问题在于，当我运行我的代码时，应用程序正在停止并出现错误（java.lang.RuntimeException：无法启动活动ComponentInfo {com.twixt.pranav.pos / com.twixt.pranav.pos.View。 Activity.Cart}：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'java.io.File android.content.Context.getDatabasePath（java.lang.String）' 在代码中你可以看到我手动插入了一个数据
我见过类似的问题，但没有解决方法
这是我的课程
class FragmentCart : Fragment() {
private val database: SQLiteHelper = SQLiteHelper(activity)
override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View {
val mView = inflater!!.inflate(R.layout.fragment_cart, container, false)
if (database != null) {
val array_list = database.cartdatas()
Toast.makeText(activity, "" , Toast.LENGTH_SHORT).show()
}
return mView
}
Sqlite帮助程序类
public class SQLiteHelper extends SQLiteOpenHelper {
private static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "POS_SQLiteDatabase.db";
public SQLiteHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("create table CATEGORIES (ID INTEGER, NAME VARCHAR, COLOR VARCHAR)");
db.execSQL("create table CART (ITEM_ID INTEGER, ITEM_NAME VARCHAR, QUANTITY FLOAT, PRICE FLOAT, ONLINE INTEGER, DATE_ VARCHAR, FLAG INTEGER)");
db.execSQL("insert into CART values(321,'test name',1.1,50,1,'test date',0)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS CATEGORIES");
db.execSQL("DROP TABLE IF EXISTS CART");
onCreate(db);
}
public Boolean insetCartData(int item_id,String item_name,float quantity,float price,int online,String date, String flag){
SQLiteDatabase database = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put("ITEM_ID",item_id);
contentValues.put("ITEM_NAME",item_name);
contentValues.put("QUANTITY",quantity);
contentValues.put("PRICE",price);
contentValues.put("ONLINE",online);
contentValues.put("DATE_",date);
contentValues.put("FLAG",flag);
database.insert("CART",null,contentValues);
return true;
}
public ArrayList<String> cartdatas(){
/* SQLiteDatabase database=this.getReadableDatabase();
Cursor cursor=database.rawQuery("SELECT * FROM CART where ITEM_ID="+id+"",null);
return cursor;*/
ArrayList<String> array_list = new ArrayList<String>();
SQLiteDatabase db = this.getReadableDatabase();
Cursor res = db.rawQuery( "select * from CART", null );
res.moveToFirst();
while(res.isAfterLast() == false){
array_list.add(res.getString(res.getColumnIndex("ITEM_NAME")));
res.moveToNext();
}
return array_list;
}
}
和错误
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.twixt.pranav.pos, PID: 4602
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.twixt.pranav.pos/com.twixt.pranav.pos.View.Activity.Cart}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getDatabasePath(java.lang.String)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getDatabasePath(java.lang.String)' on a null object reference
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:292)
at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:262)
at com.twixt.pranav.pos.Controller.SQLiteHelper.cartdatas(SQLiteHelper.java:62)
at com.twixt.pranav.pos.View.Fragment.FragmentCart.onCreateView(FragmentCart.kt:38)
at android.app.Fragment.performCreateView(Fragment.java:2508)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1279)
at android.app.FragmentManagerImpl.addAddedFragments(FragmentManager.java:2407)
at android.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2186)
at android.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2142)
at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2043)
at android.app.FragmentManagerImpl.dispatchMoveToState(FragmentManager.java:3032)
at android.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:2979)
at android.app.FragmentController.dispatchActivityCreated(FragmentController.java:178)
at android.app.Activity.performCreate(Activity.java:7006)
at android.app.Activity.performCreate(Activity.java:6991)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
Application terminated.
答案 0 :(得分：2)
传递给SQLiteHelper的param为null。将片段附加到活动后，您可以获取活动上下文。
检查片段https://developer.android.com/guide/components/fragments.html
的生命周期更改
private val database: SQLiteHelper = SQLiteHelper(activity)
到
private var database: SQLiteHelper? = null
在Fragment中覆盖onAttach并初始化数据库
database = SQLiteHelper(activity)
https://developer.android.com/reference/android/app/Fragment.html#onAttach(android.app.Activity)