在add（）上丢失最旧元素的集合

Question

I'm looking for a Java class that implements Collection and loses oldest elements when I add() a new one, if total number of elements is bigger than X. Does it exist or I have to implement it myself?

I need a thread-safe one.

Answer 1

Apart from Linkedhasmap if you are looking for list type solution, Google guava has EvictingQueue . And for thread safety you must wrap it in a synchronized wrapper ( Queues#synchronizedQueue ).

EvictingQueue<String> q = EvictingQueue.create(3);
Queue<String> syncQ =  Queues.synchronizedQueue(q);
syncQ.add("one");
syncQ.add("two");
syncQ.add("three");
syncQ.add("four");
System.out.println(q); // Prints [two, three, four]

Answer 2

CircularFifoQueue is a first-in first-out queue with a fixed size that replaces its
oldest element if it is full.

Answer 3

You can use the LinkedHashMap to do precisely that, quoting the Javadoc:

// Sample use: this override will allow the map to grow up to 100 entries and then delete the 
// eldest entry each time a new entry is added, maintaining a steady state of 100 entries.

 private static final int MAX_ENTRIES = 100;

 protected boolean removeEldestEntry(Map.Entry eldest) {
    return size() > MAX_ENTRIES;
 }

for thread safe-ness you can wrap it using Collections.synchronizedmap() .

Answer 4

I've used EvictingQueue added at v.15 of Google Guava to implement Moving Average functionality in the past.

It is not thread-safe though.