Question

这是MainActivity的onCreate方法，它位于LoginActivity之后。

public class MainActivity extends AppCompatActivity {

GridView medicineGrid;
ArrayList<Medicine> medicineArrayList;
GridAdapter adapter;
SQLiteHelper sqLiteHelper;    

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    medicineGrid = (GridView) findViewById(R.id.gv_med_list);
    try {
        sqLiteHelper = new SQLiteHelper(MainActivity.this);
    }catch (NullPointerException e){
        e.printStackTrace();
    }        
    medicineArrayList = new ArrayList<Medicine>();

    medicineArrayList = sqLiteHelper.getAllMedicine();
    adapter = new GridAdapter(MainActivity.this, medicineArrayList);
    medicineGrid.setAdapter(adapter);
}

当MainActivity被执行时，应用程序将被NullPointer异常崩溃，如下所示：

致命的例外：主要处理：com.example.amitava.medikart，PID：4662 java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.amitava.medikart / com.example.amitava.medikart.MainActivity}：java.lang.NullPointerException：尝试调用虚方法'android.content.res.Resources空对象引用上的android.content.Context.getResources（）' 在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2646）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2707）在android.app.ActivityThread.-wrap12（ActivityThread.java）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1460）在android.os.Handler.dispatchMessage（Handler.java:102）在android.os.Looper.loop（Looper.java:154）在android.app.ActivityThread.main（ActivityThread.java:6077） at java.lang.reflect.Method.invoke（Native Method）在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:866）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:756）引起：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'android.content.res.Resources android.content.Context.getResources（）' 在com.example.amitava.medikart.SQLiteHelper.onCreate（SQLiteHelper.java:57）在android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked（SQLiteOpenHelper.java:251）在android.database.sqlite.SQLiteOpenHelper.getReadableDatabase（SQLiteOpenHelper.java:187）在com.example.amitava.medikart.SQLiteHelper.getAllMedicine（SQLiteHelper.java:170）在com.example.amitava.medikart.MainActivity.onCreate（MainActivity.java:40）在android.app.Activity.performCreate（Activity.java:6662）在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1118）在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2599）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2707）在android.app.ActivityThread.-wrap12（ActivityThread.java）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1460）在android.os.Handler.dispatchMessage（Handler.java:102）在android.os.Looper.loop（Looper.java:154）在android.app.ActivityThread.main（ActivityThread.java:6077） at java.lang.reflect.Method.invoke（Native Method）在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:866）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:756）申请已终止。

这是我写的SQLiteOpenHelper类，请检查我是否在这里做错了 -

public class SQLiteHelper extends SQLiteOpenHelper{

//Database details
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "MedicineDatabase.db";

//Table details within the database
public static final String TABLE_NAME = "MEDICINE";
public static final String COLUMN_ID = "ID";
public static final String COLUMN_MEDICINE_NAME = "NAME";
public static final String COLUMN_CATEGORY = "CATEGORY";
public static final String COLUMN_DESCRIPTION = "DESCRIPTION";
public static final String COLUMN_IMAGE_BITMAP = "IMAGE";
public static final String COLUMN_STRENGTH = "STRENGTH";
public static final String COLUMN_PRICE = "PRICE";

public Context context;

public SQLiteHelper(Context context){
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}

@Override
public void onCreate(SQLiteDatabase sqLiteDatabase) {
    //Create New Table
    sqLiteDatabase.execSQL("Create Table " + TABLE_NAME + "(" + COLUMN_ID + " Integer Primary Key Autoincrement, " +
            COLUMN_MEDICINE_NAME + " Varchar, " + COLUMN_CATEGORY + " Varchar, " + COLUMN_DESCRIPTION + " Varchar, " +
            COLUMN_STRENGTH + " Varchar, " + COLUMN_PRICE + " Varchar, " + COLUMN_IMAGE_BITMAP + " Blob);");
    //Insert static data into the table which would work as a API for the dummy project
    //First row
    Bitmap bitmap1 = BitmapFactory.decodeResource(context.getResources(), R.drawable.atorvastatin);
    byte[] picture1 = getBytes(bitmap1);
    addEntry("Atorvastatin", "Cardiac", "Atorvastatin is used along with a proper diet to help" +
            " lower \"bad\" cholesterol and fats (such as LDL, triglycerides) and raise \"good\" cholesterol (HDL) in the blood." +
            " It belongs to a group of drugs known as \"statins.\" It works by reducing the amount of cholesterol made by the liver. " +
            "Lowering \"bad\" cholesterol and triglycerides and raising \"good\" cholesterol decreases the risk of heart disease and helps" +
            " prevent strokes and heart attacks.", "10 mg", "610", picture1);
    //Second Row
    Bitmap bitmap2 = BitmapFactory.decodeResource(context.getResources(), R.drawable.amlodipine_besylate_atenolol);
    byte[] picture2 = getBytes(bitmap2);
    addEntry("Amlodipine Besylate Atenolol", "Cardiac", "Amlodipine ATenolol Tablet is used for Hypertension, " +
            "Elevated blood pressure, Blood pressure, Chest pain, Coronary artery disease, High blood pressure and other conditions. Amlodipine" +
            " ATenolol Tablet works by inhibiting the influx of calcium ions into vascular smooth muscle and cardiac muscles; blocking receptors" +
            " resulting in slowing of heart rate and reducing oxygen demand", "50 mg", "350", picture2);
    //Third Row...}

请帮助我理解为什么我得到NullPointerException来调用getResources（）方法

在SQLiteOpenHelper类

0 个答案: