FindViewById返回null？

Question

I am trying to populate a listView from an array. However, when I try and find the listview in my java code, the findViewById function returns null.

Here is my XML:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/androidbg_dark"
    tools:context=".DiaryActivity" >

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

        <ListView
            android:id="@+id/lstAppts"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_weight="5"
            android:textColor="#FFFFFF" ></ListView>

        <LinearLayout
            android:layout_width="match_parent"
            android:orientation="horizontal"
            android:layout_weight="1" 
            android:layout_height="64dp">

            <Button
                android:id="@+id/btnOther"
                style="?android:attr/buttonBarButtonStyle"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_margin="2dip"
                android:layout_weight="1"
                android:background="@drawable/menu_button"
                android:onClick="newAppointment"
                android:textColor="#FFFFFF" />

            <Button
                android:id="@+id/btnOther2"
                style="?android:attr/buttonBarButtonStyle"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_margin="2dip"
                android:layout_weight="1"
                android:background="@drawable/menu_button"
                android:onClick="newAppointment"
                android:textColor="#FFFFFF" />

            <Button
                android:id="@+id/btnNewAppt"
                style="?android:attr/buttonBarButtonStyle"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_margin="2dip"
                android:layout_weight="1"
                android:background="@drawable/menu_button"
                android:onClick="newAppointment"
                android:text="@string/newappt"
                android:textColor="#FFFFFF" />
        </LinearLayout>

    </LinearLayout>

</RelativeLayout>

And here is the offending code:

ListView lstAppts = (ListView) findViewById(R.id.lstAppts);
String[] data = { "Appointment 1", "Appointment 2", "Appointment 3" };

ArrayAdapter adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, data);
lstAppts.setAdapter(adapter);

To my knowledge, I am referencing the listview correctly. It is named 'lstAppts' in my XML, and in the findViewById function, I find it using R.id.lstAppts . However when I debug the code with breakpoints, I can clearly see the the lstAppts object is null . Why is this?

Answer 1

It seems like you're trying to call findViewById() on constructor. In Android you should use onCreate() method to init your Activity instead of constructor.

Answer 2

Most likely you didn't inflate the correct layout on the onCreate method. Check the setContentView method and make sure that it match the layout name of your "listView".

Answer 3

If you are using listview in dialog,pop-up, etc where you have a parameter Context context type, you must use findViewById like this.

ListView lstAppts = (ListView) context.findViewById(R.id.lstAppts);

Or if you have: View layout

ListView lstAppts = (ListView) layout.findViewById(R.id.lstAppts);

Cheers,