将数据库记录加载到微调器中时出错

Question

I'm working on an application for my A2 Level Computer Science coursework, and have run into an error when attempting to load data from a database table into a spinner. I'm looking to include the ids, forenames and surnames of all leaders in the spinner, however I've run into an error. Here is the code I wrote for the database handler:

    private static final String CREATE_TABLE_EVENTS=("CREATE TABLE " + TABLE_EVENTS + "("
        + COLUMN_SEID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + COLUMN_SNAME + " TEXT,"
        + COLUMN_STYPE + " TEXT,"+ COLUMN_SDATE + " TEXT," + COLUMN_EDATE + " TEXT,"
        + COLUMN_STIME + " TEXT," + COLUMN_ETIME + " TEXT," + COLUMN_VENUE + " TEXT,"
        + COLUMN_DESC + " TEXT," + COLUMN_SCID + " INTEGER," + COLUMN_EMAIL + " TEXT,"
        + COLUMN_ACOST + " REAL," + COLUMN_INV + " INTEGER," + COLUMN_PCOST + " REAL,"
        + COLUMN_FOOD + " TEXT," + COLUMN_TINC + " TEXT," + COLUMN_DIST + " INTEGER,"
        + COLUMN_TCOST + " REAL )"); //The columns had been defined earlier
...
//Search for leaders for spinner for purpose of creating an event
public String[][] eventLeadersSearch() {
    int CursorPosition;
    String[][] leadersList =new String[3][];
    String ScoutSection = "Leaders";
    String query = "SELECT " + COLUMN_SCID + ", " + COLUMN_FNAME + ", " + COLUMN_LNAME +" FROM " + TABLE_SCOUTS + " WHERE " + COLUMN_SECTION + " LIKE '" + ScoutSection + "'";
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(query, null);
    int rowsFound=cursor.getCount();
    int x=0;
    if (rowsFound>0) {
        cursor.moveToFirst();
        do {
            CursorPosition = cursor.getPosition();
            leadersList[0][x] = Integer.toString((cursor.getInt(0)));//DEFINITELY SOMETHING WRONG HERE
            leadersList[1][x] = cursor.getString(1);
            leadersList[2][x] = cursor.getString(2);
            x = x + 1;
            cursor.moveToNext();
        }
        while (!cursor.isAfterLast());
    }
    else {
        Log.e("Database Error", "No Leaders Found");
    }

    cursor.close();
    db.close();
    return leadersList;
}

The reason for me using a 2d array is that later, I want to be able to go back and retrieve more details on that leader - however this will be on a different screen and thus in a different class. Therefore, I wanted to be able to retain all details individually as well.

Here is the code I wrote to put all of the values into the spinner:

    private void loadSpinnerData(){
        databaseHandler db=new databaseHandler(getApplicationContext());
        String[][] Leaders=db.eventLeadersSearch(); //Error here
        List<String> LeadersInOne=new ArrayList<>();
        for (int i=0; i<Leaders.length; i++)
        {
            for (int j=0; j<Leaders[i].length; j++)
            {
                LeadersInOne.add(Leaders[i][j]);
            }
        }
        ArrayAdapter<String> dataAdapter=new ArrayAdapter<String>(this, android.R.layout.simple_spinner_item, LeadersInOne);
        dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        leadersSpinner.setAdapter(dataAdapter);
}

These are the error details that appear in the LogCat:

01-20 18:41:02.712 16113-16113/com.example.atomi.scoutmanagerprototype E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.atomi.scoutmanagerprototype, PID: 16113
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.atomi.scoutmanagerprototype/com.example.atomi.scoutmanagerprototype.frmCreateEvent1}: java.lang.NullPointerException: Attempt to get length of null array
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2314)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2386)
    at android.app.ActivityThread.access$800(ActivityThread.java:148)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5310)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)
 Caused by: java.lang.NullPointerException: Attempt to get length of null array
    at com.example.atomi.scoutmanagerprototype.frmCreateEvent1.loadSpinnerData(frmCreateEvent1.java:44)
    at com.example.atomi.scoutmanagerprototype.frmCreateEvent1.onCreate(frmCreateEvent1.java:33)
    at android.app.Activity.performCreate(Activity.java:5953)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1128)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2386) 
    at android.app.ActivityThread.access$800(ActivityThread.java:148) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:135) 
    at android.app.ActivityThread.main(ActivityThread.java:5310) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:372) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696) 

Does anyone have any idea what may have gone wrong? It is my first time working in Android Studio, Java or XML and I am having to teach myself all of them, so any help would be much appreciated. Thanks.

Answer 1

I believe that you would have fewer issues using a Cursor Adapter , such as SimpleCursorAdapter . Cursor Adpaters are designed to handle cursors so there is no need to create an intermediate Array for an Array adapter. They also pass the ID to the onItemSelected method. However, they do require a column named _id that is the id column (the example below shows how this can be done even if the id column is named something else).

Example

The following example is based upon your code. It selects Scouts who have Leaders in the section column to populate the spinner. It also includes an onItemSelected listener that results in the details of the Scout being displayed via a toast when a scout is selected.

• For convenience/explanation/demonstration of using multiple columns the layout simple_list_item_2 has been used.

Databasehelper.java

public class DatabaseHelper extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "mydatabase";
    private static final int DATABASE_VERSION = 1;
    private static final String TABLE_EVENTS = "events";
    private static final String COLUMN_SEID = BaseColumns._ID; //<<<<<<<<<< suits Cursor Adaptor
    private static final String COLUMN_SNAME = "sname";
    private static final String COLUMN_STYPE = "stype";
    private static final String COLUMN_SDATE = "sdate";
    private static final String COLUMN_EDATE = "edate";
    private static final String COLUMN_STIME = "stime";
    private static final String COLUMN_ETIME = "etime";
    private static final String COLUMN_VENUE = "venue";
    private static final String COLUMN_DESC = "desccription";
    private static final String COLUMN_SCID = "scid";
    private static final String COLUMN_EMAIL = "email";
    private static final String COLUMN_ACOST = "acost";
    private static final String COLUMN_INV = "inv";
    private static final String COLUMN_PCOST = "pcost";
    private static final String COLUMN_FOOD = "food";
    private static final String COLUMN_TINC = "tinc";
    private static final String COLUMN_DIST = "dist";
    private static final String COLUMN_TCOST = "tcost";

    public static final String TABLE_SCOUTS = "scouts"; //<<<<<<<<<<< public so the can be used elsewhere
    public static final String COLUMN_FNAME = "fname";
    public static final String COLUMN_LNAME = "lname";
    public static final String COLUMN_SECTION = "section";

    SQLiteDatabase mDB;

    private static final String CREATE_TABLE_EVENTS=("CREATE TABLE " + TABLE_EVENTS + "("
            + COLUMN_SEID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + COLUMN_SNAME + " TEXT,"
            + COLUMN_STYPE + " TEXT,"+ COLUMN_SDATE + " TEXT," + COLUMN_EDATE + " TEXT,"
            + COLUMN_STIME + " TEXT," + COLUMN_ETIME + " TEXT," + COLUMN_VENUE + " TEXT,"
            + COLUMN_DESC + " TEXT," + COLUMN_SCID + " INTEGER," + COLUMN_EMAIL + " TEXT,"
            + COLUMN_ACOST + " REAL," + COLUMN_INV + " INTEGER," + COLUMN_PCOST + " REAL,"
            + COLUMN_FOOD + " TEXT," + COLUMN_TINC + " TEXT," + COLUMN_DIST + " INTEGER,"
            + COLUMN_TCOST + " REAL )");

    private static final String CREATE_TABLE_SCOUTS = "CREATE TABLE " + TABLE_SCOUTS + "(" +
            COLUMN_SCID + " INTEGER PRIMARY KEY," +
            COLUMN_FNAME + " TEXT," +
            COLUMN_LNAME + " TEXT," +
            COLUMN_SECTION + " TEXT" +
            ")";

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
        mDB = this.getWritableDatabase();
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_TABLE_EVENTS);
        db.execSQL(CREATE_TABLE_SCOUTS);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int i, int i1) {

    }

    public long addScout(String fname, String lname, String section) {
        ContentValues cv = new ContentValues();
        cv.put(COLUMN_FNAME,fname);
        cv.put(COLUMN_LNAME,lname);
        cv.put(COLUMN_SECTION,section);
        return mDB.insert(TABLE_SCOUTS,null,cv);
    }

    public Cursor getLeaders() {
        String where_clause = COLUMN_SECTION + "=?";
        String[] where_args =  new String[]{"Leaders"};
        //<<<<<<<<<< NOTE SELECT scid AS _id, fname, lname, section so that the id column in the cursor is _id
        String[] columns = new String[]{COLUMN_SCID + " AS " + BaseColumns._ID,COLUMN_FNAME, COLUMN_LNAME,COLUMN_SECTION};
        return mDB.query(TABLE_SCOUTS,columns,where_clause,where_args,null,null,null);
    }
}

MainActivity.java

public class MainActivity extends AppCompatActivity {

    DatabaseHelper databaseHelper;
    Spinner leadersSpinner;
    SimpleCursorAdapter leaderAdapter;
    Cursor leaders;
    Context context;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        context = this;
        databaseHelper = new DatabaseHelper(this); //<<<<<<<<<< prepre to use Database
        leadersSpinner = this.findViewById(R.id.leaders_spinner); //<<<<<<<<<< get the Spinner
        addSomeScoutsForTesting(); //<<<<<<<<<< Add some data for testing
        setOrRefreshSpinner(); //<<<<<<<<<< prepare the spinner
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        leaders.close(); //<<<<<<<<<< Close the Cursor when the activity is done with
    }

    private void setOrRefreshSpinner() {
        leaders = databaseHelper.getLeaders();
        if (leaderAdapter == null) {
            String[] columns = new String[]{DatabaseHelper.COLUMN_FNAME, DatabaseHelper.COLUMN_LNAME};
            leaderAdapter = new SimpleCursorAdapter(this,android.R.layout.simple_list_item_2,leaders,columns,new int[]{android.R.id.text1,android.R.id.text2},0);
            leadersSpinner.setAdapter(leaderAdapter);
            leadersSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
                @Override
                public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
                    String fname = leaders.getString(leaders.getColumnIndex(DatabaseHelper.COLUMN_FNAME));
                    String lname = leaders.getString(leaders.getColumnIndex(DatabaseHelper.COLUMN_LNAME));
                    Toast.makeText(
                            context,
                            "You clicked the Scout " +
                                    fname + " " + lname +
                                    " whoose ID is " + String.valueOf(l) +
                                    " at position " + String.valueOf(i) +
                                    " in the Spinner.",
                            Toast.LENGTH_SHORT
                    ).show();
                }

                @Override
                public void onNothingSelected(AdapterView<?> adapterView) {

                }
            });
        } else {
            leaderAdapter.swapCursor(leaders);
        }
    }

    private void addSomeScoutsForTesting() {

        // Only add if there are none
        if (DatabaseUtils.queryNumEntries(databaseHelper.getWritableDatabase(),databaseHelper.TABLE_SCOUTS) > 0) return;

        databaseHelper.addScout("Tony","Smith","Leaders");
        databaseHelper.addScout("Anne","Taylor","Something else");
        databaseHelper.addScout("Mary","Bloggs","Leaders");
        databaseHelper.addScout("Sue","Franks","Leaders");
        databaseHelper.addScout("Alan","Jones","Cubs Scouts");
        databaseHelper.addScout("Belinda","Forrestor","Venturers");
    }
}

• Note some comments have been added