Centering a button in a linear layout

Question

Using gravity on LinearLayout centers the button correctly:

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

        <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/new"
        android:paddingLeft="40dp"
        android:paddingRight="40dp"/>
    </LinearLayout>

But using layout_gravity=center on the Button only centers the button horizontally and not vertically, why?

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

            <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="TEST"
            android:paddingLeft="40dp"
            android:paddingRight="40dp"
            android:layout_gravity="center"/>
        </LinearLayout>

Answer 1

I think the layout_gravity can make button center in LinearLayout.
Helpful article: LinearLayout gravity and layout_gravity explained
Check the simple code below. It will give you a button in center horizontal of LinearLayout

<?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" >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="#f00"
        android:orientation="vertical" >

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:text="Button Center Hozizontal" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="100dp"
        android:background="#00f"
        android:orientation="horizontal" >

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:text="Button Center Vertical" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#0ff"
        android:orientation="horizontal" >

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

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:text="Button Center Vertical/ Horizontal" />
        </LinearLayout>
    </LinearLayout>
</LinearLayout>

Hope this help.