Java collection keeping only n last inserted members

Question

As the title states, I am looking for a java collection keeping only the N last objects inserted into the collection. This FIFO collection does not need to implement random access or support changing N.

All collections I can find are either blocking (LinkedBlockingQueue) or of unlimited size (ArrayDeque). I found org.eclipse.jetty.util.ArrayQueue but as you could guess this brings quite an unwanted dependency on my project and also is very complicated since it support changing N so its not what I need.

Do you know if there is a way to have that with a quite common java library or do I have to write it myself?

Check this out: http://stackoverflow.com/questions/7266042/java-ring-buffer — theglauber, Aug 07 '12 at 16:58
See the answer of this question: http://stackoverflow.com/questions/422181/how-do-i-limit-the-number-of-entries-in-a-java-hashtable — YMomb, Aug 07 '12 at 17:52
possible duplicate of [Size-limited queue that holds last N elements in Java](http://stackoverflow.com/questions/5498865/size-limited-queue-that-holds-last-n-elements-in-java) — om-nom-nom, May 22 '13 at 13:25

Brian Agnew · Accepted Answer · 2012-08-07T18:25:26.887

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Check out Apache Commons CircularFifoBuffer

CircularFifoBuffer is a first in first out buffer with a fixed size that replaces its oldest element if full.

The removal order of a CircularFifoBuffer is based on the insertion order; elements are removed in the same order in which they were added. The iteration order is the same as the removal order.

edited Aug 07 '12 at 18:25

answered Aug 07 '12 at 16:59

Brian Agnew

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Hmmm -- that looks like it won't let you add elements when it's full, though, which isn't what OP wants. – Sam Mussmann Aug 07 '12 at 17:09
@SamMussmann - Whoops. Was thinking of the CircularFifoBuffer. Now corrected – Brian Agnew Aug 07 '12 at 18:25
See CircularFifoQueue for a generic solution – cquezel Oct 30 '18 at 13:24

score 2 · Answer 2 · answered Aug 07 '12 at 17:45

I would just write a wrapper class which contains a private Queue or Deque instance and a public insert method which behaves as you need if the queue is already full when the client tries to insert an extra member. The size could be passed in via the constructor, and any methods belonging to Queue or Deque which you need available but which already behave as you need you could simply forward to the private instance.

score 2 · Answer 3 · answered Mar 26 '14 at 01:01

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After Guava 15.0 there's EvictingQueue which come with a fixed size that replaces its oldest element if full.

answered Mar 26 '14 at 01:01

sendon1982

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Java collection keeping only n last inserted members

3 Answers3