java - In Java8 functional style, how can i map the values to already existing key value pair

Question

I have a map:

Map<String, List<Object>> dataMap;

Now i want to add new key value pairs to the map like below:

if(dataMap.contains(key)) {
    List<Object> list = dataMap.get(key);
    list.add(someNewObject);
    dataMap.put(key, list);
} else {
    List<Object> list = new ArrayList();
    list.add(someNewObject)
    dataMap.put(key, list);
}

How can i do this with Java8 functional style?

Answer 1

You can use computeIfAbsent.

If the mapping is not present, just create one by associating the key with a new empty list, and then add the value into it.

dataMap.computeIfAbsent(key, k -> new ArrayList<>()).add(someNewObject);

As the documentation states, it returns the current (existing or computed) value associated with the specified key so you can chain the call with ArrayList#add. Of course this assume that the values in the original map are not fixed-size lists (I don't know how you filled it)...

By the way, if you have access to the original data source, I would grab the stream from it and use Collectors.groupingBy directly.