Question

I want to have an object that allows other objects of a specific type to register themselves with it. Ideally it would store the references to them in some sort of set collection and have .equals() compare by reference rather than value. It shouldn’t have to maintain a sort at all times, but it should be able to be sorted before the collection is iterated over.

Looking through the Java Collection Library, I’ve seen the various features I’m looking for on different collection types, but I am not sure about how I should go about using them to build the kind of collection I’m looking for.

This is Java in the context of Android if that is significant.

Answer 1

Java’s built-in tree-based collections won’t work.

To illustrate, consider a tree containing weak references to nodes ‘B’, ‘C’, and ‘D’:

   C
B     D

Now let the weak reference ‘C’ get collected, leaving null behind:

   -
B     D

Now insert an element into the tree. The TreeMap/TreeSet doesn’t have sufficient information to select the left or right subtree. If your comparator says null is a small value, then it will be incorrect when inserting ‘A’. If it says null is a large value, it will be incorrect when inserting ‘E’.

Sort on demand is a good choice.

A more robust solution is to use an ArrayList<WeakReference<T>> and to implement a Comparator<WeakReference<T>> that delegates to a Comparator<T>. Then call Collections.sort() prior to iteration.

Android’s Collections.sort uses TimSort behind-the-scenes and so it runs quite efficiently if the input is already partially sorted.