嘗試插入SQLite時發生NullPointerException-Android

Question

遵循Android標簽的任何人都可能對我很熟悉。 我很難為自己的高分實現SQLite數據庫。 這也是我第一次使用SQLite。

我正在嘗試僅插入兩個變量-int和long。 我的insert()方法無法正常工作。 除了上面討論的內容外，任何其他建議也應受到贊賞。

預先感謝您的幫助。

Highscores.java

public class Highscores extends Activity {

    DatabaseHelper dh;
    SQLiteDatabase db;

    @Override
    public void onCreate(Bundle savedInstanceState) {

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

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

        TextView rank = (TextView)findViewById(R.id.rank);
        TextView percentage = (TextView)findViewById(R.id.percentage);
        TextView score = (TextView)findViewById(R.id.score);

        TextView r1r = (TextView)findViewById(R.id.r1r);
        TextView r1p = (TextView)findViewById(R.id.r1p);
        TextView r1s = (TextView)findViewById(R.id.r1s);

        dh.insert(x, y, db);  //Line 55
        scores = dh.getScore(db);
        percentages = dh.getPercentage(db);

        rank.setText("Rank Column - TEST");
        percentage.setText("Percentage Column - TEST ");
        score.setText("Score Column - Test");
        r1r.setText("test..rank");
        r1p.setText("" + percentages);
        r1s.setText("test..score");

        table = (TableLayout)findViewById(R.id.tableLayout);

        dh.closeDB(db);
    }
}

DatabaseHelper.java

public class DatabaseHelper extends SQLiteOpenHelper { 

    SQLiteDatabase db;

    // Table columns names. 
    private static final String RANK = "_id"; 
    private static final String SCORE = "score"; 
    private static final String PERCENTAGE = "percentage";

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

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

    public int getPercentage(SQLiteDatabase db) {
        //Cursor c = db.rawQuery("SELECT " + PERCENTAGE + " FROM " + TABLE + " WHERE " + PERCENTAGE + " = " + 22 + ";", null);
        Cursor c = db.rawQuery("SELECT " + PERCENTAGE + " FROM " + TABLE + ";", null);
        int 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("PERCENTAGE"));
               if(columnIndex != -1) {
                   i = c.getInt(columnIndex);
               } else { 
                   i = 999; 
               }*/
        }
        c.close();
        return i;
   }

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

        return db.insert(TABLE, null, values);  //Line 120
    }

    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"
                + ");");
}

LogCat輸出

01-03 17:09:18.232: E/AndroidRuntime(1712): FATAL EXCEPTION: main
01-03 17:09:18.232: E/AndroidRuntime(1712): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test/com.example.test.Highscores}: java.lang.NullPointerException
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.os.Handler.dispatchMessage(Handler.java:99)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.os.Looper.loop(Looper.java:137)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.app.ActivityThread.main(ActivityThread.java:5039)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at java.lang.reflect.Method.invokeNative(Native Method)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at java.lang.reflect.Method.invoke(Method.java:511)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at dalvik.system.NativeStart.main(Native Method)
01-03 17:09:18.232: E/AndroidRuntime(1712): Caused by: java.lang.NullPointerException
01-03 17:09:18.232: E/AndroidRuntime(1712):     at com.example.test.DatabaseHelper.insert(DatabaseHelper.java:120)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at com.example.test.Highscores.onCreate(Highscores.java:55)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.app.Activity.performCreate(Activity.java:5104)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
01-03 17:09:18.232: E/AndroidRuntime(1712):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
01-03 17:09:18.232: E/AndroidRuntime(1712):     ... 11 more

Answer 1

在這行上，打開數據庫：

dh.openDB(db);

這會將數據庫的成員變量存儲在DatabaseHelper.db （但沒有其他地方，尤其是在Highscores.db ）。 然后，您致電：

dh.insert(x, y, db);

使用Highscores的null db對象。 最后，在insert() ，使用此null db對象：

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

相反，您應該使用以下代碼作為insert()方法：

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

    return db.insert(TABLE, null, values); // This will use the member variable
}

然后，調用dh.insert(x, y); 代替。

您還應該將openDB(SQLiteDatabase db)更改為openDB()以便它可以寫入正確的數據庫。 就目前而言，它只會覆蓋局部變量的值（一旦函數作用域完成，它就不會改變）。

Answer 2

代替

dh.openDB(db);

說

db = dh.openDB(db);

（盡管openDB方法實際上不需要參數）。 您對openDB調用未將數據庫對象引用存儲在db ，因此當您開始調用dh.insert(..., db);時，它為NULL dh.insert(..., db); 。