Java排序图按键说明 [英] Java Sort map by key explanation

问题描述

我最近发现了一种对包含GregorianCalendar作为键的地图进行排序的好方法.

I recently found a great way to sort a map that contains GregorianCalendar as keys.

Map<GregorianCalendar, String> map = new HashMap<>();

Map<GregorianCalendar, String> sortedMap = map.entrySet().stream().sorted(Map.Entry.comparingByKey()).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
            (oldValue, newValue) -> oldValue, LinkedHashMap::new));

有人可以帮助我理解此命令中调用的每个过程，尤其是从流函数进行的过程吗?

Can someone help me to understand each procedure called in this command especially going on from the stream function ?

谢谢

推荐答案

map.entrySet().stream()生成输入Map的条目(即Stream<Map.Entry<GregorianCalendar,String>>.)的Stream.

map.entrySet().stream() produces a Stream of the entries of the input Map (i.e. a Stream<Map.Entry<GregorianCalendar,String>>.

.sorted(Map.Entry.comparingByKey())通过条目的键对Stream的元素进行排序(这取决于键类型-GregorianCalendar-实现Comparable的事实).

.sorted(Map.Entry.comparingByKey()) sorts the elements of this Stream by the keys of the entries (it relies on the fact that the key type - GregorianCalendar - implements Comparable).

.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new))

.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new))

生成一个LinkedHashMap，其中包含与输入Map相同的条目.由于使用的是LinkedHashMap，因此将保持插入顺序，并且由于通过键对Stream的条目进行了排序，因此会得到一个Map，其插入顺序(和迭代顺序)是根据LinkedHashMap的顺序排列的.键.

produces a LinkedHashMap that contains the same entries as the input Map. Since you are using LinkedHashMap, the insertion order is maintained, and since you sorted the entries of the Stream by the keys, you get a Map whose insertion order (and iteration order) is according to the order of the keys.

当然，通过将输入Map的所有条目放在TreeMap中，您可以更轻松地获得排序的Map:

Of course you can get a sorted Map more easily by putting all the entries of the input Map in a TreeMap:

Map<GregorianCalendar, String> sortedMap = new TreeMap<>(map);

即使将新条目添加到Map，这也可以保持顺序.

This has the advantage of maintaining the order even if you add new entries to the Map.

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