how can setup default value in ftl?

bhupen 

Have you tried:

<#assign link    = notification.target.entity.@view_href />

<#if link??>

  <a href="${link}">

<#else>

  <a href="http://www.google.com">

</#if>

  <h4>${notification.target.actor.login?string},
  ${text.format('pdc6.notifications.mentioned_string')},                                
  ${subject?string}</h4>
 <div class="details">
  <b class="notification-icon mono-icons-bg notification-mention"></b> ${date}
    </div>
  </a>

If it doesn't work then the error is with 'notification.target.entity.@view_href /'