Question

我想填写一个列表，并从活动中打开的数据库中执行此操作。我创建了一个自定义适配器，'helper'类调用listview包装器的另一个'helper'类。在那个listview包装器中，我想打开另一个（不同的）数据库，但是我得到了一个nullpointer异常。

所以在我打电话的活动中：

 adapter = new ListViewAdapter(this, ListViewAdapter.TPL_HISTORY, historydatasource.getAllCalls());
             list.setAdapter(adapter);

比historydatasource类打开一个listviewwrapper（helper）类，它处理我为listview创建的模板。在那里我想打开另一个数据库，但我得到一个nullpointer异常。

有人知道我该怎么做吗？

        DB_DataSource cardsdatasource=new DB_DataSource(myContext);
cardsdatasource.open(); (GIVES NULLPOINTER, because myContext=null)

有没有办法获得上下文？

错误：

03-20 14:03:51.919: E/AndroidRuntime(15492): FATAL EXCEPTION: main
03-20 14:03:51.919: E/AndroidRuntime(15492): java.lang.NullPointerException
03-20 14:03:51.919: E/AndroidRuntime(15492):    at com.myapp.testapp.ListViewWrapper.populateFrom(ListViewWrapper.java:91)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at com.myapp.testapp.ListViewAdapter.getView(ListViewAdapter.java:121)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.widget.AbsListView.obtainView(AbsListView.java:1536)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.widget.ListView.makeAndAddView(ListView.java:1793)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.widget.ListView.fillDown(ListView.java:718)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.widget.ListView.fillFromTop(ListView.java:775)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.widget.ListView.layoutChildren(ListView.java:1646)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.widget.AbsListView.onLayout(AbsListView.java:1366)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.view.View.layout(View.java:7175)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.widget.RelativeLayout.onLayout(RelativeLayout.java:912)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.view.View.layout(View.java:7175)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.widget.FrameLayout.onLayout(FrameLayout.java:338)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.view.View.layout(View.java:7175)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.widget.FrameLayout.onLayout(FrameLayout.java:338)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.view.View.layout(View.java:7175)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.view.ViewRoot.performTraversals(ViewRoot.java:1146)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.view.ViewRoot.handleMessage(ViewRoot.java:1866)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.os.Looper.loop(Looper.java:123)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at android.app.ActivityThread.main(ActivityThread.java:3687)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at java.lang.reflect.Method.invokeNative(Native Method)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at java.lang.reflect.Method.invoke(Method.java:507)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
03-20 14:03:51.919: E/AndroidRuntime(15492):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

这确实是一个上下文问题，我将来自CustomAdapter的activity.getApplicationContext（）传递给包装器，并且已经解决了。

Answer 2

我制作DataSource课程的方式如下：

public class StudyManagerDataSource {

    public static final String LOG_TAG = "studymanager";

    SQLiteOpenHelper dbHelper;
    SQLiteDatabase database;        

    // --------------------------------------------------------------------------------------------

    public StudyManagerDataSource(Context context) {
        dbHelper = new DBOpenHelper(context);

    }

    // --------------------------------------------------------------------------------------------

    public void open() {
        database = dbHelper.getWritableDatabase();
        Log.i(LOG_TAG, "Database Opened");
    } // end method open

    public void close() {
        Log.i(LOG_TAG, "Database Closed");
        dbHelper.close();
    } // end method close
    /*
    Other methods for manipulating tables goes here...
       */    
}

告诉我它是否解决了你的问题！

Nullpointer database.open（），getwritabledatabase

2 个答案: