实现和集合 [英] Implementations and Collections

问题描述

为什么这样无效...

  public ArrayList< Edge> getEdges（）{
 
 return A; 
 
 // A是类型为Action的数组列表。动作实现边缘。 
 
} 
  

接口Edge包括：public ArrayList getEdges / p>

   
 return B; 
 
 // B是一个'Action'。动作实现边缘。 
 
} 
  

接口Edge包括：public Edge getEdges / p>

谢谢，
Chet

解决方案

code> Edge 是 Action ， ArrayList< Action> 的子类型不是 ArrayList< Edge> 的子类型。

使用 ArrayList <

您可以查看本教程的4.通配符部分，虽然我建议只读一遍，因为它是非常有帮助的。

Why does this not work...

public ArrayList<Edge> getEdges() {

return A;

//A is an Arraylist of type 'Action'. Action implements Edge.

}

the interface Edge includes: public ArrayList getEdges();

even though this does.

public Edge getEdges() {

return B;

//B is an 'Action'. Action implements Edge.

}

the interface Edge includes: public Edge getEdges();

Thank You, Chet

解决方案

Because while Edge is a subtype of Action, ArrayList<Action> is not a subtype of ArrayList<Edge>.

Use ArrayList<? extends Edge> instead.

You could take a look at this tutorial's 4. Wildcard section, although I'd suggest to just read through it, because it is really helpful.

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