NullPointerException in FragmentTransaction

Question

我有一个框架输出和一个LinearLayout在MainActivity布局。线性布局由7幅图像组成。我要在点击这些图片的时候换一些片段。在其中一个片段中，我从json获得值，并将它们添加到2个旋转器中。当我第一次将片段从这个片段更改为任何其他片段时，它工作得很好。但是，当我第二次回到这个片段，然后改变片段，应用程序崩溃与NPE。我的MainActivity是不是做错了什么？

Logcat:

FATAL EXCEPTION: main
Process: com.rishta.rishtabliss, PID: 26506
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
at android.view.LayoutInflater.from(LayoutInflater.java:228)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:178)
at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:163)
at com.rishta.rishtabliss.fragment.FragmentTwo$1.onResponse(FragmentTwo.java:81)
at com.rishta.rishtabliss.fragment.FragmentTwo$1.onResponse(FragmentTwo.java:64)
at com.android.volley.toolbox.StringRequest.deliverResponse(StringRequest.java:60)
at com.android.volley.toolbox.StringRequest.deliverResponse(StringRequest.java:30)
at com.android.volley.ExecutorDelivery$ResponseDeliveryRunnable.run(ExecutorDelivery.java:99)
at android.os.Handler.handleCallback(Handler.java:746)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5443)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
    <android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/main_content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fitsSystemWindows="true"
        tools:context=".activity.MainActivity">

        <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

            <LinearLayout
                android:id="@+id/llfooter"
                android:background="#ffffff"
                android:layout_alignParentBottom="true"
                android:orientation="horizontal"
                android:layout_width="match_parent"
                android:layout_height="45dp">

                <ImageView
                    android:id="@+id/img1"
                    android:layout_weight="1"
                    android:src="@drawable/number_one"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:onClick="selectFrag"/>
                <ImageView
                    android:id="@+id/img2"
                    android:layout_weight="1"
                    android:src="@drawable/number_two"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:onClick="selectFrag"/>
                <ImageView
                    android:id="@+id/img3"
                    android:layout_weight="1"
                    android:src="@drawable/number_three"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:onClick="selectFrag"/>
                <ImageView
                    android:id="@+id/img4"
                    android:layout_weight="1"
                    android:src="@drawable/number_four"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:onClick="selectFrag"/>
                <ImageView
                    android:id="@+id/img5"
                    android:layout_weight="1"
                    android:src="@drawable/number_five"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:onClick="selectFrag"/>
                <ImageView
                    android:id="@+id/img6"
                    android:layout_weight="1"
                    android:src="@drawable/number_six"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:onClick="selectFrag"/>
                <ImageView
                    android:id="@+id/img7"
                    android:layout_weight="1"
                    android:src="@drawable/number_seven"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:onClick="selectFrag"/>

            </LinearLayout>

            <FrameLayout
                android:id="@+id/frame_main"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_alignParentTop="true"
                android:layout_above="@+id/llfooter" />

        </RelativeLayout>
    </android.support.design.widget.CoordinatorLayout>

主要活动：

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (findViewById(R.id.frame_main) != null) {
            if (savedInstanceState != null) {
                return;
            }

            FragmentOne firstFragment = new FragmentOne();
            getSupportFragmentManager().beginTransaction().add(R.id.frame_main, firstFragment).commit();
        }


    } //End OnCreate Method


    public void selectFrag(View view) {
        FragmentManager fm = getSupportFragmentManager();

        Fragment currentFragment = fm.findFragmentById(R.id.frame_main);
        Fragment newFragment = null;

        switch (view.getId()) {
            case R.id.img1:
                if (currentFragment instanceof FragmentOne) {
                    return;
                }
                newFragment = new FragmentOne();
                break;
            case R.id.img2:
                if (currentFragment instanceof FragmentTwo) {
                    return;
                }
                newFragment = new FragmentTwo();
                break;
            case R.id.img3:
                if (currentFragment instanceof FragmentThree) {
                    return;
                }
                newFragment = new FragmentThree();
                break;
            case R.id.img4:
                if (currentFragment instanceof FragmentFour) {
                    return;
                }
                newFragment = new FragmentFour();
                break;
            case R.id.img5:
                if (currentFragment instanceof FragmentFive) {
                    return;
                }
                newFragment = new FragmentFive();
                break;
            case R.id.img6:
                if (currentFragment instanceof FragmentSix) {
                    return;
                }
                newFragment = new FragmentSix();
                break;
            case R.id.img7:
                if (currentFragment instanceof FragmentSeven) {
                    return;
                }
                newFragment = new FragmentSeven();
                break;
        }

        FragmentTransaction fragmentTransaction = fm.beginTransaction();
        fragmentTransaction.replace(R.id.frame_main, newFragment);
        fragmentTransaction.commit();
    }


}

片段：

public class FragmentTwo extends Fragment  {

        private Spinner religion, subreligion, mtoungue;
        private List<String> religionlist;

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_two, container, false);

        }

        @Override
        public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
            super.onViewCreated(view, savedInstanceState);

            religion = (Spinner)view.findViewById(R.id.spinner_religion);
            fetchlist();

        }

        private void fetchlist() {

            StringRequest stringRequest = new StringRequest(Request.Method.GET, URLUtils.SIGNUP_COTAIN_URL, new Response.Listener<String>() {

                @Override
                public void onResponse(String response) {
                    Log.e("ResponceFragmentTwo","***********************"+response);
                    try {


                        religionlist = new ArrayList<String>();

                        JSONObject jo = new JSONObject(response);
                        JSONArray arr = jo.getJSONArray("all_religion");
                        for (int i = 0; i < arr.length(); i++) {
                            JSONObject obj = arr.getJSONObject(i);
                            religionlist.add(obj.getString("name"));
                        }

                        religion.setAdapter(new ArrayAdapter<String>(getActivity(), R.layout.spinner_item, religionlist));



                    } catch (JSONException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }

                }
            }, new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    Toast.makeText(getActivity(), "VolleyError" + error.toString(), Toast.LENGTH_LONG).show();
                }


            }) {



            };

            RequestQueue requestQueue = Volley.newRequestQueue(getActivity());
            requestQueue.add(stringRequest);

        }


    }

Answer 1

在深入研究代码之后，我确信您的问题的核心是getActivity()在交付响应时返回null。这很可能是因为片段不再附加到您的活动中(也就是说，您已经用另一个片段调用了replace )。

最好在调用null之前检查setAdapter (虽然我不知道这是否会对您的功能产生任何不期望的后果)：

if (getActivity() != null) {
    religion.setAdapter(new ArrayAdapter<String>(getActivity(), R.layout.spinner_item, religionlist));
}