super(context，textviewid)；NullPointerException

Question

public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus);
    if (hasFocus) {
        ((DownloadFragment) downloads).refreshlist();
    }
}

public class MyAdapter extends ArrayAdapter<AdapterItem> {
    public MyAdapter(Context context, int textviewid) {
        super(context, textviewid); // NPE here
    }

    public void refreshlist() {
        items.clear();
        adapter = new MyAdapter(getActivity(), 0);
        // ...
    }
}

java.lang.NullPointerException
at android.widget.ArrayAdapter.init(ArrayAdapter.java:310)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:104)
at com.DownloadFragment$MyAdapter.<init>(DownloadFragment.java:142)
at com.DownloadFragment.refreshlist(DownloadFragment.java:479)
at com.MainActivity.onWindowFocusChanged(MainActivity.java:387)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onWindowFocusChanged(PhoneWindow.java:2466)
at android.view.View.dispatchWindowFocusChanged(View.java:7456)
at android.view.ViewGroup.dispatchWindowFocusChanged(ViewGroup.java:935)
at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:3264)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:153)
at android.app.ActivityThread.main(ActivityThread.java:5297)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
at dalvik.system.NativeStart.main(Native Method)

downloadfragment.java

Cursor a = mDbHelper.getEachDownload(2);

Answer 1

我相信这是因为您传递了一个空上下文。

尝试：

public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus);
    if (hasFocus) {
        ((DownloadFragment) downloads).refreshlist(this);
    }
}

downloadfragment.java

public class MyAdapter extends ArrayAdapter<AdapterItem> {
    public MyAdapter(Context context, int textviewid) {
        super(context, textviewid); // NPE here
    }

    public void refreshlist(Context context) {
        items.clear();
        adapter = new MyAdapter(context, 0);
        // ...
    }
}

public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus); 
    if (hasFocus) { 
       DownloadFragment.adapter.notifyDataSetChanged(); 
    } 
}