freemarker:强制转换"Interface"；反对真正的秘密阶层 [英] freemarker : cast "Interface" Object to the real concret class

问题描述

我迭代到一个IColonne对象列表.但是我需要转换为concret类以获取特定的属性.我的列表中有"Colonne"和"ColonneGroup"对象.

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.

IColonne界面:

The IColonne interface :

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

科隆大学的具体班级:

public class Colonne implements IColonne {
    ....
}

ColonneGroup的具体课程:

The ColonneGroup concret class :

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

我需要访问文本"属性.但是我有唯一的" IColonne.因此，我需要转换为ColonneGroup.我该怎么办?

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?

推荐答案

我想您需要从FreeMarker模板访问texts.由于FTL是动态键入的，因此您不需要强制转换.如果对象具有公共getTexts()方法，则只能访问成员.可以使用colonne.texts??进行测试(也可以是非null)，也可以使用!运算符为其提供默认值.所以可能是这样的:

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>

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