Add email id to display table and manage it on edit display

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I want to add email id to the display table and manage it on display edit form. Done all, but it always show 0, ie integer value because it showed 1 when I update email id to 1. But it is not showing string values. But it updates string values, not showing in the edit form. That is the issue. I used string type in lib/Entity/Display.php

 /**
 * @SWG\Property(description="The Email Id for the device Display")
 * @var string
 */
public $emailId;

Please help me how can I fix this issue as it is very difficult to understand the code?