Top50.java

Map<String, Integer> map = // ...
int limit = 50;
PriorityQueue<Map.Entry<String, Intger>> queue = new PriorityQueue<>(limit, Comparator.comparing(Map.Entry::getValue).reversed());

for (var entry: map.entrySet()) {
    if (queue.size() == limit) {
        queue.remove(); // Remove MAXIMAL element
    }
    queue.add(entry);
}

// After this loop queue contains limit MINIMAL elements
Map<String, Integer> result = new HashMap();
while (!queue.isEmpty()) {
    var entry = queue.remove();
    result.put(entry.getKey(), entry.getValue());
}

// Total time is O(n * log k) where k is limit,
// considering that limit is small, total time can be seen as O(n)