co.cask.cdap.common.collect.LastNCollector.java Source code

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Here is the source code for co.cask.cdap.common.collect.LastNCollector.java

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/*
 * Copyright  2014 Cask Data, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

package co.cask.cdap.common.collect;

import com.google.common.base.Preconditions;
import com.google.common.collect.Lists;

import java.util.Collection;
import java.util.Deque;

/**
 * This collector will keep only the most recent N elements. It will
 * never return false, but keeps a bound on the memory it uses.
 *
 * @param <Element> Type of element.
 */
public class LastNCollector<Element> implements Collector<Element> {

    private final Deque<Element> elements;
    private final int maxCount;

    public LastNCollector(int n) {
        Preconditions.checkArgument(n > 0, "n must be greater than 0");
        this.maxCount = n;
        this.elements = Lists.newLinkedList();
    }

    @Override
    public boolean addElement(Element element) {
        if (elements.size() < maxCount) {
            elements.addLast(element);
        } else {
            elements.removeFirst();
            elements.addLast(element);
        }
        return true;
    }

    @Override
    public <T extends Collection<? super Element>> T finish(T collection) {
        collection.addAll(elements);
        elements.clear();
        return collection;
    }
}