title: Why does a class implements Serializable interface?
body: <pre><code>@Entity
  public class Husband implements Serializable {
  
     @Id
     private int id;
  
     private String name;
  
     @OneToOne
     private Wife wife;
  
  }
  
  @Entity
  public class Wife implements Serializable {
  
     @Id
     private int id;
  
     private String name;
  
     @OneToOne(mappedBy="wife")
     private Husband husband;
  
  }
  </code></pre>
  
  <ol>
  <li>What is <code>Serializable</code> in broad term?</li>
  <li>Why does a class implements <code>Serializable</code> interface?</li>
  <li>Why does the husband member alone have @OnetoOne(mappedBy ="Wife"), but the wife member does not have @OnetoOne(mappedBy="husband")</li>
  </ol>