Android - NullPointerException时，试图实现适配器从片段

Question

我正在加载Fragment，然后Fragment创建Adapter。Adapter有一个interface。加载implements和Adapter的Fragment。启动mCallback时没有错误

适配器中的代码:

Scanlist mScanlist; 
ScanlistListener mCallback;
public Scanadapter(Activity a, ArrayList<Scanmodel> d, Resources resLocal){
    mActivity = a;
    mData = d;
    res = resLocal;
    sInflater = (LayoutInflater)mActivity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    mScanlist = (Scanlist) mActivity.getFragmentManager().findFragmentById(R.layout.scanlist);
    try {
        mCallback = (ScanlistListener) mScanlist;
        Log.v(TAG, String.valueOf(mCallback)+ " here");
    } catch (ClassCastException e) {
        throw new ClassCastException(mScanlist.toString()
                + " must implement ScanlistListener");
    }
}
private class OnItemClickListener implements OnClickListener{
    private int mPosition;
    OnItemClickListener(int position){
        mPosition = position;
    }
    @Override
    public void onClick(View v) {
        mCallback.scanlistclick(v, mPosition);
    }
}
public interface ScanlistListener{
    public void scanlistclick(View v, int i);
}

当我点击列表中的一个项目，我得到一个NullPointerException，但try statement不给error exception启动mCallback变量。

08-22 10:19:09.099: E/AndroidRuntime(8112): java.lang.NullPointerException
08-22 10:19:09.099: E/AndroidRuntime(8112):     at net.xxxxx.adapter.Scanadapter$OnItemClickListener.onClick(Scanadapter.java:140)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.view.View.performClick(View.java:4489)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.view.View$PerformClick.run(View.java:18803)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.os.Handler.handleCallback(Handler.java:730)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.os.Handler.dispatchMessage(Handler.java:92)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.os.Looper.loop(Looper.java:137)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at android.app.ActivityThread.main(ActivityThread.java:5455)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at java.lang.reflect.Method.invokeNative(Native Method)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at java.lang.reflect.Method.invoke(Method.java:525)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
08-22 10:19:09.099: E/AndroidRuntime(8112):     at dalvik.system.NativeStart.main(Native Method)

我做错了什么?

Answer 1

如果findFragmentByID没有找到它正在寻找的布局元素，它将返回null。然后将null赋值给mCallback。用强制类型转换赋值null不会触发异常。也许可以使用调试器逐步检查代码，并确保找到了视图，并且mCallback没有设置为null。