游标 RawQuery 上的 NullPointerException - Android

Question

我正在尝试创建一个数据库，插入数据库，从数据库中提取并将内容（长）显示到 TextView。

我认为我正在正确地创建和插入数据库。在我的 getScore() 方法中，我得到了 NullPointerException。

也非常感谢任何有关代码结构的帮助！提前谢谢你。

DatabaseHelper.java

public class DatabaseHelper extends SQLiteOpenHelper { 

    SQLiteDatabase db;

    // Table columns names. 
    private static final String SCORE = "score"; 

    public DatabaseHelper(Context context) { 
        super(context, DB_NAME, null, DATABASE_VERSION); 
    }

    public SQLiteDatabase openDB() {
        db = this.getWritableDatabase();
        return db;
    }

    public long getScore(SQLiteDatabase db) {
        //Cursor c = db.rawQuery("SELECT " + SCORE + " FROM " + TABLE + " WHERE " + SCORE + " = " + 11 + ";", null);  //Line 45
        Cursor c = db.rawQuery("SELECT " + SCORE + " FROM " + TABLE + ";", null);  //Line 39
        long i = 0;
        if(c.getCount() == 0) {
            i = 333;
        } else if (c.getCount() == 1) {
            i = 444;
        } else {
            i = 888;
               /*c.moveToFirst();
               int columnIndex = c.getInt(c.getColumnIndex("SCORE"));
               if(columnIndex != -1) {
                   i = c.getLong(columnIndex);
               } else { 
                   i = 999; 
               }*/
        }
        c.close();
        return i;
    }

    //Insert new record.
    public long insert(long score, int percentage) {
        ContentValues values = new ContentValues();
        values.put(SCORE, score);
        values.put(PERCENTAGE, percentage);

        return db.insert(TABLE, null, values);
    }

    public synchronized void closeDB(SQLiteDatabase db) {
        if(db != null) {
            db.close();
        }
        super.close();
    }

    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE " + TABLE + " ("
                + RANK + " INTEGER PRIMARY KEY AUTOINCREMENT,"
                + SCORE + " LONG,"
                + PERCENTAGE + " INTEGER"
                + ");");
    }
}

Highscores.java

public class Highscores extends Activity {

    DatabaseHelper dh;
    SQLiteDatabase db;
    long scores;

    TextView score;

    @Override
    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.highscoresmain);

        dh = new DatabaseHelper(this);
        dh.openDB();
        long x = 11;
        int y = 22;

        TextView score = (TextView)findViewById(R.id.score);        
        TextView r1s = (TextView)findViewById(R.id.r1s);

        dh.insert(x, y);
        scores = dh.getScore(db);  //Line 56            
        score.setText("Score Column - TEST");
        r1s.setText("" + score);

        dh.closeDB(db);
    }
}

LogCat 输出

01-03 17:36:01.172: E/AndroidRuntime(1941): FATAL EXCEPTION: main
01-03 17:36:01.172: E/AndroidRuntime(1941): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test/com.example.test.Highscores}: java.lang.NullPointerException
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.os.Handler.dispatchMessage(Handler.java:99)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.os.Looper.loop(Looper.java:137)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.app.ActivityThread.main(ActivityThread.java:5039)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at java.lang.reflect.Method.invokeNative(Native Method)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at java.lang.reflect.Method.invoke(Method.java:511)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at dalvik.system.NativeStart.main(Native Method)
01-03 17:36:01.172: E/AndroidRuntime(1941): Caused by: java.lang.NullPointerException
01-03 17:36:01.172: E/AndroidRuntime(1941):     at com.example.test.DatabaseHelper.getScore(DatabaseHelper.java:39)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at com.example.test.Highscores.onCreate(Highscores.java:56)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.app.Activity.performCreate(Activity.java:5104)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
01-03 17:36:01.172: E/AndroidRuntime(1941):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
01-03 17:36:01.172: E/AndroidRuntime(1941):     ... 11 more

编辑：新的getScore() 方法。

public long getScore() {
    Cursor c = db.rawQuery("SELECT " + SCORE + " FROM " + TABLE + " WHERE " + SCORE + " = " + 11 + ";", null);
    //Cursor c = db.rawQuery("SELECT " + SCORE + " FROM " + TABLE + ";", null);
    long i = 0;
    if(c.getCount() == 0) {
        i = 333;
    } else if (c.getCount() == 1) {
        i = 444;
    } else if (c.getCount() == 2) {
        i = 555;
    } else {
        i = 666;
           /*c.moveToFirst();
           int columnIndex = c.getInt(c.getColumnIndex("SCORE"));
           if(columnIndex != -1) {
               i = c.getLong(columnIndex);
           } else { 
               i = 999; 
           }*/
    }
    c.close();
    return i;
}

这仍然返回值为 666 的 i。

Answer 1

您在这里覆盖了您的本地 db 值：public long getScore(SQLiteDatabase db) { ...。删除该参数，它应该可以工作。

Answer 2

您传递给数据库类的数据库从未初始化并且为空。 scores = dh.getScore(db); //Line 56这里的db变量是一个成员变量，你应该在传递给这个方法之前初始化它。

Answer 3

查看documentation，也许这是你的问题。

SQL 字符串不能是 ;终止