来自数据库的微调器 - 首先添加空值

Question

我有代码：

public class TesterActivity extends Activity {
public Context mContext;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    // just for this example:
    // create database table with an id field and a text field and add some data
    class MyDBHelper extends SQLiteOpenHelper {
        public MyDBHelper(Context context) {
            super(context, "someDB", null, 2);
        }
        @Override
        public void onCreate(SQLiteDatabase db) {
            db.execSQL("CREATE TABLE someTable (someIDF INTEGER, someTextF TEXT)");
        }
        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            db.execSQL("DROP TABLE IF EXISTS someTable");
            onCreate(db);
            db.execSQL("INSERT INTO someTable (someIDF, someTextF) VALUES (54, 'Some text')");
            db.execSQL("INSERT INTO someTable (someIDF, someTextF) VALUES (99, 'Some more text')");
            db.execSQL("INSERT INTO someTable (someIDF, someTextF) VALUES (173, 'Even more text')");
        }
    }
    SQLiteDatabase db = new MyDBHelper(this).getWritableDatabase();

    // get a cursor from the database with an "_id" field
    Cursor c = db.rawQuery("SELECT someIDF AS _id, someTextF FROM someTable", null);

    // make an adapter from the cursor
    String[] from = new String[] {"someTextF"};
    int[] to = new int[] {android.R.id.text1};
    SimpleCursorAdapter sca = new SimpleCursorAdapter(this, android.R.layout.simple_spinner_item, c, from, to);

    // set layout for activated adapter
    sca.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item); 

    // get xml file spinner and set adapter 
    Spinner spin = (Spinner) this.findViewById(R.id.spinner1);
    spin.setAdapter(sca);

    // set spinner listener to display the selected item id
    mContext = this;
    spin.setOnItemSelectedListener(new OnItemSelectedListener() {
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id){
            Toast.makeText(mContext, "Selected ID=" + id, Toast.LENGTH_LONG).show();
        }
        public void onNothingSelected(AdapterView<?> parent) {}
        });
    }
}

来自：Populating Spinner From SQLite Database Android

我想为这个空值添加，所以我做：

    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS someTable");
        onCreate(db);
        db.execSQL("INSERT INTO someTable (someIDF, someTextF) VALUES (54, ' ')");
        db.execSQL("INSERT INTO someTable (someIDF, someTextF) VALUES (54, 'Some text')");
        db.execSQL("INSERT INTO someTable (someIDF, someTextF) VALUES (99, 'Some more text')");
        db.execSQL("INSERT INTO someTable (someIDF, someTextF) VALUES (173, 'Even more text')");
    }

这向我显示微调器中的空值，但如果我想通过此微调器获取此微调器

TextView textView = (TextView)spinner.getSelectedView();
String result = textView.getText().toString();

我有错误：

java.lang.NullPointerException

这个错误总是 - 如果我选择了非空项目。

为什么？我可以使用其他方式添加空值，但是呢？

Answer 1

首先我看不到您是否在您的活动 onCreate 方法 ((TextView) findbyid(R.id.etc..)) 上定义了您的文本视图？如果那个没问题，那么，

在您的 spin.setOnItemSelectedListener 上，您可以使用：

String result = parent.getItemAtPosition(position).toString();

结果应该是“”。