Google Collections (Guava Libraries): ImmutableSet

Question

Assume that you want to build a copy of an ImmutableSet/List/Map object but filter out some of the original entries. One way to implement that is the following:

ImmutableList.copyOf(Iterables.filter(myObject, myObject.EQUALS));

where myObject.EQUALS is a predicate for the Iterables.filter() operation. I think this is a pretty elegant and easy-to-read implementation. However, one builds two list objects (first through the Iterables.filter(...) call, second through ImmutableList.copyOf(...)) which is very inefficient.

Does anybody know of a more efficient way to do it?

I guess the best thing would be to add filter predicates to the ImmutableSet/List/Map builders so that the object must be constructed only once. But unfortunately there is no such parameter.

Answer 1

Look at Guava's Iterators

Specifically filter(Iterator unfiltered, Predicate predicate)

Answer 2

The result of Iterables.filter() is just a view over the data in myObject: the new list is only built by ImmutableList.copyOf() using the filtering iterator provided by the Iterable