Using Java8 Stream to find the highest values from map

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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.

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