Java：有效地跟踪使用过的对象

Question

I have a program that collects objects over time. Those objects are often, but not always duplicates of objects the program has already received. The number of unique objects can sometimes be up in the tens of thousands. As my lists grow, it takes more time to identify whether an object has appeared or not before.

My current method is to store everything in an ArrayList, al; use Collections.sort(al); and use Collections.binarySearch(al, key) to determine whether I've used an object. Everytime I come across a new object I have to insert and sort however.

I'm wondering if there's just a better way to do this. Contains tends to slow up too quickly. I'm looking for something as close to O(1) as possible.

Thanks much.

This is java. For the purpose of understanding what I'm talking about, I basically need a method that does this:

public boolean objectAlreadyUsed(Object o) {
  return \\ Have we seen this object already?

}

Answer 1

This begs the question - why not use a data structure that doesn't allow duplicates (eg Set )? If you attempt to add a duplicate item, the method will return false and the data structure will remain unchanged.

Answer 2

Instead of using an ArrayList , why wouldn't you use a Set implementation (likely a HashSet )? You'll get constant-time lookup , no sorting needed.

NB your objects will need to correctly override hashCode() and equals() .

Answer 3

Make sure the objects have correct equals() and hashCode() methods, and store them in a HashSet . Lookup then becomes constant time.

If retaining unwanted objects becomes an issue, by the way, you could consider using one of the many WeakHashSet implementations available on the Internet -- it will hold the objects but still allow them to be garbage collected if necessary.