Android SQLite 数据获取

Question

Every time I run a "select *" query from my SQLite, the app keeps stopping. I've deduced it to it calling the database helper every time, but I don't know how to fix it.

MainActivity.java

public class MainActivity extends AppCompatActivity {
    DatabaseHelper databaseHelper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        getWindow().setStatusBarColor(Color.WHITE);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        displayUsers();
    }

    public void displayUsers() {
        Cursor cursor = databaseHelper.getAllUsers(); //here's where the error keeps on happening
        if(cursor.getCount() == 0) {
            Toast.makeText(this, "none", Toast.LENGTH_SHORT).show();
        }
    }
}

DatabaseHelper.java

public class DatabaseHelper extends SQLiteOpenHelper {
    public static final String DATABASE_NAME = "reminders.db";
    public static final String T_1 = "tbl_users";
    public static final String T1_COL_1 = "ID";
    public static final String T1_COL_2 = "FIRST_NAME";
    public static final String T1_COL_3 = "MIDDLE_INITIAL";
    public static final String T1_COL_4 = "LAST_NAME";
    public static final String T1_COL_5 = "PHONE";
    public static final String T1_COL_6 = "EMAIL";
    public static final String T1_COL_7 = "USERNAME";
    public static final String T1_COL_8 = "PASSWORD";

    public DatabaseHelper(@Nullable Context context) {
        super(context, DATABASE_NAME, null, 1);
        SQLiteDatabase database = this.getWritableDatabase();
    }
    ...//other functions

    public Cursor getAllUsers() {
        SQLiteDatabase database = this.getWritableDatabase();
        Cursor cursor = database.rawQuery("select * from " + T_1, null);
        return cursor;
    }
}

Answer 1

Before returning cursor close your database

public Cursor getAllUsers() {
   SQLiteDatabase database = this.getWritableDatabase();
   Cursor cursor = database.rawQuery("select * from " + T_1, null);

   database.close(); // Added

   return cursor;
}

Answer 2

Your DatabaseHelper class doesn't create any object cause you didn't call new DatabaseHelper (this). Thats why databasehelper return null. You can try those line in onCreate method, databaseHelper= new DatabaseHelper(this);

Answer 3

In following function at first your have to initialize database to get WritableDatabase in DatabaseHelper class.

public void displayUsers() {
databaseHelper = DatabaseHelper(MainActivity.this)
        Cursor cursor = databaseHelper.getAllUsers();
        if(cursor.getCount() == 0) {
            Toast.makeText(this, "none", Toast.LENGTH_SHORT).show();
        }
}