java-tutorials/java-collections-maps/src/main/java/com/baeldung/map/MapUtil.java

/**
 * 
 */
package com.baeldung.map;

import java.util.HashSet;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.stream.Stream;

/**
 * @author swpraman
 *
 */
public class MapUtil {
    
    public static <K, V> Stream<K> keys(Map<K, V> map, V value) {
        return map.entrySet()
                .stream()
                .filter(entry -> value.equals(entry.getValue()))
                .map(Map.Entry::getKey);
    }
    
    public static <K, V> Set<K> getKeys(Map<K, V> map, V value) {
        Set<K> keys = new HashSet<>();
        for (Entry<K, V> entry : map.entrySet()) {
            if (entry.getValue().equals(value)) {
                keys.add(entry.getKey());
            }
        }
        return keys;
    }
    
    public static <K, V> K getKey(Map<K, V> map, V value) {
        for (Entry<K, V> entry : map.entrySet()) {
            if (entry.getValue().equals(value)) {
                return entry.getKey();
            }
        }
        return null;
    }

}