title: Problem with mapping String to Class (MapStruct)
body: <p>I can't figure out how to map this exactly. I need the one to be String and the other one to be object, so I can't unfortunately make it easier for myself.</p>
  
  <p>I have a RqgisterRequest class: </p>
  
  <pre><code>public class RegisterRequest {
      private String firstName;
      private String lastName;
      private String email;
      private String password;
  
      private Set&lt;String&gt; roles;
  </code></pre>
  
  <p>Here is my register 
  method with Instance </p>
  
  <pre><code> @PostMapping("/register")
      public HttpEntity authenticate(@Valid @RequestBody RegisterRequest registerRequest) {
  
          // create new user account
          User user = UserMapper.INSTANCE.registerRequestoUser(registerRequest);
  
          etc..
  </code></pre>
  
  <p>Here is my userDTO</p>
  
  <pre><code>@Data
  @NoArgsConstructor
  @AllArgsConstructor
  public abstract class UserDTO extends BaseDTO {
  
      @JsonProperty("first_name")
      private String firstName;
  
      @JsonProperty("last_name")
      private String lastName;
  
      @JsonProperty("email")
      private String email;
  
      @JsonProperty("password")
      private String password;
  
      private Set&lt;Role&gt; roles;
  
      private Set&lt;Appointment&gt; appointments;
  </code></pre>
  
  <p>(my user entity is the same)</p>
  
  <p>Here is my UserMapper class:</p>
  
  <pre><code>     User registerRequestoUser(RegisterRequest registerRequest);
  </code></pre>
  
  <p>And lastly the error I have when trying to run the program: Error:(20, 11) java: Can't map property "java.util.Set roles" to "java.util.Set roles". Consider to declare/implement a mapping method: "java.util.Set map(java.util.Set value)".</p>
  
  <p>How should I tackle this problem?</p>