android - 有关null对象引用上android.view.View android.view.View.findViewById(int)的onCreateOptionsMenu的问题

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.material.components, PID: 11563
                  java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
                      at com.material.components.activity.dashboard.Dashboard.onCreateOptionsMenu(Dashboard.java:251)

这条线上的问题：

ui_hot = menu_hotlist.findViewById(R.id.hotlist_hot);

发生了什么？我试图在菜单中findViewById，但是android找不到对象引用。有人可以帮助我吗？谢谢。

完整代码：

Dashboard.java：

@Override
public boolean onCreateOptionsMenu(final Menu menu) {

    MenuInflater menuInflater = getMenuInflater();
    menuInflater.inflate(R.menu.menu_dashboard, menu);
    final View menu_hotlist = menu.findItem(R.id.action_notification).getActionView();
    ui_hot = menu_hotlist.findViewById(R.id.hotlist_hot); //Problem location
    updateHotCount(hot_number);
    new MyMenuItemStuffListener(menu_hotlist, "Show hot message") {
        @Override
        public void onClick(View v) {

        }
    };
    return super.onCreateOptionsMenu(menu);
}

menu_dashboard.xml：

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <item
        android:id="@+id/action_search"
        android:icon="@drawable/ic_search"
        android:orderInCategory="100"
        android:title="Search"
        app:showAsAction="always" />

    <item
        android:id="@+id/action_notification"
        android:actionLayout="@layout/action_bar_notifitcation_icon"
        app:showAsAction="always"
        android:icon="@drawable/ic_notifications"
        android:title="Notifications"
        />


    <item
        android:id="@+id/lessons_discuss_with_teacher"
        android:orderInCategory="100"
        android:title="Ask Teacher List"
        app:showAsAction="never" />
</menu>

action_bar_notification_icon.xml：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="wrap_content"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    android:gravity="center"
    android:layout_gravity="center"
    android:clickable="true"
    style="@android:style/Widget.ActionButton">

    <ImageView
        android:id="@+id/hotlist_bell"
        app:srcCompat="@drawable/ic_notifications"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:layout_margin="0dp"
        android:contentDescription="bell"
        />

    <TextView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/hotlist_hot"
        android:layout_width="wrap_content"
        android:minWidth="17sp"
        android:textSize="12sp"
        android:textColor="#ffffffff"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:text="@null"
        android:layout_alignTop="@id/hotlist_bell"
        android:layout_alignRight="@id/hotlist_bell"
        android:layout_marginRight="0dp"
        android:layout_marginTop="3dp"
        android:paddingBottom="1dp"
        android:paddingRight="4dp"
        android:paddingLeft="4dp"
        android:background="@drawable/rounded_square"/>
</RelativeLayout>

最佳答案

当应用程序尝试使用具有空值的对象引用时，抛出NullPointerException。


试试看

MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.menu_dashboard, menu);
    MenuItem menuITEM = menu.findItem(R.id.action_notification);
    View view         = menuITEM.getActionView();
    ui_hot            = view.findViewById(R.id.hotlist_hot);

费耶

您应该致电app:actionLayout