How do I specify eg. Roboto-Medium or Roboto-Black in styles.xml

On Android 5.0 you can set Roboto Medium with sans-serif-medium.

This solution, taken from Google iosched 2014, uses sans-serif on Android pre-v21:

values/styles.xml

<style name="MyStyle">
    <item name="android:fontFamily">@string/font_fontFamily_medium</item>
</style>

values/fonts.xml

<string name="font_fontFamily_medium">sans-serif</string>

values-v21/fonts.xml

<string name="font_fontFamily_medium">sans-serif-medium</string>

NEW SOLUTION

After Google I/O '17 you can create multiple font family in res/font folder

<?xml version="1.0" encoding="utf-8"?>
<font-family xmlns:app="http://schemas.android.com/apk/res-auto">
<font
    app:font="@font/roboto_light"
    app:fontStyle="normal"
    app:fontWeight="300" />
<font
    app:font="@font/roboto_bold"
    app:fontStyle="normal"
    app:fontWeight="700" />

</font-family>

and you can use wherever want

 <style name="AppTheme.DefaultTextView" parent="android:style/Widget.TextView">
    <item name="android:textColor">@color/colorBlack</item>
    <item name="android:fontFamily">@font/font_family_roboto</item>
</style>

OLD ANSWER

Thanks for Jakob Eriksson

original answer

android:fontFamily="sans-serif"           // roboto regular
android:fontFamily="sans-serif-light"     // roboto light
android:fontFamily="sans-serif-condensed" // roboto condensed
android:fontFamily="sans-serif-black"     // roboto black
android:fontFamily="sans-serif-thin"      // roboto thin (android 4.2)
android:fontFamily="sans-serif-medium"    // roboto medium (android 5.0)

in style:

<style name="AppTheme.DefaultTextView" parent="android:style/Widget.TextView">
    <item name="android:textSize">@dimen/text_view_text_size</item>
    <item name="android:textColor">@color/black</item>
    <item name="android:fontFamily">sans-serif-light</item>
    <item name="android:textStyle">normal</item>
</style>

in app theme:

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
    <item name="colorAccent">@color/gray</item>
    <item name="android:textViewStyle">@style/AppTheme.DefaultTextView</item>
</style>

Support Library 26 introduced using fonts in XML, and it's backwards compatible to Android API 14.

To add fonts as resources, perform the following steps in the Android Studio:

1. Right-click the res folder and go to New > Android resource directory. The New Resource Directory window appears.

2. In the Resource type list, select font, and then click OK. Note: The name of the resource directory must be font.

3. Add your font files in the font folder. The folder structure below generates R.font.dancing_script, R.font.lobster, and R.font.typo_graphica.

4. Double-click a font file to preview the file's fonts in the editor.

To set a font for the TextView, in the layout XML file, set the fontFamily attribute to the font file you want to access.

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:fontFamily="@font/lobster"/>

The Android Studio layout preview allows you to preview the font set in the TextView.

To add a font to a style, Open the styles.xml, and set the fontFamily attribute to the font file you want to access.

<style name="customfontstyle" parent="@android:style/TextAppearance.Small">
<item name="android:fontFamily">@font/lobster</item>

If you don't want to bundle the fonts in your app, you can always look into Downloadable Fonts

I suggest you to use custom libraries like Android-RobotoTextView or Calligraphy. With one of these you can set the font of a view in xml with an attribute so you can put in in styles.xml . And they works with previous version of Android than 4.0