How to call bind function in wordpress actions or hooks

I have made a class file in my plugin folder.
Then I wrote,

class Sample{

  function footer_content(){
    echo "show any data";
  }

}

Then I added the function footer_content to wordpress footer by using add_action command. But it didn’t show anything. maybe I am missing something in this, as it is not working.

add_action('wp_footer', Sample::footer_content());

Here’s reference from Codex

To use the add_action hook when your plugin or theme is built up using
classes, add $this to your add_action call together with the function
name within that class, like so:

class MyPluginClass {

    public function __construct() {
         //add your actions to the constructor!
         add_action( 'save_post', array( $this, 'save_posts' ) );
    }

    public function save_posts() {
         //do stuff here...
    }
}