android - 无法访问 fragment 内的 Textview

Question

我有一个 Activity ，上面有一个 fragment ，里面有一个 TextView。在 CourseFragment.java 中，我创建了一个更改 TextView 文本的简单方法。但是，当我尝试从 Activity 调用 fragment 的方法时，它说 TextView 对象为空。这是我的一些代码:

CourseFragment.java

public class CourseFragment extends Fragment {

private Subject subject;
View rootView;
private TextView title;

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

    title = (TextView) rootView.findViewById(R.id.course_title);

    return rootView;
}

public void setTitle() {
    title.setText("My Title");
}

}

这是在我的 Activity 中创建新 fragment 并放置在正确位置的代码:

CourseFragment fragment = new CourseFragment();
FragmentManager fm = getFragmentManager();
FragmentTransaction ft = fm.beginTransaction();
ft.replace(R.id.course1, fragment);
fragment.setTitle();
ft.addToBackStack(null);
ft.commit();

错误来自 fragment 中的 setTitle() 方法

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference

这看起来很简单，但我不明白为什么我总是得到 NPE。我查看了关于此的类似问题并尝试了所有可接受的解决方案但无济于事。有什么想法吗？

提前致谢

编辑:这里是根据要求的更多代码:

fragment_course.xml 的开头

<LinearLayout 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/course_fragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.stagenda.stagenda.CourseFragment"
    android:padding="16dp"
    android:orientation="vertical"
    android:background="@android:color/background_light">

    <RelativeLayout
        android:layout_width="match_parent"
        tools:ignore="UselessParent"
        android:background="@color/colorAccent"
        android:padding="16dp"
        android:layout_height="wrap_content"
        android:id="@+id/course_container">
<TextView
        android:text="Course Given Title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/course_given_title"
        android:textColor="@android:color/background_light"
        android:textSize="18sp" />

    <TextView
    android:text="Course Title"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/course_title"
    android:layout_below="@+id/course_given_title"
    android:textColor="@android:color/background_light" />

    <TextView
        android:text="Course Code"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/course_code"
        android:textColor="@android:color/background_light"
        android:layout_toRightOf="@+id/course_title"
        android:layout_below="@+id/course_given_title"
        android:layout_alignParentEnd="false"
        android:layout_marginStart="10dp" />

完整错误:

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.stagenda.stagenda, PID: 1654
                  java.lang.IllegalStateException: Could not execute method for android:onClick
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
                      at android.view.View.performClick(View.java:5639)
                      at android.view.View$PerformClick.run(View.java:22387)
                      at android.os.Handler.handleCallback(Handler.java:751)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6088)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
                   Caused by: java.lang.reflect.InvocationTargetException
                      at java.lang.reflect.Method.invoke(Native Method)
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
                      at android.view.View.performClick(View.java:5639) 
                      at android.view.View$PerformClick.run(View.java:22387) 
                      at android.os.Handler.handleCallback(Handler.java:751) 
                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                      at android.os.Looper.loop(Looper.java:154) 
                      at android.app.ActivityThread.main(ActivityThread.java:6088) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                      at com.stagenda.stagenda.CourseFragment.setTitle(CourseFragment.java:34)
                      at com.stagenda.stagenda.MainPage.confirmAddCourse(MainPage.java:435)
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
                      at android.view.View.performClick(View.java:5639) 
                      at android.view.View$PerformClick.run(View.java:22387) 
                      at android.os.Handler.handleCallback(Handler.java:751) 
                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                      at android.os.Looper.loop(Looper.java:154) 
                      at android.app.ActivityThread.main(ActivityThread.java:6088) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 
I/Process: Sending signal. PID: 1654 SIG: 9
Disconnected from the target VM, address: 'localhost:8612', transport: 'socket'

explanation of layout (image)

Answer 1

采用这种方法。

 Bundle bundle = new Bundle();
    bundle.putString("title", "whatever you want to pass");

        CourseFragment fragment = new CourseFragment();
        fragment.setArguments(bundle);
        FragmentManager fm = getFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();
        ft.replace(R.id.course1, fragment);
        ft.addToBackStack(null);
        ft.commit();



    public class CourseFragment extends Fragment {

    private Subject subject;
    View rootView;
    private TextView title;
    Bundle bundle;

      @Override
        public void onCreate(@Nullable Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);

            bundle = getArguments();
         }

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

        title = (TextView) rootView.findViewById(R.id.course_title);

        String text = bundle.getString("title");
        title.setText(text);

        return rootView;
    }


    }