Editable multimap index

Question

I am having hard time looking for constructor to build mutable Multimap. My code is:

Multimap<String, DbEntity> multimapByKey = Multimaps.index(goodHosts, instanceGetKeyfunction);

...

multimapByKey.removeAll(someKey); 
// throws 
// java.lang.UnsupportedOperationException
//     at com.google.common.collect.ImmutableListMultimap.removeAll(Unknown Source)
//     at com.google.common.collect.ImmutableListMultimap.removeAll(Unknown Source)

Since index returns ImmutableListMultimap I really cannot modify it. However, I cannot see other options to do group by keyFunction for Multimaps on the official documentation.

Answer 1

You can create a method that return a mutable Multimap, like the index function, like this:

public static <K, V> Multimap<K, V> indexMutable(Iterable<V> values,
        Function<? super V, K> function) {

    // check null value, function
    Multimap<K, V> map = ArrayListMultimap.create();

    for (V v : values) {
        // check null V
        map.put(function.apply(v), v);
    }

    return map;
}

And use like this:

@Test
public void testMutableMap() throws Exception {

    List<String> badGuys = Arrays.asList("Inky", "Blinky", "Pinky",
            "Pinky", "Clyde");
    Function<String, Integer> stringLengthFunction = new Function<String, Integer>() {

        public Integer apply(String input) {
            return input.length();
        }
    };

    Multimap<Integer, String> multipmap = indexMutable(badGuys,
            stringLengthFunction);

    System.out.println(multipmap);
    multipmap.clear();
    System.out.println("It's mutable!");

    for (String guy : badGuys) {
        multipmap.get(stringLengthFunction.apply(guy)).add(guy);
    }

    System.out.println(multipmap);
}

It's output:

{4=[Inky], 5=[Pinky, Pinky, Clyde], 6=[Blinky]}
It's mutable!
{4=[Inky], 5=[Pinky, Pinky, Clyde], 6=[Blinky]}

This example, is the same for the Javadoc of Multimaps#index.