Java通过value获取Map中key的三种实现过程

@Test
public void loop() {
	Map<String, Integer> map = new HashMap<>();
	map.put("A", 1);
	map.put("B", 2);
	map.put("C", 3);
	map.put("D", 2);
	//找到一个值
	getKeyByLoop(map, 1);
	//找到多个值
	getKeysByLoop(map, 2);
	//找不到
	getKeyByLoop(map, 4);
}

private <K, V> Set<K> getKeysByLoop(Map<K, V> map, V value) {
	Set<K> set = Sets.newHashSet();
	for (Map.Entry<K, V> entry : map.entrySet()) {
		if (Objects.equals(entry.getValue(), value)) {
			set.add(entry.getKey());
		}
	}
	return set;
}

@Test
public void stream() {
	Map<String, Integer> map = ImmutableMap.of("A", 1, "B", 2, "C", 3, "D", 2);
	getKeysByStream(map, 2);
}

private <K, V> Set<K> getKeysByStream(Map<K, V> map, V value) {
	return map.entrySet()
		.stream()
		.filter(kvEntry -> Objects.equals(kvEntry.getValue(), value))
		.map(Map.Entry::getKey)
		.collect(Collectors.toSet());
}

<dependency>
	<groupId>org.apache.commons</groupId>
	<artifactId>commons-collections4</artifactId>
	<version>4.0</version>
</dependency>

@Test
public void apacheCommons() {
	BidiMap<String, Integer> bidiMap = new DualHashBidiMap<>();
    bidiMap.put("A", 1);
    bidiMap.put("B", 3);
    bidiMap.put("B", 2);
    bidiMap.put("E", 9);
    bidiMap.put("C", null);
    bidiMap.put("D", 2);
    System.out.println(bidiMap.getKey(1)); // A
    System.out.println(bidiMap.getKey(2)); // D
    System.out.println(bidiMap.getKey(3)); // null
    System.out.println(bidiMap.getKey(4)); // null
    System.out.println(bidiMap.getKey(null)); // C
    System.out.println(bidiMap.getKey(9)); // E
    
	System.out.println(bidiMap.getOrDefault("A",null)); // 1
    System.out.println(bidiMap.getOrDefault("B",null)); // null
    System.out.println(bidiMap.getOrDefault("C",null)); // null
    System.out.println(bidiMap.getOrDefault("D",null)); // 2
    System.out.println(bidiMap.getOrDefault("E",null)); // 9
   
    System.out.println(bidiMap); // {A=1, C=null, D=2, E=9}
}