1. Go to the (project folder)
2. Then app -> src -> main.
3. Create folder 'assets -> fonts' into the main folder.
4. Put your 'fontfile.ttf' into the fonts folder.

private Typeface myFont;

// A good practice might be to call this in onCreate() of a custom
// Application class and pass 'this' as Context. Your font will be ready to use
// as long as your app lives
public void initFont(Context context) {
    myFont = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-Light.ttf");
}

public void setFont(TextView textView) {
    textView.setTypeface(myFont);    
}

TextViewPlus.java:

public class TextViewPlus extends TextView {
    private static final String TAG = "TextView";

    public TextViewPlus(Context context) {
        super(context);
    }

    public TextViewPlus(Context context, AttributeSet attrs) {
        super(context, attrs);
        setCustomFont(context, attrs);
    }

    public TextViewPlus(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        setCustomFont(context, attrs);
    }

    private void setCustomFont(Context ctx, AttributeSet attrs) {
        TypedArray a = ctx.obtainStyledAttributes(attrs, R.styleable.TextViewPlus);
        String customFont = a.getString(R.styleable.TextViewPlus_customFont);
        setCustomFont(ctx, customFont);
        a.recycle();
    }

    public boolean setCustomFont(Context ctx, String asset) {
        Typeface typeface = null;
        try {
            typeface = Typeface.createFromAsset(ctx.getAssets(), asset);
        } catch (Exception e) {
            Log.e(TAG, "Unable to load typeface: "+e.getMessage());
            return false;
        }

        setTypeface(typeface);
        return true;
    }
}

attrs.xml: (Where to place res/values)

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="TextViewPlus">
        <attr name="customFont" format="string"/>
    </declare-styleable>
</resources>

How to use:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:foo="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical" android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <com.mypackage.TextViewPlus
        android:id="@+id/textViewPlus1"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        android:text="@string/showingOffTheNewTypeface"
        foo:customFont="my_font_name_regular.otf">
    </com.mypackage.TextViewPlus>
</LinearLayout>

Drawing text in canvas with your font from assets.

Typeface typeface = Typeface.createFromAsset(getAssets(), "fonts/SomeFont.ttf");
Paint textPaint = new Paint();
textPaint.setTypeface(typeface);
canvas.drawText("Your text here", x, y, textPaint);

Loading custom fonts can be lead to a bad performance. I highly recommend to use this little helper which saves/loads your already used fonts into a Hashtable.

public class TypefaceUtils {

private static final Hashtable<String, Typeface> sTypeFaces = new Hashtable<>();

/**
 * Get typeface by filename from assets main directory
 *
 * @param context
 * @param fileName the name of the font file in the asset main directory
 * @return
 */
public static Typeface getTypeFace(final Context context, final String fileName) {
    Typeface tempTypeface = sTypeFaces.get(fileName);

    if (tempTypeface == null) {
        tempTypeface = Typeface.createFromAsset(context.getAssets(), fileName);
        sTypeFaces.put(fileName, tempTypeface);
    }

    return tempTypeface;
}

}

Usage:

Typeface typeface = TypefaceUtils.getTypeface(context, "RobotoSlab-Bold.ttf");
setTypeface(typeface);

public class ReplaceFont {

public static void changeDefaultFont(Context context, String oldFont, String assetsFont) {
    Typeface typeface = Typeface.createFromAsset(context.getAssets(), assetsFont);
    replaceFont(oldFont, typeface);
}

private static void replaceFont(String oldFont, Typeface typeface) {
    try {
        Field myField = Typeface.class.getDeclaredField(oldFont);
        myField.setAccessible(true);
        myField.set(null, typeface);
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
}

Then in your activity, in onCreate() method:

// Put your font to assets folder...

ReplaceFont.changeDefaultFont(getApplication(), "DEFAULT", "LinLibertine.ttf");

Android O changes the way to work with fonts.

Android O introduces a new feature, called Fonts in XML, which allows you to use fonts as resources. This means, that there is no need to bundle fonts as assets. Fonts are now compiled in an R file and are automatically available in the system as a resource.

In order to add a new font, you have to do the following:

• Create a new resource directory: res/font.
• Add your font files into this font folder. For example, by adding myfont.ttf, you will be able to use this font via R.font.myfont.

You can also create your own font family by adding the following XML file into the res/font directory:

<?xml version="1.0" encoding="utf-8"?>
<font-family xmlns:android="http://schemas.android.com/apk/res/android">
    <font
        android:fontStyle="normal"
        android:fontWeight="400"
        android:font="@font/lobster_regular" />
    <font
        android:fontStyle="italic"
        android:fontWeight="400"
        android:font="@font/lobster_italic" />
</font-family>

You can use both the font file and the font family file in the same way:

• In an XML file, by using the android:fontFamily attribute, for example like this:

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

  Or like this:

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

• In your code, by using the following lines of code:

  Typeface typeface = getResources().getFont(R.font.myfont);
  textView.setTypeface(typeface);