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java - 更新方法返回 null

问题描述

使用以下代码

SQLiteDatabase db;

MainData myDBHlpr = new MainData(getActivity());

String MMS = "(SELECT Name FROM UserData WHERE MessagesSent=(SELECT max(MessagesSent) FROM UserData))";
Log.d("Tager", MMS);
db.execSQL("UPDATE MainData SET MostMessagesSent = "+ MMS + " WHERE Data = MyData");

我在 db.execSQL 行出现错误...有人告诉我这是因为没有初始化 db 但我还应该怎么做？

这是错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.database.sqlite.SQLiteDatabase.execSQL(java.lang.String)' on a null object reference

at com.appmaster.akash.messageplus.Settings.UsageSettings.onCreateView(UsageSettings.java:44)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:2354)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1419)
at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1740)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1809)
at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:799)
at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2580)
at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2367)
at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2322)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2229)
at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:700)
at android.os.Handler.handleCallback(Handler.java:742)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:157)
at android.app.ActivityThread.main(ActivityThread.java:5571)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:745)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:635)

标签： javaandroidsqldatabasesqlite

解决方案

你得先打开SQLiteDatabase看Android Docs

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