How can I stream a Map<String, List<Object>> into a Stream<Entry<String, Object>>?

Question

I have a Map<String, List<Object>>.

How can I make it into a Stream of Entry<String, Object> so that I can construct a concatenated query String?

q1    a, b
q2    c, d

into

q1=a&q1=b&q2=c&q2=d

I'm, currently, doing this.

if (params != null && !params.isEmpty()) {
    final boolean[] flag = new boolean[1];
    params.forEach((n, vs) -> {
        vs.forEach(v -> {
            builder.append(flag[0] ? '&' : '?')
                    .append(n)
                    .append('=')
                    .append(v);
            if (!flag[0]) {
                flag[0] = true;
            }
        });
    });
}

Answer 1

Well, you don't have to produce a Entry<String, Object>. You can use flatMap to obtain the key=value Strings and directly construct the query String using Collectors.joining:

String query =
    map.entrySet()
       .stream()
       .flatMap(e -> e.getValue().stream().map(v -> e.getKey() + '=' + v))
       .collect(Collectors.joining("&"));

Input :

{q1=[a, b], q2=[c, d]}

Output :

q1=a&q1=b&q2=c&q2=d

Answer 2

If you have Guava, you might want to consider using a ListMultimap<String, Object> instead of Map<String, List<Object>>, and create your string like so:

String query = Joiner.on("&").withKeyValueSeparator("=").join(map.entries());