使用Stream对HashMap进行排序

话不多说。直接贴代码

        HashMap<String, BigDecimal> map = new HashMap<>();
        map.put("a", BigDecimal.valueOf(1231));
        map.put("d", BigDecimal.valueOf(55));
        map.put("c", BigDecimal.valueOf(223));
        LinkedHashMap<String, BigDecimal> collect = map.entrySet().stream()
                .sorted(Map.Entry.comparingByValue())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                        (oldValue, newValue) -> oldValue, LinkedHashMap::new));
        collect.entrySet().stream().forEach(c-> System.out.println(c.getKey()+" "+c.getValue()));

Result

output

按照特定序列进行排序

        List<String> rules = Arrays.asList("a", "d", "c");
        HashMap<String, BigDecimal> map = new HashMap<>();
        map.put("a", BigDecimal.valueOf(1));
        map.put("d", BigDecimal.valueOf(2));
        map.put("c", BigDecimal.valueOf(3));
        LinkedHashMap<String, BigDecimal> collect = map.entrySet().stream().sorted(Comparator.comparingInt(k -> rules.indexOf(k.getKey())))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                        (oldValue, newValue) -> oldValue, LinkedHashMap::new));
        collect.entrySet().stream().forEach(c-> System.out.println(c.getKey()+" "+c.getValue()));

Result

rules

Collectors.toMap()有三个签名:
1.Collects.toMap(Map.Entry::getKey, Map.Entry::getValue);
参数一是你转化后的key，参数二是你转化后的value,此方法下，发生碰撞会直接丢异常
2.Collects.toMap(Map.Entry::getKey, Map.Entry::getValue,(oldValue, newValue) -> oldValue);
参数一二同上，参数三是发生碰撞时进行什么处理。这里是保持旧值
3.Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new)
参数一二三同上，最后一个是你想生成的Map结构，这里采用的是LinkedHashMap.如果想实现排序后仍然保持Map结构，那么LinkedHashMap是不错的选择。

Collects.toMap的妙用

给定一个key列表，获取hashMap中的子Map

        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("a","a");
        hashMap.put("b","b");
        hashMap.put("c","c");
        hashMap.put("d","d");
        hashMap.put("e","e");
        hashMap.put("f","f");
        List<String> re = Arrays.asList("a", "b", "c");
        Map<String, Object> resultMap = re.stream().collect(Collectors.toMap(key -> key, hashMap::get));
        resultMap.forEach((k,v)-> System.out.println(k+","+v));

输出:

image.png