What would be a good way to implement a set collection with weak references, compares by reference, and is also sortable in Java?

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.

Answer 2

Perhaps the collections classes are a level of abstraction below what you're looking for? It sounds like the end product you want is a cache with the ability to iterate in a user-defined sort order. If so, perhaps the cache interface in the Google Guava library is close enough to what you want:

http://code.google.com/p/guava-libraries/source/browse/trunk/guava/src/com/google/common/cache/Cache.java

At a glance, it looks like CacheBuilder in that package doesn't allow you to build an implementation with user-defined iteration order. However, it does provide a Map view that might be good enough for your needs:

List<Thing> cachedThings = Lists.newArrayList(cache.asMap().values());
Collections.sort(cachedThings, YOUR_THING_COMPARATOR);
for (Thing thing : cachedThings) { ... }

Even if this isn't exactly what you want, the classes in that package might give you some useful insights re: using References with Collections.

Answer 3

DISCLAIMER: This was a comment but it got kinda big, sorry if it doesn't solve your problem:

References in Java

Just to clarify what I mean when I say reference, since it isn't really a term commonly used in Java: Java does not really use references or pointers. It uses a kind of pseudo-reference that can be (and is by default) assigned to the special null instance. That's one way to explain it anyway. In Java, these pseudo-references are the only way that an Object can be handled. When I say reference, I mean these pseudo-references.

Sets

Any Set implementation will not allow two references to the same object to be included in it since it uses identity equality for this check. That violates the mathematical concept of a set. The Java Set s ignore any attempt to add duplicate references.

You mention a Map in your comment though... Could you clarify what kind of collection you are after? And why you need that kind of equality checking within it? Are you thinking in C++ terms? I'll try to edit my answer to be more helpful then:)

EDIT: I thought that might have been your goal;) So a TreeSet should do the trick then! I would not get concerned about performance until there is a performance issue. Simplicity is fantastic for readability, maintenance and preventing bugs. If performance does become a problem, ideally you should profile your code and only optimize the areas that are proven to be the problem.