Extending a class in Java

Question

Hi

I got a list of objects like this

ArrayList <Page> pageList = aForeignObject.getAllPages();

And a child class

class MyPage extends Page
{

    public void newFunction()
    {
        // A new Feature
    }
}

Is it possible somehow to convert the Page objects into MyPage objects?

I would love to do sth like this:

MyPage page = pages.get(1); // This will obviously fail 
page.newFunction();

Answer 1

If the Page objects coming out of the getAllPages() method are actually MyPage objects, then simply use a cast (e.g. MyPage page = (MyPage) pages.get(1);. If they're not (as is likely if you're using external code), you can't use sub classes. What you could do, however, is use composition:

class MyPage{
    private Page p;

    public MyPage(Page aPage){
        p = aPage;
    }

    public void newFunction(){
        //stuff
    }

    public Object oldFunction(){
        return p.oldFunction();
    }
}

Then you could do stuff like:

MyPage page = new MyPage(pages.get(1));
page.newFunction();

Answer 2

This is obviously messy but can work...

Page page = pages.get(1);
if (page instanceof MyPage) {
  ((MyPage)page).newFunction();
}

You can hold a list of all classes that extends Page, i.e.

List <? extends Page> pageList = aForeignObject.getAllPages(); //Bear in mind that Page objects will never be found here.

Now, if the list only contains MyPage, then you can use my example above. Like I said, it's not an effective nor is it an efficient solution.

Answer 3

I think the solution is to implement your "new feature" in Page. I had also a few of these situations before and for me, implementing it in the base class was the solution. But I don't know exactly in what kind of situation your are....

Second solution (not recommend): make your "new feature" a static method. Something like this:

public static void feature(Page page)
{
     ....
}

Then, you can make a shortcut in MyPage:

public void invokeFeatureMethod()
{
     feature(this);
}