如何在Java中对Map的键进行排序？

Question

This is a very basic question, I'm just not that good with Java. I have a Map and I want to get a list or something of the keys in sorted order so I can iterate over them.

Answer 1

Use a TreeMap , which is an implementation of the SortedMap interface. It presents its keys in sorted order.

Map<String, Object> map = new TreeMap<String, Object>();
/* Add entries to the map in any order. */
...
/* Now, iterate over the map's contents, sorted by key. */
for (Map.Entry<String, ?> entry : map.entrySet()) {
  System.out.println(entry.getKey() + ": " + entry.getValue());
}

If you are working with another Map implementation that isn't sorted as you like, you can pass it to the constructor of TreeMap to create a new map with sorted keys.

void process(Map<String, Object> original) {
  Map<String, Object> copy = new TreeMap<String, Object>(original);
  /* Now use "copy", which will have keys in sorted order. */
  ... 
}

A TreeMap works with any type of key that implements the Comparable interface, putting them in their "natural" order. For keys that aren't Comparable , or whose natural ordering isn't what you need, you can implement your own Comparator and specify that in the constructor .

Answer 2

You have several options. Listed in order of preference:

1. Use a SortedMap :
   SortedMap<whatever> myNewMap = new TreeMap<whatever>(myOldMap);
   This is vastly preferable if you want to iterate more than once. It keeps the keys sorted so you don't have to sort them before iterating.
2. There is no #2.
3. There is no #3, either.
4. SortedSet<whatever> keys = new TreeSet<whatever>(myMap.keySet());
5. List<whatever> keys = new ArrayList<whatever>(myMap.keySet()); Collections.sort(keys);

The last two will get you what you want, but should only be used if you only want to iterate once and then forget the whole thing.

Answer 3

You can create a sorted collection when iterating but it make more sense to have a sorted map in the first place. (As has already been suggested)

All the same, here is how you do it.

Map<String, Object> map;
for(String key: new TreeSet<String>(map.keySet()) {
  // accessed in sorted order.
}

Answer 4

Apart from the methods mentioned in other answers, with Java 8 streams, another shorthand to get a sorted key list from a map would be -

List<T> sortedKeys = myMap.keySet().stream().sorted().collect(Collectors.toList());

One could actually get stuff done after .sorted() as well (like using a .map(...) or a .forEach(...) ), instead of collecting it in the list and then iterating over the list.