Question

I have lists containing lists in three levels. Im using JPA to get this data from three different database tables.

On more details, I have three different objects (Group, Type, Item): a group that contains a List of Types that contains a List of Items.

I want to get only a few Items from different Types and Groups and return this Items like a List.

The question is: Is there a elegant and simple way to do this? Or I really need to get all the Items and group them level by level?

Answer 1

Guava might provide a nice solution:

 Predicate<Group> matchingGroup = ...;
 Predicate<Type> matchingType = ...;

 Function<Group, Type> getType = new Function<Group, Type>(){};
 Function<Type, Item> getItem = new Function...;

 // starting point
 List<Group> myGroups;
 Iterable<Item> filteredItems =
    Iterables.transform(
       Iterables.filter( 
          Iterables.transform(
              Iterables.filter(myGroups, matchingGroup),
              getType),
          matchingType),
       getItem);

or expanded:

 // starting point
 List<Group> myGroups;
 Iterable<Type> types = Iterables.transform(
              Iterables.filter(myGroups, matchingGroup),
              getType);
 Iterable<Item> items = Iterables.transform(
              Iterables.filter(types, matchingType),
              getItem);

There might be a way to use Predicates / Functions compose methods to shorten this.

Guava is here