src/main/java/com/authlete/jaxrs/util/JaxRsUtils.java
public static <TKey, TValue> MultivaluedMap<TKey, TValue>
            putAll(MultivaluedMap<TKey, TValue> target, Map<TKey, TValue[]> data)
    {
        if (target == null || data == null)
        {
            return target;
        }

        for (Map.Entry<TKey, TValue[]> entry : data.entrySet())
        {
            target.addAll(entry.getKey(), entry.getValue());
        }

        return target;
    }
public static <TKey, TValue> MultivaluedMap<TKey, TValue>
            putAll(MultivaluedMap<TKey, TValue> target, Map<TKey, TValue[]> data)
    {
        if (target == null || data == null)
        {
            return target;
        }

        for (Map.Entry<TKey, TValue[]> entry : data.entrySet())
        {
            target.addAll(entry.getKey(), entry.getValue());
        }

        return target;
    }
Put all entries in a {@code Map<TKey, TValue[]>} instance
into an existing {@code MultivaluedMap<TKey, TValue>}.

@param target
Target {@code MultivaluedMap} instance.

@param data
Input data.

@return
The object given as {@code target}.