Using Java8 Stream to find the highest values from map

I wrote following method to find the keys mapped to the highest values and trying to convert to java Streams. Can you please help?

private List<Integer> testStreamMap(Map<Integer, Long> mapGroup) 
{
    List<Integer> listMax = new ArrayList<Integer>();
    Long frequency = 0L;
    for (Integer key : mapGroup.keySet()) {
        Long occurrence = mapGroup.get(key);
        if (occurrence > frequency) {
            listMax.clear();
            listMax.add(key);
            frequency = occurrence;
        } else if (occurrence == frequency) {
            listMax.add(key);
        }
    }
    return listMax;
}

You can get a single key via

Integer max=mapGroup.entrySet().stream().max(Map.Entry.comparingByValue()).get().getKey();

but unfortunately, there is no built-in function for getting all equivalent maximums.

The simplest, straight-forward solution is to find the maximum value first and retrieve all keys mapping to that value afterwards:

private List<Integer> testStreamMap(Map<Integer, Long> mapGroup) {
    if(mapGroup.isEmpty())
        return Collections.emptyList();
    long max = mapGroup.values().stream().max(Comparator.naturalOrder()).get();
    return mapGroup.entrySet().stream()
        .filter(e -> e.getValue() == max)
        .map(Map.Entry::getKey)
        .collect(Collectors.toList());
}

Solutions for getting all maximum values of a stream in a single pass, are discussed in “How to force max() to return ALL maximum values in a Java Stream?”. You will see that single-pass solutions are much more complicated and not worth the effort if your input is an ordinary Map (e.g. HashMap), which can be iterated multiple times cheaply.

I'm not sure what half your code is trying to do, but to answer your question as per the title, which I'm guessing was meant to be "find the entry with the highest value":

Map.Entry<Integer, Long> maxEntry = map.entrySet().stream()
  .max(Map.Entry.comparingByValue()).get();