java8 map how to add some element to a list value simply [duplicate]

问题

This question already has answers here:

In Java8 functional style, how can i map the values to already existing key value pair (4 answers)

Closed 4 years ago.

I want to use Map<String, List<String>> to record something, e.g. each city have how many users of some kind .

Now my code is

    Map<String, List<String>> map = new HashMap<>();
    if(map.get("city_1")==null){
        map.put("city_1", new ArrayList<>());
    }
    map.get("city_1").add("aaa");

but I feel it's a little cumbersome, I want this effect

    Map<String, List<String>> map = new HashMap<>();
    map.compute("city_1", (k,v)->v==null?new ArrayList<>():v.add("aaa"));

but it has compile error:

Type mismatch: cannot convert from boolean to List<String>

So have any other manner could simplify it?

回答1:

Use computeIfAbsent:

map.computeIfAbsent(work, k -> new ArrayList<>()).add("aaa");

Which stores a new list in the map if it does not already exist and returns the new or existing list.

来源：https://stackoverflow.com/questions/37165934/java8-map-how-to-add-some-element-to-a-list-value-simply