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));;
}

参考资料

How to swap keys and values in a Map elegantly