Override widget in function WordPress

Im trying to override the text widget in WordPress. My code looks like this:

class WP_Widget_Text_Custom extends WP_Widget {

    function __construct() {
        $widget_ops = array('classname' => 'widget_text', 'description' => __('Arbitrary text or HTML'));
        $control_ops = array('width' => 400, 'height' => 350);
        parent::__construct('text', __('Text'), $widget_ops, $control_ops);
    }

    function WP_Widget_Calendar( $args, $instance ) {
        extract($args);
        $title = apply_filters( 'widget_title', empty( $instance['title'] ) ? '' : $instance['title'], $instance, $this->id_base );
        $text = apply_filters( 'widget_text', empty( $instance['text'] ) ? '' : $instance['text'], $instance );
        echo $before_widget;
        if ( !empty( $title ) ) { echo $before_title . $title . $after_title; } ?>
            <div class="textwidget test"><?php echo !empty( $instance['filter'] ) ? wpautop( $text ) : $text; ?></div>
        <?php
        echo $after_widget;
    }

    function update( $new_instance, $old_instance ) {
        $instance = $old_instance;
        $instance['title'] = strip_tags($new_instance['title']);
        if ( current_user_can('unfiltered_html') )
            $instance['text'] =  $new_instance['text'];
        else
            $instance['text'] = stripslashes( wp_filter_post_kses( addslashes($new_instance['text']) ) ); // wp_filter_post_kses() expects slashed
        $instance['filter'] = isset($new_instance['filter']);
        return $instance;
    }

    function form( $instance ) {
        $instance = wp_parse_args( (array) $instance, array( 'title' => '', 'text' => '' ) );
        $title = strip_tags($instance['title']);
        $text = esc_textarea($instance['text']);
?>
        <p><label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?></label>
        <input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo esc_attr($title); ?>" /></p>

        <textarea class="widefat" rows="16" cols="20" id="<?php echo $this->get_field_id('text'); ?>" name="<?php echo $this->get_field_name('text'); ?>"><?php echo $text; ?></textarea>

        <p><input id="<?php echo $this->get_field_id('filter'); ?>" name="<?php echo $this->get_field_name('filter'); ?>" type="checkbox" <?php checked(isset($instance['filter']) ? $instance['filter'] : 0); ?> />&nbsp;<label for="<?php echo $this->get_field_id('filter'); ?>"><?php _e('Automatically add paragraphs'); ?></label></p>
<?php
    }
}

function custom_register_widgets() {
    register_widget( 'WP_Widget_Text_Custom' );
}

add_action( 'widgets_init', 'custom_register_widgets' );

Im getting the following error message:
*function WP_Widget::widget() must be over-ridden in a sub-class.*

I have copied the WP_Widget_Text from default-widgets.php to functions.php and added _Custom in class name.

Why am I getting this error and how do I fix it?