Size-limited queue that holds last N elements in Java
Guava now has an EvictingQueue, a non-blocking queue which automatically evicts elements from the head of the queue when attempting to add new elements onto the queue and it is full.
import java.util.Queue;
import com.google.common.collect.EvictingQueue;
Queue<Integer> fifo = EvictingQueue.create(2);
fifo.add(1);
fifo.add(2);
fifo.add(3);
System.out.println(fifo);
// Observe the result:
// [2, 3]
Apache commons collections 4 has a CircularFifoQueue<> which is what you are looking for. Quoting the javadoc:
CircularFifoQueue is a first-in first-out queue with a fixed size that replaces its oldest element if full.
import java.util.Queue;
import org.apache.commons.collections4.queue.CircularFifoQueue;
Queue<Integer> fifo = new CircularFifoQueue<Integer>(2);
fifo.add(1);
fifo.add(2);
fifo.add(3);
System.out.println(fifo);
// Observe the result:
// [2, 3]
If you are using an older version of the Apache commons collections (3.x), you can use the CircularFifoBuffer which is basically the same thing without generics.
Update: updated answer following release of commons collections version 4 that supports generics.
I like @FractalizeR solution. But I would in addition keep and return the value from super.add(o)!
public class LimitedQueue<E> extends LinkedList<E> {
private int limit;
public LimitedQueue(int limit) {
this.limit = limit;
}
@Override
public boolean add(E o) {
boolean added = super.add(o);
while (added && size() > limit) {
super.remove();
}
return added;
}
}