WordPress Menu Walker with div elements

<?php
class Nav_Menu_Div_Walker extends Walker_Nav_Menu {
    private $_submenu_buffer = '';
    private $_clear_buffer = false;
    function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        global $wp_query;
        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $class_names = $value = '';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;
        $classes[] = 'menu-item-' . $item->ID;

        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
        $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

        $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
        $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

        $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
        $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
        $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
        $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;

        if ( $depth > 0 ) {
            $this->_submenu_buffer .= $indent . '<div' . $id . $value . $class_names .'>';
            $this->_submenu_buffer .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
        } else {
            $output .= $indent . '<div' . $id . $value . $class_names .'>';
            $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
        }
    }

    function end_el( &$output, $item, $depth = 0, $args = array() ) {
        if ( $depth > 0 ) {
            $this->_submenu_buffer .= "</div>\n";
        } else {
            $output .= "</div>\n";
        }
        if ( $this->_clear_buffer ) {
            $output .= $this->_submenu_buffer;
            $this->_submenu_buffer = '';
        }
    }

    function start_lvl(&$output, $depth, $item) {
        $indent = str_repeat("\t", $depth);
        if ( $depth >= 0 ) {
            $this->_submenu_buffer .= "\n$indent<div class=\"sub-menu\">\n";
        }
    }
    function end_lvl(&$output, $depth) {
        $indent = str_repeat("\t", $depth);
        $this->_submenu_buffer .= "$indent</div>\n";
        if ( $depth == 0 ) {
            $this->_submenu_buffer .= "</div>\n";
            $this->_clear_buffer = true;
        }
    }
}

// Example wp_nav_menu() call
wp_nav_menu(array(
    'theme_location' => 'main-menu',
    'walker' => new Nav_Menu_Div_Walker(),
    'items_wrap' => '<div id="%1$s" class="%2$s">%3$s</div>'
)) ?>