title: How do we unit test this lambda expression?
body: <p>We have a REST end point (JAX-RS) that is invoked from the browser. We are passing around OutputStream so that we could have the browser display the result of the call. </p>
  
  <p>Here is the method.</p>
  
  <pre><code>@Path("/mypath/{userId}")
  @POST
  public Response createUser(@PathParam("userId") final int userId ) {
      StreamingOutput stream = (outputStream) -&gt; {
          User user = userHelper.findUser(userId);
          userHelper.updateUser(user,outputStream);
      };
  
      return Response.ok(stream).build();
  }
  </code></pre>
  
  <p><strong>Using Junit and Mockito, how do we verify if <code>userHelper.findUser</code> and <code>userHelper.updateUser</code> has been called</strong> ? </p>
  
  <p>Basically we just want to verify the interactions.</p>