互换Map键与值

一般方法

//map must be a bijection in order for this to work properly
public static <K,V> HashMap<V,K> reverse(Map<K,V> map) {
    HashMap<V,K> rev = new HashMap<V, K>();
    for(Map.Entry<K,V> entry : map.entrySet())
        rev.put(entry.getValue(), entry.getKey());
    return rev;
}

优雅方法

public static <K,V> HashMap<V,K> reverse(Map<K,V> map) {
    return map.entrySet().stream().collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey));;
}