Question

I know that the Collection framework allows for the creation of “views”, that is lightweight “wrappers” for a Collection object.
What I am especially interested in is, given a List, to return a view for only a subset of elements matching some conditions.
Basically, what I want to emulate is the functionality of the subList() method, only not based on start and end indexes, but on some parameters of the elements.

The first approach I thought about was simply to create another List, go through the first List and check each element…
While this wouldn’t be actually copy any MyObject but only their references, I would anyways create a new List object, with its overhead. Isn’t that right?
Is there any lightweight method of doing what I need?

N.B. My original List is a really big collection…

Thank you all

Answer 1

You can do this easily in Java using the Guava collections (Collections2 has a filter method http://docs.guava-libraries.googlecode.com/git-history/v11.0.1/javadoc/index.html).

You can also do this in groovy using the findAll method, for example

myList.findAll { it.contains("aValue") }

Any of these methods will create a new collection under the hood. So they are just doing the work for you of iterating over the elements and checking them. The overhead of creating a new list is minimal (it’s just instantiating one new object).