freemarker : cast “Interface” Object to the real concret class

问题

I iterate to a list of IColonne object. But I need to cast to the concret class to get specific attribut. My list have "Colonne" and "ColonneGroup" object.

The IColonne interface :

public interface IColonne {
    String getFtlName();
    int getWidthPx(final int tableSize);
}

The Colonne concrete class:

public class Colonne implements IColonne {
    ....
}

The ColonneGroup concret class :

public class ColonneGroup implements IColonne {
    private List<String> texts;
}

I need to access to access to the "texts" attribut. But I have "only" IColonne. So I need to cast to ColonneGroup. How can I do this?

回答1:

I suppose you need to access texts from a FreeMarker template. As FTL is dynamically typed, you need no casting for that. You can just access the member, if the object has a public getTexts() method. Wether it has it (and that it's also non-null) you can test as colonne.texts??, or you can use the ! operator to give it a default. So it could be something like this:

<#list colonnes as colonne>
  ...
  <#if colonne.texts??>
    Do something with colonne.texts
  </#if>
  ...
  <#!-- Or if you would #list the texts anyway: -->
  <#list colonne.texts!>
     <p>We have some texts here:
     <ul>
       <#items as text>
         <li>${text}</li>
       </#items>
     </ul>
  </#list>
  ...
</#list>

来源：https://stackoverflow.com/questions/41099386/freemarker-cast-interface-object-to-the-real-concret-class