title: how to map java class having parent class as field with graphql
body: <p>I have Java class modeled like:</p>
  <pre class="lang-java prettyprint-override"><code>public class Tree {
      private Integer countLeave;
      private String type;
  }
      
  public class BananaTree extends Tree {
      private Integer countBanana;
  }
      
  public class MangoTree extends Tree {
      private Integer countMango;
  }
      
  public class Forest {
      private List&lt;Tree&gt; trees;
  }
  </code></pre>
  <p>I want to map above model in GraphQL schema. And have query getForest where it will load all type of tree and return to caller.</p>
  <pre><code>type Query {
      getForest:Forest
  }
  
  interface Tree {
      countLeave:Integer
      type:String
  }
      
  type BananaTree implements Tree {
      countBanana:Integer
  }
      
  type MangoTree implements Tree {
      countMango:Integer
  }
  
  </code></pre>
  <p>But on doing so getting:
  There is no type resolver defined for interface / union 'Tree' type</p>