将条目从一个Multimap复制到另一个Multimap

Question

This question relates to the Multimap from com.google.common.collect.Multimap.

I have one Multimap, is there a simpler and more convenient way to copy entries whose key starts with a keyword to another temporary Multimap? - below is my current solution.

private Multimap<String, String> copyDesiredMetadata(Multimap<String, String> metadata)
{
    Multimap<String, String> returnedMap = new CaseInsensitiveKeyMultimap<>();

    // Iterate through the entries in the metadata
    for (Map.Entry entry : metadata.entries()) {
        String key =entry.getKey().toString();
        // If the entry has the field key we are looking for add to returned map.
        if (key.startsWith("AAA") || key.startsWith("BBB") || key.startsWith("CCC") || key.startsWith("DDD")) {
            returnedMap.put(key, entry.getValue().toString());
        }
    }
    return returnedMap;
}

Answer 1

I believe "more convenient" should be defined but here a more functional approach :

ImmutableMultimap<String, String> yourNewMap = metadata.entries()
            .stream()
            .filter(entry -> entry.getKey().matches("(AAA|BBB|CCC|DDD).*"))
            .collect(Collector.of(ImmutableMultimap.Builder<String, String>::new, 
                    ImmutableMultimap.Builder<String, String>::put, 
                    (left, right) -> {
                        left.putAll(right.build());
                        return left;
                    }, 
                    ImmutableMultimap.Builder::build));

Notice that I used ImmutableMultimap instead of CaseInsensitiveKeyMultimap as I don't know about this implementation but you should be able to adapt it easily.

Personally, I would extract the collector in an utility class so the code would look cleaner ... .collect(MoreCollectors.toCaseInsensitiveKeyMultimap())