Convert Multivalued map to HashMap

I have a Multivalued map (javax.ws.rs.core.MultivaluedMap) which I want to convert to regular HashMap so I got below code:

private Map convertMultiToRegularMap(MultivaluedMap m) {
    Map map = new HashMap();
    if (m == null) {
        return map;
    }
    for (Entry> entry : m.entrySet()) {
        String qKey = entry.getKey();
        List values = entry.getValue();
        if (values.size() > 1) {
            String val = "";
            int i = 0;
            for (String s : values) {
                if (i > 0) {
                    val += ",";
                }
                val += s;
                i++;
            }
            map.put(qKey, val);
        } else {
            map.put(qKey, values.get(0));
        }
    }
    return map;
}

Is there any improvement I can do here? I am using Java7.

The main part of the code is converting the List value into a String delimited by a comma. Instead of using an external i variable, you can use a StringBuilder and append the comma or not depending on whether it is empty or not.

Regarding your current code:

• Try to avoid concatenating String with +. You should use a StringBuilder when necessary.

• You don't need to make a separate code path for the case where the list is empty in the map value: the regular path handles it also.

• You don't need to store the key and the value in local variables.

• It is indeed a very good idea to return a new empty map instead of null when the incoming map is null.

• If the value stored in the multimap is an empty list, then your code will throw an exception because it tries to access the element 0 when there is no such element.

This would be a proposed code for Java 7:

private Map convertMultiToRegularMap(MultivaluedMap m) {
    Map map = new HashMap();
    if (m == null) {
        return map;
    }
    for (Entry> entry : m.entrySet()) {
        StringBuilder sb = new StringBuilder();
        for (String s : entry.getValue()) {
            if (sb.length() > 0) {
                sb.append(',');
            }
            sb.append(s);
        }
        map.put(entry.getKey(), sb.toString());
    }
    return map;
}

As a side-note, when you'll upgrade to Java 8, this code can be made simpler by using the new String.join(delimiter, elements) method.

StackExchange Code Review Q#117641, answer score: 7