将数据库记录加载到微调器中时出错

Question

我正在为我的A2级计算机科学课程编写应用程序，并且在尝试将数据从数据库表加载到微调器时遇到错误。 我希望在微调器中包含所有领导者的ID，姓名和姓氏，但是我遇到了一个错误。 这是我为数据库处理程序编写的代码：

    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;
}

我之所以使用2d数组，是因为稍后，我希望能够返回并检索有关该领导者的更多详细信息-但是，这将在不同的屏幕上，因此在不同的类中。 因此，我也希望能够单独保留所有细节。

这是我编写的将所有值放入微调器的代码：

    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);
}

这些是出现在LogCat中的错误详细信息：

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) 

有谁知道可能出了什么问题？ 这是我第一次在Android Studio，Java或XML中工作，而且我必须自学所有这些内容，因此对您的帮助将不胜感激。 谢谢。

Answer 1

我相信使用Cursor适配器 （例如SimpleCursorAdapter）会遇到的问题更少。 Cursor Adpaters设计用于处理游标，因此无需为Array适配器创建中间Array。 他们还将ID传递给onItemSelected方法。 但是，它们确实需要一个名为_id的列作为id列（下面的示例显示了即使id列使用其他名称也可以做到这一点）。

例

以下示例基于您的代码。 它选择在部分列中具有“ 领导者”的侦察员来填充微调器。 它还包括一个onItemSelected侦听器，当选择侦察员时，该侦听器将通过吐司显示侦察员的详细信息。

• 为了方便/解释/演示使用多列，已使用布局simple_list_item_2。

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");
    }
}

• 请注意，已添加一些注释