How can I add keys to a list based on the size of the value of an ArrayListMultiMap?

user10751899 :

I have an ArrayListMultimap like

ArrayListMultimap<Long, String> multiMap =  ArrayListMultimap.create();

multiMap.put(100L,"test1");
multiMap.put(750L,"test0");
multiMap.put(50L, "test2");
multiMap.put(500L, "test3");
multiMap.put(200L, "test45");
multiMap.put(200L, "test000");
multiMap.put(60L, "test48");

which gives me {50=[test2], 100=[test1], 500=[test3], 200=[test45, test000], 60=[test48], 750=[test0]} .

I want to add all the keys to a list but the keys with multiple values need to be added based on the size of the value list. For example, in the above example, I want 200 to be added twice to the list since the size of its value is 2. I have tried using ArrayList<Long> keyList = pathSizeMap.keySet() but this gives me just the keys. How can I achieve this preferably using Java 8 streams?

Malt :

Note that keySet() specifically says Set (as in the collection that has no duplicate entries). That's why you're getting a single copy of each key.

Instead of using keySet() just stream the entries and get the key of each one:

List<Long> list = multiMap.entries().stream().map(Entry::getKey).collect(Collectors.toList());
System.out.println(list)

Output:

[100, 200, 200, 750, 50, 500, 60]