Java8 對Map進行排序

前言：

隨着Java 8 的

Stream

以後推出後能夠很優雅實現Map的排序。

示例：

final 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);

//{USA=100, software=50, jobs=200, opportunity=200, technology=70}複製代碼

按升序對值進行排序，使用

LinkedHashMap

存儲排序結果來保留結果映射中元素的順序

1、正向 
final 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));

//{software=50, technology=70, USA=100, jobs=200, opportunity=200}

2、反向 reversed
final Map<String, Integer> sortedByCount1 = wordCounts.entrySet()
        .stream()
        .sorted((Map.Entry.<String, Integer>comparingByValue().reversed()))
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));

//{jobs=200, opportunity=200, USA=100, technology=70, software=50}複製代碼

推薦

sorted()

方法將

Comparator

做爲參數使用任何類型的值對映射進行排序。上面的排序能夠用

Comparator

寫成：

//正向 
  final 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));
//{software=50, technology=70, USA=100, jobs=200, opportunity=200}


//反向 == reversed()
final 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));

//{jobs=200, opportunity=200, USA=100, technology=70, software=50}
複製代碼