Question

Is it possible to parameterize an ArrayList from a method parameter? For instance, I have a method getAllViewsFromRoot(View root) which retrieves all Views from a layout. Now I want a method that gets all ProgressBars in a layout, so I can just use this (toolkit)method:

public static ArrayList<ProgressBar> getAllProgressBars(View root) {
  ArrayList<View> allviews = getAllViewsFromRoot(root);
  ArrayList<ProgressBar> results = new ArrayList<ProgressBar>();
  for (View view : allviews)
    if (view instanceof ProgressBar)
      results.add((ProgressBar)view);
  return results;
}

But since I need to do this for different kinds of Views, I wondered if this is possible in a more generic way, passing on the class in the method. I hoped this would work, but it doesn’t:

public static ArrayList<?> getViewsFromViewGroup(ViewGroup root, Class clazz) {
    ArrayList<View> views = getAllViewsFromRoots(root);
    ArrayList<View> result = new ArrayList<clazz>();
    for (View view : views) {
        if (clazz.isInstance(view.getClass())) {
            result.add(view);
        }
    }
    return result;
}

And somewhere else I would call ArrayList<ProgressBar> pbs = Toolkit.getViewsFromViewGroup(root, ProgressBar.class).

In other words, can I specify how I want to strong type the ArrayList with a method parameter?

Is any sort of what I want possible? I don’t necessarily need it badly, but I like to solve problems in a generic way.

EDIT

I’ve managed to get the code working with the answer below (Thanks to thkala). It’s actually way easier that I suspected.

public static <T> ArrayList<T> getViewsFromViewGroup(View root, Class<T> clazz) {
    ArrayList<T> result = new ArrayList<T>();
    for (View view : getAllViewsFromRoots(root)) 
        if (clazz.isInstance(view)) 
            result.add(clazz.cast(view));
    return result;
}

So if you want to retrieve all ProgressBars from a certain point in your layout, just call:

ArrayList<ProgressBar> myPbs = Toolkit.getViewFromViewGroup(myRootView, ProgressBar.class);

Answer 1

How about this:

public static <T> ArrayList<T> getViewsFromViewGroup(ViewGroup root, Class<T> clazz) {
    ...
}

Class is a parametric type, so you can use its parameter to specify a generic parameter in a method. Naturally, you will have to use the T parameter in the method body – unless you plan on using reflection you could even completely ignore the clazz argument.