find key in map java code example

Example 1: check if map contains key java

if (map.containsKey(key)) {
       // Okay, there's a key but the value is null
    } else {
       // Definitely no such key
    }

Example 2: java map get the key from value

public static <T, E> Set<T> getKeyByValue(Map<T, E> map, E value) {
    return map.entrySet()
              .stream()
              .filter(entry -> Objects.equals(entry.getValue(), value))
              .map(Map.Entry::getKey)
              .collect(Collectors.toSet());
}