Question

Given a List of items of different types, how do I efficiently separate those, so each group only contains item of the same type?

More details

In java, I have a List of about 10000 items. There are 10 different types of item. Type as in property of value. For example, List<Foo>, then type is Foo#getType().

I need to create zip-files that contain only one type of item.

Please give me some idea. Code are most-welcomed and appreciated, even pseudo-code. Thank you.

Accepted Code taking the idea from Jon Skeet

List<Foo> list = ...;
ImmutableListMultimap<String, Foo> grouped = MultiMaps.index(list,
    new Function<Foo, String>() {
        public String apply(Foo input) {
            return input.getType();
        }
});
for(String type : grouped.keySet()){
    //The below list will contains items with the same type.
    ImmutableList<Foo> fooListWithSameType = grouped.get(type);
    String zipName = getZipName(type);
    //Zip the list to a given file name
    zipList(fooListWithSameType, zipName);
}

Answer 1

You could use Guava‘s MultiMap functionality with MultiMaps.index, so using your Foo.getType() example, if it returns a string:

ImmutableListMultimap<String, Foo> grouped = MultiMaps.index(list,
    new Function<Foo, String>() {
        public String apply(Foo input) {
            return input.getType();
        }
});