Cargar fuente TTF

    <TextView
        android:id="@+id/textview"
        android:textSize="42sp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!" />

//se declara texview
TextView textview = (TextView) findViewById( R.id.textview );
//se carga la fuente de la carpeta ASSETS
Typeface myTypeface = Typeface.createFromAsset(getAssets(), "fonts/Amatic-Bold.ttf");
//se asigna fuente al control Textview
textview.setTypeface(myTypeface);

import android.content.Context;
import android.graphics.Typeface;
import android.util.AttributeSet;
import android.widget.TextView;

/**
 * Clase MyTextView
 */
public class MyTextView extends TextView {

    private final String nameFont="OldLondon.ttf";

    public MyTextView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        Typeface tf = Typeface.createFromAsset(getContext().getAssets(), "fonts/" + nameFont);
        setTypeface(tf);
    }

    public MyTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
        Typeface tf = Typeface.createFromAsset(getContext().getAssets(), "fonts/" + nameFont);
        setTypeface(tf);
    }

    public MyTextView(Context context) {
        super(context);
        Typeface tf = Typeface.createFromAsset(getContext().getAssets(), "fonts/" + nameFont);
        setTypeface(tf);
    }

}

    <net.jc_mouse.ejemplofuente.MyTextView
        android:textSize="42sp"
        android:layout_below="@+id/textview"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!" />