How do I limit the number of entries in a java hashtable?

Question

Is there a technique such that I can specify a number n such that when the (n + 1)th entry is inserted, the oldest entry is removed first ensuring that the size of the hashtable is always limited to n?

Answer 1

You're looking for a LRU cache perhaps? Here's a blog post on one based on LinkedHashMap.

Answer 2

You may want to consider using Apache Collections. They have a bunch of LRU implementations. Otherwise, you can easily write a similar wrapper for the standard library collections; I don't think there's one you can use directly.

Answer 3

You could use a double-ended queue, or Deque, and just remove the first item when you're at the max count.

Answer 4

If you're caching you could use a WeakHashMap or WeakReference and then not have to worry about the size of the cache.

Answer 5

LinkedHashMap does exactly that, see javadoc for the removeEldestEntry method.

Something like this should do the trick, this will remove the oldest inserted entry:

Map map = new LinkedHashMap() {
 @Override
 protected boolean removeEldestEntry(Entry eldest) {
  return size() > N;
 }
};

You can also remove oldest accessed entry by specifying it in the constructor:

    Map map = new LinkedHashMap(16, 0.75f, true) {
     @Override
     protected boolean removeEldestEntry(Entry eldest) {
      return size() > N;
     }
    };