1、Map 与 List 的转换
2、Map 的排序
3、参考文章：
http://www.mkyong.com/java8/java-8-convert-list-to-map/
http://www.mkyong.com/tutorials/java-8-tutorials/

    @Test
    public void testToMapSort() {

        Map<String, Integer> unsortMap = new HashMap<>();
        unsortMap.put("z", 10);
        unsortMap.put("b", 5);
        unsortMap.put("a", 6);
        unsortMap.put("c", 20);
        unsortMap.put("d", 1);

        unsortMap.put("e", 7);
        unsortMap.put("y", 8);
        unsortMap.put("n", 99);
        unsortMap.put("g", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

        System.out.println("Original...");
        System.out.println(unsortMap);

        // sort by keys, a,b,c..., and return a new LinkedHashMap
        // toMap() will returns HashMap by default, we need LinkedHashMap to keep the order.
        Map<String, Integer> result = unsortMap.entrySet().stream()
                .sorted(Map.Entry.comparingByKey())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                        (oldValue, newValue) -> oldValue, LinkedHashMap::new));


        // Not Recommend, but it works.
        //Alternative way to sort a Map by keys, and put it into the "result" map
        Map<String, Integer> result2 = new LinkedHashMap<>();
        unsortMap.entrySet().stream()
                .sorted(Map.Entry.comparingByKey())
                .forEachOrdered(x -> result2.put(x.getKey(), x.getValue()));

        System.out.println("Sorted...");
        System.out.println(result);
        System.out.println(result2);
    }

    @Test
    public void testToMapSort2() {
        Map<String, Integer> unsortMap = new HashMap<>();
        unsortMap.put("z", 10);
        unsortMap.put("b", 5);
        unsortMap.put("a", 6);
        unsortMap.put("c", 20);
        unsortMap.put("d", 1);
        unsortMap.put("e", 7);
        unsortMap.put("y", 8);
        unsortMap.put("n", 99);
        unsortMap.put("g", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

        System.out.println("Original...");
        System.out.println(unsortMap);

        //sort by values, and reserve it, 10,9,8,7,6...
        Map<String, Integer> result = unsortMap.entrySet().stream()
                .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                        (oldValue, newValue) -> oldValue, LinkedHashMap::new));

        //Alternative way
        Map<String, Integer> result2 = new LinkedHashMap<>();
        unsortMap.entrySet().stream()
                .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
                .forEachOrdered(x -> result2.put(x.getKey(), x.getValue()));

        System.out.println("Sorted...");
        System.out.println(result);
        System.out.println(result2);
    }


    @Test
    public void testMap2List() {

        Map<Integer, String> map = new HashMap<>();
        map.put(10, "apple");
        map.put(20, "orange");
        map.put(30, "banana");
        map.put(40, "watermelon");
        map.put(50, "dragonfruit");

        System.out.println("\n1. Export Map Key to List...");

        List<Integer> result = new ArrayList(map.keySet());

        result.forEach(System.out::println);
        System.out.println("\n2. Export Map Value to List...");

        List<String> result2 = new ArrayList(map.values());
        result2.forEach(System.out::println);
    }

    @Test
    public void testConvertMapToList() {
        Map<Integer, String> map = new HashMap<>();
        map.put(10, "apple");
        map.put(20, "orange");
        map.put(30, "banana");
        map.put(40, "watermelon");
        map.put(50, "dragonfruit");

        // split a map into 2 List
        List<Integer> resultSortedKey = new ArrayList<>();

        List<String> resultValues = map.entrySet().stream()
                //sort a Map by key and stored in resultSortedKey
                .sorted(Map.Entry.<Integer, String>comparingByKey().reversed())
                .peek(e -> resultSortedKey.add(e.getKey()))
                .map(x -> x.getValue())
                // filter banana and return it to resultValues
                .filter(x -> !"banana".equalsIgnoreCase(x))
                .collect(Collectors.toList());

        resultSortedKey.forEach(System.out::println);
        resultValues.forEach(System.out::println);
    }

    //统计字符的出现次数
    @Test
    public void testFrequency() {
        List<String> list = new ArrayList<String>();
        list.add("a");
        list.add("b");
        list.add("c");
        list.add("d");
        list.add("b");
        list.add("c");
        list.add("a");
        list.add("a");
        list.add("a");

        System.out.println("\nExample 1 - Count 'a' with frequency");
        System.out.println("a : " + Collections.frequency(list, "a"));

        System.out.println("\nExample 2 - Count all with frequency");
        Set<String> uniqueSet = new HashSet<String>(list);
        System.out.println("==========>" + uniqueSet);
        for (String temp : uniqueSet) {
            System.out.println(temp + ": " + Collections.frequency(list, temp));
        }


    }