Java8--Lambda表达式对Map的操作

根据Map的键名、键值进行升序、降序：

public class StudyMap {

     public static void main(String[] args) {

        Map<String, Integer> wordCounts = new HashMap<>();
        wordCounts.put("USA", 100);
        wordCounts.put("jobs", 200);
        wordCounts.put("software", 50);
        wordCounts.put("technology", 70);
        wordCounts.put("opportunity", 200);


        //按升序对值进行排序，使用LinkedHashMap存储排序结果来保留结果映射中元素的顺序
        Map<String, Integer> sortedByCount = wordCounts.entrySet()
                .stream()
                .sorted(Map.Entry.comparingByValue())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
        soutMap(sortedByCount);

        //sorted()方法将Comparator作为参数使用任何类型的值对映射进行排序。上面的排序可以用Comparator写成：
        //正向
        Map<String, Integer> sortedByCount3 = wordCounts.entrySet()
                .stream()
                .sorted((e1, e2) -> e1.getValue().compareTo(e2.getValue()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
        soutMap(sortedByCount3);

        //反向 == reversed()
        Map<String, Integer> sortedByCount2 = wordCounts.entrySet()
                .stream()
                .sorted((e1, e2) -> e2.getValue().compareTo(e1.getValue()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
        soutMap(sortedByCount2);

    }

    public static void soutMap(Map<String, Integer> sortedByCount){
        for (Map.Entry<String, Integer> entry : sortedByCount.entrySet()) {
            System.out.println("key:" + entry.getKey()+"\tvalue:"+entry.getValue());
        }
    }
}

在上述代码中对于map的重新转换，建议转换成LinkedHashMap，因为HashMap是无序的