27 18:43:34.151: E/AndroidRuntime(4338):    at    android.os.Handler.dispatchMessage(Handler.java)
     04-27 18:43:34.151: E/AndroidRuntime(4338):    at android.os.Looper.loop(Looper.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at android.app.ActivityThread.main(ActivityThread.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at java.lang.reflect.Method.invokeNative(Native Method)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at java.lang.reflect.Method.invoke(Method.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java)
    04-27 18:43:34.151: E/AndroidRuntime(4338):     at dalvik.system.NativeStart.main(Native Method)

Thanks.

Answer 1

Problem is that in AddPlaylist class the PlaylistDbAdapter dh is not instantiated. And your are trying to access the dh.open() method in onClick.

You need to create instance for PlaylistDbAdapter to access its methods or variable.