Java: Transforming List<List> into a Map

When using Collectors.toMap, you need to specify how to get the key and the value from each element of the stream. Integer::intValue won't work here, because your stream elements are lists, not integers.

For the key, get the first element of the list. (This assumes that the inner lists are all non-empty.) For the value, pass the size method reference.

values.stream()
    .collect(Collectors.toMap(list -> list.get(0), List::size));

You could flatten the list and then use something like Collectors.groupingBy(Function.identity(), Collectors.counting()). However in this case i would say a good old (nested) for loop might be simpler to write and read.

List<List<Integer>> lst = Arrays.asList(Arrays.asList(1,1,1),Arrays.asList(2),Arrays.asList(4,4),Arrays.asList(7));

Map<Integer,Integer> result= new HashMap<Integer,Integer>();
System.out.println(lst);
//[[1, 1, 1], [2], [4, 4], [7]]

for(List<Integer> sub:lst){
  for(int n:sub){
    Integer last=result.get(n);
    int newCount=(last==null?0:last)+1;
    result.put(n, newCount);
  }
}
System.out.println(result);
//{1=3, 2=1, 4=2, 7=1}

I think @rgettman's answer is the most elegant. However, it makes the presumption that all lists are non-empty. Of course this is easily fixed by simply adding .filter(list -> !list.isEmpty()) before the collect reduction operation.

This is another approach, which no longer considers the list to be 'two-dimensional', that is, List<List<Integer>>, but instead flattens it to just a stream of Integers.

Map<Integer, Long> map = lists.stream()
    .flatMap(Collection::stream)
    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));