无法访问包含的布局中的元素：Android

Question

I have an xml file which contains a button and its called button/xml

<Button
    android:id="@+id/loginButton1"
    style="?android:attr/buttonStyleSmall"
    android:layout_width="match_parent"
    android:layout_height="40dp"
    android:background="@drawable/button_background"
    android:text="Button" />

I have another layout called login.xml , which includes the button.xml in it twice.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" 
    android:paddingLeft="30dp"
    android:paddingRight="30dp"
    android:paddingTop="30dp">

    <include
        android:id="@+id/loginUser1"
        android:layout_width="match_parent"
        android:layout_height="30dp"
        layout="@layout/button" />

        <include
            android:id="@+id/loginUser2"
            android:layout_width="match_parent"
            android:layout_height="30dp"
            android:layout_marginTop="20dp"
            layout="@layout/button" />

</LinearLayout>

Now when I try to access each of the buttons seperately in my Java class, I get an error while pointing to loginUser1 . The error says NullPointerException . Since I know for sure that loginUser1 exists, why am I still getting the error?

    final LinearLayout layout = (LinearLayout)findViewById(R.id.loginUser1); //null pointer exception HERE!
    final Button button = (Button)layout.findViewById(R.id.loginButton1);
    button.setText("button one");

Answer 1

Check out your log cat. You should get ClassCastException instead of NullPointerException . The problem is that view with id R.id.loginUser1 is actually a Button and not a LinearLayout . The following code should work well:

final Button first = (Button) findViewById(R.id.loginUser1);
final Button second = (Button) findViewById(R.id.loginUser2);

first.setText("button one");
second.setText("button two");

Also, please note that there is no button with id R.id.loginButton1 anymore because its id was overridden by include tag

Answer 2

You should do this

final Button first = (Button) findViewById(R.id.loginButton1); first.setText("your text");

as loginUser1 is a button and not a LinearLayout.