private class AnimatedMinimalTestBorder
    implements AnimatedBorder
{
    @Override
    public void paintBorderAnimated( Component c, Graphics g, int x, int y, int width, int height, float animatedValue ) {
        int lh = UIScale.scale( 2 );

        g.setColor( Color.blue );
        g.fillRect( x, y + height - lh, Math.round( width * animatedValue ), lh );
    }

    @Override
    public float getValue( Component c ) {
        return c.isFocusOwner() ? 1 : 0;
    }

    @Override
    public Insets getBorderInsets( Component c ) {
        return UIScale.scale( new Insets( 4, 4, 4, 4 ) );
    }

    @Override public boolean isBorderOpaque() { return false; }
}

// sample usage
JTextField textField = new JTextField();
textField.setBorder( new AnimatedMinimalTestBorder() );