Java Android应用程序写入SQLite数据库

我正在尝试在我的android应用程序上写入本地SQLlite数据库。 我在处理程序中创建了一个打开数据库的方法，但它正在使应用程序崩溃。 通过使用日志，我知道这是一行：

db = helper.getWritableDatabase();

这是导致坠机的原因，但我不知道为什么

下面是处理程序类：

public static final String TAG = "TableSubandObAssessment";

    public static final String KEY_TABLE = "assessments";

    private static final String KEY_APPOINTMENT_ID = "appointmentID";
    private static final String KEY_TYPE = "type";
    private static final String KEY_ASSESSMENT = "assessment";
    private static final String KEY_VARIABLE1 = "variable1";
    private static final String KEY_VARIABLE2 = "vriable2";
    private static final String KEY_VARIABLE3 = "vriable3";
    private static final String KEY_VARIABLE4 = "vriable4";


    private static DatabaseHelper helper;
    private static Context context;
    public static SQLiteDatabase db;


    static public void createTable(SQLiteDatabase db) {
        String createTbl = 
                "CREATE TABLE " + KEY_TABLE
                + " (" + KEY_APPOINTMENT_ID + " INTEGER PRIMARY KEY NOT NULL, " 
                + KEY_TYPE + " TEXT NOT NULL, " 
                + KEY_ASSESSMENT + " TEXT NOT NULL, "
                + KEY_VARIABLE1 + " TEXT, "
                + KEY_VARIABLE2 + " TEXT NOT NULL, "
                + KEY_VARIABLE3 + " TEXT, "
                + KEY_VARIABLE4 + " TEXT " + ");";
    db.execSQL(createTbl);
    }




    public static void dropTable(SQLiteDatabase db) {
        db.execSQL("DROP TABLE IF EXISTS " + KEY_TABLE);
        createTable(db);
    }   


    public TableSubandObAssessment(Context context) {
        this.context = context;
    }

    public static SQLiteDatabase open() throws SQLException {
        helper = new DatabaseHelper(context);
        db = helper.getWritableDatabase();
        return db;
    }

    public static void close() {
        helper.close();
    }


      public static long insertTableSubandObAssessment(int appointmentID, String type, String assessment, String variable1, String variable2, String variable3, String variable4) {
            ContentValues values = new ContentValues();
            values.put(KEY_APPOINTMENT_ID, appointmentID);
            values.put(KEY_TYPE, type);
            values.put(KEY_ASSESSMENT, assessment);
            values.put(KEY_VARIABLE1, variable1);
            values.put(KEY_VARIABLE2, variable2);
            values.put(KEY_VARIABLE3, variable3);
            values.put(KEY_VARIABLE4, variable4);
            return db.insert(KEY_TABLE, null, values);
        }

下面是日志：

08-07 14:34:55.806: E/AndroidRuntime(1995): FATAL EXCEPTION: main
08-07 14:34:55.806: E/AndroidRuntime(1995): java.lang.NullPointerException
08-07 14:34:55.806: E/AndroidRuntime(1995):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at com.ucl.pga.db.TableSubandObAssessment.open(TableSubandObAssessment.java:58)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at com.ucl.pga.db.Objective.Summary.saveToDb(Summary.java:965)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at com.ucl.pga.db.Objective.Summary$2$1.onClick(Summary.java:480)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at android.os.Looper.loop(Looper.java:137)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at android.app.ActivityThread.main(ActivityThread.java:4745)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at java.lang.reflect.Method.invokeNative(Native Method)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at java.lang.reflect.Method.invoke(Method.java:511)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-07 14:34:55.806: E/AndroidRuntime(1995):     at dalvik.system.NativeStart.main(Native Method)

---

传递给

SQLiteOpenHelper

构造函数的

Context

是

null