title: Best way to check if an email exists
body: <p>I developped an application java using spring-boot framework. In my class, before saving the user I need to check if his email exists in a list of 1000 emails, if the check is ok I have to set a boolean that the user is a loyel user.</p>
  
  <p>My question is what is the best implementation for checking that if the user email exist in that list or not in short time:</p>
  
  <pre><code>1- Read a file that contain the 1000 emails each time when a user will be created. What is the best way to do that without read every time the file ?
  using a singleton....
   2- Create an email ArrayList and parse it each time ???
   3. create a database and make a request to check each time if the email exists 
  </code></pre>
  
  <p>Would you have any suggestion ?</p>
  
  <p>Best regards</p>