Java Collection Element Get getByMostFrequent(Collection collection)

Description

Returns a list of unique elements by the most frequent.

License

Open Source License

Parameter

Parameter	Description
collection	a parameter

Exception

Parameter	Description
Exception	an exception

Declaration

public static <T> List<T> getByMostFrequent(Collection<T> collection) 

Method Source Code

//package com.java2s;
/*//www .j  a  v a  2  s  .  com
 * Copyright (c) 2015 Christopher Ritchie
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to 
 * deal in the Software without restriction, including without limitation the 
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or 
 * sell copies of the Software, and to permit persons to whom the Software is 
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in 
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER 
 * DEALINGS IN THE SOFTWARE.
 */

import java.util.ArrayList;

import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;

import java.util.List;
import java.util.Map;
import java.util.Set;

public class Main {
    /**
     * Returns a list of unique elements by the most frequent.
     * @param collection
     * @return
     * @throws Exception
     */
    public static <T> List<T> getByMostFrequent(Collection<T> collection) {
        if (collection == null || collection.isEmpty()) {
            return null;
        }

        final Map<T, Integer> histogram = getHistogram(collection);
        List<T> list = new ArrayList<T>();
        list.addAll(histogram.keySet());
        Collections.sort(list, new Comparator<T>() {
            @Override
            public int compare(T o1, T o2) {
                if (histogram.get(o1) > histogram.get(o2)) {
                    return -1;
                } else if (histogram.get(o1) < histogram.get(o2)) {
                    return 1;
                }
                return 0;
            }
        });
        return list;
    }

    /**
     * Returns a histogram of items in the collection.
     * @param collection
     * @return
     * @throws Exception
     */
    public static <T> Map<T, Integer> getHistogram(Collection<T> collection) {
        if (collection == null || collection.isEmpty()) {
            return null;
        }
        Map<T, Integer> histogram = new HashMap<T, Integer>();
        for (T item : collection) {
            if (histogram.containsKey(item)) {
                histogram.put(item, histogram.get(item) + 1);
            } else {
                histogram.put(item, 1);
            }
        }
        return histogram;
    }

    /**
     * Returns a collection of values based on the supplied collection of keys.
     * Ignores keys not in the map.
     * @param map
     * @param keys
     * @return
     * @throws Exception
     */
    public static <T, U> Set<U> get(final Map<T, U> map, final Collection<T> keys) throws Exception {
        Set<U> set = new HashSet<U>();
        for (T t : keys) {
            if (map.containsKey(t)) {
                set.add(map.get(t));
            }
        }
        return set;
    }
}

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