I use the following LinkedHashMap and get the occurences of numbers as <number, occurences>.

Map<Integer, Integer> map = new LinkedHashMap<>();

The values stored in the map are as in the following:

key | value
4   | 2
10  | 2
5   | 1
3   | 1

I want to get the first key that has value of 1 (in the example, it is 5). If there is not a key that has value of 1, it returns -1. I use the following approach, but I think it is not a proper way and there may be a better or short way to do this:

int key = map.containsValue(1) ?
                map.entrySet().stream()
                   .filter(entry -> entry.getValue() == 1)
                   .map(Map.Entry::getKey).findFirst().get() : -1;

So, is there a better way to achieve this?

CodePudding user response：

Rather than checking containsValue first, use orElse on the optional returned by findFirst, which is one fewer iteration through the map.

int key = map.entrySet().stream()
               .filter(entry -> entry.getValue() == 1)
               .map(Map.Entry::getKey).findFirst().orElse(-1);

findFirst would return an empty optional if there is nothing left after the filter. This only happens if there is no value 1 in the map to begin with. orElse returns its argument, if the optional is empty, otherwise it returns the wrapped value.

CodePudding user response：

1. Swap key, value (prepare): Map<Integer, List<Integer>> swapped = map.entrySet() .stream() .collect(Collectors.groupingBy(Map.Entry::getValue, Collectors.mapping(Map.Entry::getKey, Collectors.toList())));
2. Get value: swapped.containsKey(1) ? swapped.get(1).get(0) : -1;