001/*-------------------------------------------------------------------------+
002|                                                                          |
003| Copyright 2005-2011 The ConQAT Project                                   |
004|                                                                          |
005| Licensed under the Apache License, Version 2.0 (the "License");          |
006| you may not use this file except in compliance with the License.         |
007| You may obtain a copy of the License at                                  |
008|                                                                          |
009|    http://www.apache.org/licenses/LICENSE-2.0                            |
010|                                                                          |
011| Unless required by applicable law or agreed to in writing, software      |
012| distributed under the License is distributed on an "AS IS" BASIS,        |
013| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
014| See the License for the specific language governing permissions and      |
015| limitations under the License.                                           |
016+-------------------------------------------------------------------------*/
017package org.conqat.lib.commons.collections;
018
019import java.util.Comparator;
020import java.util.PriorityQueue;
021
022import org.conqat.lib.commons.assertion.CCSMAssert;
023
024/**
025 * Priority queue that retains only the n last/largest elements. Order is
026 * determined by the employed comparator.
027 */
028public class BoundedPriorityQueue<T> extends PriorityQueue<T> {
029
030        /** Version used for serialization. */
031        private static final long serialVersionUID = 1;
032
033        /** Capacity of the collection */
034        private int capacityBound;
035
036        /**
037         * Constructor using natural order
038         * 
039         * @param capacityBound
040         *            maximal number of elements that are retained
041         */
042        public BoundedPriorityQueue(int capacityBound) {
043                super(capacityBound);
044                setCapacityBound(capacityBound);
045        }
046
047        /** Assert that capacityBound is positive and store in field */
048        private void setCapacityBound(int capacityBound) throws AssertionError {
049                CCSMAssert.isTrue(capacityBound > 0, "Capacity bound must be positive but was: " + capacityBound);
050                this.capacityBound = capacityBound;
051        }
052
053        /**
054         * Constructor.
055         * 
056         * @param capacityBound
057         *            maximal number of elements that are retained
058         * 
059         * @param comparator
060         *            Comparator used to compare elements
061         */
062        public BoundedPriorityQueue(int capacityBound, Comparator<T> comparator) {
063                super(capacityBound, comparator);
064                setCapacityBound(capacityBound);
065        }
066
067        /** Add a single element */
068        @Override
069        public boolean offer(T element) {
070                boolean result = super.offer(element);
071
072                if (size() > capacityBound) {
073                        remove();
074                }
075                CCSMAssert.isTrue(size() <= capacityBound, "Size exceeds capacity bound");
076
077                return result;
078        }
079}