每当我尝试从 Fragment java 文件中更改片段的某些属性时，应用程序就会崩溃

Question

所以基本上我面临的问题是，每当我尝试为某些视图分配 .settext() 等值时，我的应用程序就会崩溃。

基本上，我从 Main activity 创建了一个导航抽屉。在导航抽屉中，我有两个菜单选项，单击每个选项都会打开一个片段。此外，由于第一个片段也是主屏幕，因此每当我打开 Main activity.

时，我都会启动第一个片段

现在，第二个片段是我面临的问题。我有一个带有一些视图的片段布局以及一个片段 java 文件。每当我尝试从 java 文件中为这些视图分配一些值时，应用程序就会崩溃。我已经粘贴了我的片段 xml 文件和 java 文件的代码以及下面的错误。

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".ProfileFragment">

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

        <RelativeLayout
            android:id="@+id/up"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="1.4">

            <com.scwang.wave.MultiWaveHeader
                app:mwhVelocity="2"
                app:mwhStartColor="@color/colorAccent"
                app:mwhCloseColor="@color/colorPrimaryDark"
                app:mwhGradientAngle="-90"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:id="@+id/waveview"
                />


            <TextView
                android:id="@+id/name_display"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_centerHorizontal="true"
                android:layout_margin="10dp"
                android:text="User"
                android:textColor="#ffff"
                android:textSize="25sp" />

        </RelativeLayout>

      
    </LinearLayout>
</FrameLayout>

我的 java 文件是：

public class ProfileFragment extends Fragment {

    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    public ProfileFragment() {
        // Required empty public constructor
    }

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment ProfileFragment.
     */
    // TODO: Rename and change types and number of parameters
    public static ProfileFragment newInstance(String param1, String param2) {
        ProfileFragment fragment = new ProfileFragment();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);

        }
    final TextView profile_name = getActivity().findViewById(R.id.name_display);
    profile_name.setText("Hi");
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_profile, container, false);
    }

}

我得到的错误是：

2021-01-27 15:31:22.641 29226-29226/com.appname.app E/AndroidRuntime: FATAL EXCEPTION: main Process: com.appname.app, PID: 29226 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference at com.appname.app.ProfileFragment.onCreate(ProfileFragment.java:71) at androidx.fragment.app.Fragment.performCreate(Fragment.java:2586) at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:838) at androidx.fragment.app.FragmentTransition.addToFirstInLastOut(FragmentTransition.java:1197) at androidx.fragment.app.FragmentTransition.calculateFragments(FragmentTransition.java:1080) at androidx.fragment.app.FragmentTransition.startTransitions(FragmentTransition.java:119) at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1866) at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1824) at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727) at androidx.fragment.app.FragmentManagerImpl.run(FragmentManagerImpl.java:150) at android.os.Handler.handleCallback(Handler.java:883) at android.os.Handler.dispatchMessage(Handler.java:100) at android.os.Looper.loop(Looper.java:221) at android.app.ActivityThread.main(ActivityThread.java:7542) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)

所以我猜错误大概在：

  profile_name.setText("Hi");

为简单起见，我删除了不必要的代码。

Answer 1

您在此处定义了代码：

  @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);

        }
    final TextView profile_name = getActivity().findViewById(R.id.name_display);
    profile_name.setText("Hi");
    }

但这不是用于片段的正确生命周期方法，相反，您应该覆盖并使用 onViewCreated:

@Override
public void onViewCreated(@NotNull View view, @org.jetbrains.annotations.Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    // your code here
    
}

更多信息，但具体来说：

The onCreate() method in a Fragment is called after the Activity's onAttachFragment() but before that Fragment's onCreateView().

所以此时，还没有视图，这就是您的代码崩溃的原因

每当我尝试从 Fragment java 文件中更改片段的某些属性时，应用程序就会崩溃

App crashes whenever I try to change some attribute of a fragment from inside the Fragment java file

android

android-fragments

navigation-drawer