通用返回方法，由方法中的通用参数定义

Question

I have an interface in Java:

public interface IElement{

    public String getId();
    public String getName();

    /***
     * adds an IElement to this elements children
     * @param child
     */
    void getAddChild(IElement child);

    /***
     * returns all children
     * @return
     */
    Collection<IElement> getChildren( );

    /***
     * returns all children which match the Class of the passed IElement
     * @param eType - the class type to match
     */ 
    Collection<IElement> getChildren( Class<? extends IElement> eType);
}

The idea is that an IElement may contain a collection of other IElement s.

I need the method Collection<IElement> getChildren(Class<? extends IElement> eType) to return a collection of matching siblings.

Say I had a three classes which all extend IElement . One was called Room , another was called Box ( box1 , box2 , box3 ), and another was called Shelve ( shelve1 )

Now with an instance of a Room I can do the following:

room1.addChildren(box1);
room1.addChildren(box2);
room1.addChildren(shelve1,sheleve2);
room1.addChildren(box3);

Now I have a Room ( room1 ) with three different boxes in it and a Shelve ( shelve1 ). Now I would like to get all Box objects inside room1 using the Collection<IElement> getChildren(Class<? extends IElement> eType); method, as in room1.getChildren(Box.class) . However, the method only returns a collection of IElement s. I would like it to return Collection<Box> . If I passed Shelve then it should return a collection of Shelve objects.

Is this possible? And if so, how do you do it? I know this seems odd, but I have a lot of different objects which can all hold other Elements, and I need a quick and easy way to filter for different types.

Answer 1

You can declare a generic method with a named generic type, instead of using a wildcard.

<E extends IElement> Collection<E> getChildren(Class<E> eType);

That way, the generic type of the collection you get out will match the class that you passed in.

Your implementation will have to construct and return a collection of the appropriate generic type for its argument.