Wie bekomme ich Daten von Android-Datenbank in eine Arraylist

Question

Ich bin sehr neu bei Android-Programmierung und das ist meine erste Frage auf Stack-Overflow überhaupt. Ich habe Probleme zu verstehen, wo ich in meiner Code-Implementierung falsch gelaufen bin. Ich versuche, Daten in einer Datenbank zu speichern und sie dann in eine Arraylist zu extrahieren.

Dies ist die Klasse, wo ich Daten in die Datenbank in der Schaltfläche OnClickListener hinzufügen:

public class KarmaDescription extends AppCompatActivity { 

Button button; 
TasksDBHandler dbHandler; 
int tId = 1; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_karma_desc2min); 
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 
    getSupportActionBar().setHomeButtonEnabled(true); 
    getSupportActionBar().setDisplayHomeAsUpEnabled(true); 
    getSupportActionBar().setTitle("Overview"); 

    button = (Button) findViewById(R.id.button); 
    button.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      MyTasksGS myTask = new MyTasksGS(tId, "New Task Title", 2); 
      dbHandler.addTask(myTask); 
      Toast.makeText(KarmaDescription.this, "Task Added", Toast.LENGTH_SHORT).show(); 
     } 
    }); 

} 

@Override 
public boolean onOptionsItemSelected(MenuItem item) { 

    int id = item.getItemId(); 

    if (id == android.R.id.home) { 
     onBackPressed(); 
    } 

    return super.onOptionsItemSelected(item); 
} 

}

Dies ist die Klasse, die die Datenbank verwaltet:

public class TasksDBHandler extends SQLiteOpenHelper { 

private static final int DATABASE_VERSION = 1; 
private static final String DATABASE_NAME = "tasks.db"; 
public static final String TABLE_TASKS = "tasks"; 
public static final String COLUMN_ID = "id"; 
public static final String COLUMN_TASKNAME = "taskname"; 
public static final String COLUMN_DAYSLEFT = "daysleft"; 

public TasksDBHandler(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) { 
    super(context, DATABASE_NAME, factory, DATABASE_VERSION); 
} 

@Override 
public void onCreate(SQLiteDatabase db) { 

    String query = "CREATE TABLE " + TABLE_TASKS + "(" + 
      COLUMN_ID + " INTEGER PRIMARY KEY," + 
      COLUMN_TASKNAME + " TEXT," + 
      COLUMN_DAYSLEFT + " INTEGER" + 
      ")"; 
    db.execSQL(query); 
} 

@Override 
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 

    db.execSQL("DROP TABLE IF EXISTS " + TABLE_TASKS); 
    onCreate(db); 
} 

//Add a new row to the database 
public void addTask(MyTasksGS myTask) { 

    ContentValues values = new ContentValues(); 
    values.put(COLUMN_ID, myTask.getId()); 
    values.put(COLUMN_TASKNAME, myTask.getTitle()); 
    values.put(COLUMN_DAYSLEFT, myTask.getDaysRemaining()); 
    SQLiteDatabase db = getWritableDatabase(); 
    db.insert(TABLE_TASKS, null, values); 
    db.close(); 
} 


//Delete row from the database 
public void deleteTask(String taskID) { 

    SQLiteDatabase db = getWritableDatabase(); 
    db.execSQL("DELETE FROM " + TABLE_TASKS + " WHERE " + COLUMN_ID + "=\"" + taskID + "\";"); 
} 


//To put data into an arraylist 
public List<MyTasksGS> getDataFromDB() 
{ 
    int id, daysRemaining; 
    String title; 

    List<MyTasksGS> tasksList = new ArrayList<MyTasksGS>(); 
    String query = "SELECT * FROM " + TABLE_TASKS; 

    SQLiteDatabase db = getReadableDatabase(); 
    Cursor cursor = db.rawQuery(query, null); 

    cursor.moveToFirst(); 
    while (cursor.moveToNext()) 
    { 
     id = cursor.getInt(cursor.getColumnIndexOrThrow(COLUMN_ID)); 
     title = cursor.getString(cursor.getColumnIndexOrThrow(COLUMN_TASKNAME)); 
     daysRemaining = cursor.getInt(cursor.getColumnIndexOrThrow(COLUMN_DAYSLEFT)); 
     MyTasksGS myTask = new MyTasksGS(id, title, daysRemaining); 
     tasksList.add(myTask); 
    } 

    return tasksList; 
} 

}

Ich versuche, die Arraylist Daten zu kopieren, die von der ABO zurückgegeben wird ve Klasse in myTaskList hier und im immer diesen Fehler (getDataFromDB Funktion):

java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.example.prateek.karma.TasksDBHandler.getDataFromDB()' on a null object reference

public class MyTasksFragment extends Fragment { 

Button taskComplete; 
RecyclerView RVMyTasks; 
static MyTasksAdapter mtAdapter; 
List<MyTasksGS> myTaskList = new ArrayList<>(); 
TasksDBHandler dbHandler; 


public MyTasksFragment() { 
    // Required empty public constructor 
} 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
} 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
    // Inflate the layout for this fragment 
    View view = inflater.inflate(R.layout.fragment_my_tasks, container, false); 

    taskComplete = (Button) view.findViewById(R.id.taskComplete); 

    RVMyTasks = (RecyclerView) view.findViewById(R.id.RVMyTasks); 
    mtAdapter = new MyTasksAdapter(myTaskList); 
    RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getContext()); 
    RVMyTasks.setLayoutManager(mLayoutManager); 
    RVMyTasks.setItemAnimator(new DefaultItemAnimator()); 
    RVMyTasks.setAdapter(mtAdapter); 

    myTaskList = dbHandler.getDataFromDB(); 
    mtAdapter.notifyDataSetChanged(); 

    return view; 
} 

}

Und das ist die MyTasksGS Klasse:

public class MyTasksGS { 
String title; 
int daysRemaining; 
int id; 

public MyTasksGS() { 

} 

public MyTasksGS(int id, String title, int daysRemaining) { 
    this.title = title; 
    this.daysRemaining = daysRemaining; 
    this.id = id; 
} 

public int getId() { 
    return id; 
} 

public void setId(int id) { 
    this.id = id; 
} 

public String getTitle() { 
    return title; 
} 

public void setTitle(String title) { 
    this.title = title; 
} 

public int getDaysRemaining() { 
    return daysRemaining; 
} 

public void setDaysRemaining(int daysRemaining) { 
    this.daysRemaining = daysRemaining; 
} 

}

Ich habe vielleicht irgendwo einen sehr dummen Fehler gemacht, aber ich kann ihn nicht finden. Jede Hilfe wird geschätzt

Answer 1

Sie müssen den dbHandler instanziieren, Sie versuchen, es ohne eine gültige Instanz zu verwenden. Es verursacht die NullPointerException.

In Ihrem auf onCreateView vor myTaskList = dbHandler.getDataFromDB();

add: statt Pass in Konstruktor dbHandler = new TasksDBHandler(getActivity());

Klasse Eigenschaft DATABASE_NAME und Database_Version verwenden.

Ihre ändern TaskDBHandler Konstruktor wie diese

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