How to use multiset in flink?

I use collect() function in flink sql, and the return type is multiset. However, common functions such as cardinality, char_length, multiset[item] all cannot use for multiset. How can I get the element in multiset?

I saw the Multiset type is bridging to JVM java.util.Map<t, java.lang.Integer>, so I try to write a udf to convert Multiset to String like this,

public String eval(Map<String, Integer> map){
    StringBuffer result = new StringBuffer();
    for(String key: map.keySet()){
        result.append(key);
        result.append(";");
    }
    if(result.length() > 0){
        result.deleteCharAt(result.length()-1);
    }
    return result.toString();
}

However, I got error, Map<String, Integer> could not be used Multiset.

How can I modify this udf?