Textview tv = new TextView(this);
tv.setTextSize(20);

<TextView android:text="@+id/TextView01"
 android:id="@+id/TextView01"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:textSize="30px"
 android:textStyle="italic"
 android:typeface="serif" />

TextView tv=(TextView)findViewById(R.id.custom);
  Typeface face=Typeface.createFromAsset(getAssets(), "fonts/HandmadeTypewriter.ttf");
  tv.setTypeface(face);

TextView text = new TextView(this);
text.setTypeface(Typeface.createFromAsset(getAssets(), "default.ttf"));

text.setTextSize(20);

public class TypefaceHelper {

    public static void setViewGroupTypeface(ViewGroup container, Typeface typeface) {
        final int children = container.getChildCount();

        for (int i = 0; i < children; i++)
            View child = container.getChildAt(i);

            if (child instanceof TextView) {
                setTextViewTypeface((TextView) child, typeface);
            } else if (child instanceof ViewGroup) {
                setViewGroupTypeface((ViewGroup) child, typeface);
            }
        }
    }

    public static void setTextViewTypeface(TextView textView, Typeface typeface) {
        textView.setTypeface(typeface);
    }

}

For Custom font:

 TextView tv = ((TextView) v.findViewById(R.id.select_item_title));
 Typeface face=Typeface.createFromAsset(getAssets(),"fonts/mycustomfont.ttf");
 tv.setTypeface(face);

For Default font:

 tv.setTypeface(Typeface.create("sans-serif-medium",Typeface.NORMAL));

FONT FAMILY                    TTF FILE

1  casual                      ComingSoon.ttf
2  cursive                     DancingScript-Regular.ttf
3  monospace                   DroidSansMono.ttf
4  sans-serif                  Roboto-Regular.ttf
5  sans-serif-black            Roboto-Black.ttf
6  sans-serif-condensed        RobotoCondensed-Regular.ttf
7  sans-serif-condensed-light  RobotoCondensed-Light.ttf
8  sans-serif-light            Roboto-Light.ttf
9  sans-serif-medium           Roboto-Medium.ttf
10  sans-serif-smallcaps       CarroisGothicSC-Regular.ttf
11  sans-serif-thin            Roboto-Thin.ttf
12  serif                      NotoSerif-Regular.ttf
13  serif-monospace            CutiveMono.ttf

private static final String FONT_NAME = "fonts/Roboto-Regular.ttf";
private static Typeface m_font = null;

public static Typeface getFont(Context p_context)
{
    if (null == m_font && null != p_context)
    {
        m_font = Typeface.createFromAsset(p_context.getApplicationContext().getAssets(), FONT_NAME);
    }
    return m_font;
}

public static void setViewGroupFont(ViewGroup p_viewGroup)
{
    if (null != p_viewGroup)
    {
        for (int currChildIndex = 0; currChildIndex < p_viewGroup.getChildCount(); currChildIndex++)
        {
            View currChildView = p_viewGroup.getChildAt(currChildIndex);

            if (ViewGroup.class.isInstance(currChildView))
            {
                setViewGroupFont((ViewGroup) currChildView);
            }
            else
            {
                try
                {
                    Method setTypefaceMethod = currChildView.getClass().getMethod("setTypeface", Typeface.class);

                    setTypefaceMethod.invoke(currChildView, getFont(p_viewGroup.getContext()));
                }
                catch (NoSuchMethodException ex)
                {
                    // Do nothing
                }
                catch (Exception ex)
                {
                    // Unexpected error setting font
                }
            }
        }
    }
}