Tabs in TabLayout not filling up entire ActionBar

Simple answer which I got from here.

You just put this in your xml code :

<android.support.design.widget.TabLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:tabMaxWidth="0dp"
            app:tabGravity="fill"
            app:tabMode="fixed" />

You can refer to the TabLayout.

GRAVITY_CENTER Gravity used to lay out the tabs in the center of the TabLayout.

GRAVITY_FILL Gravity used to fill the TabLayout as much as possible.

MODE_FIXED Fixed tabs display all tabs concurrently and are best used with content that benefits from quick pivots between tabs.

MODE_SCROLLABLE Scrollable tabs display a subset of tabs at any given moment, and can contain longer tab labels and a larger number of tabs.

Set this in your code or your layout xml.

app:tabGravity="center"
app:tabMode="fixed"

or

tabLayout.setTabGravity(TabLayout.GRAVITY_CENTER);
tabLayout.setTabMode(TabLayout.MODE_FIXED);    

Generally, using the code like blow can work without setting tabGravity and tabMode.

<android.support.design.widget.AppBarLayout
    android:id="@+id/appbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
        app:layout_scrollFlags="scroll|enterAlways" />

    <android.support.design.widget.TabLayout
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</android.support.design.widget.AppBarLayout>