Question

I have a List<ParentObject> where ParentObject is defined as:

public class ParentObject
{
  ChildObject child1;
  // and other stuff...
}

I would like to expose that List<ParentObject> to others as an unmodifiable List<ChildObject> by constructing a wrapper list:

public class WrapperList implements List<ChildObject>
{
  List<ParentObject> delegateList;

  public WrapperList( List<ParentObject> delegateList )
  {
    this.delegateList = delegateList;
  }

  // fill in java.util.List methods...
}

I can take this approach and make it work, but this seems like a problem which may have been solved generally. Is there an existing library which provides a list wrapper like what I am proposing? Presumably I would define a translation method telling the wrapper list how to retrieve the ChildObject from a given ParentObject.

The basic question is: is this a good approach or is there a library (Guava or Apache Commons Collections or something) which already provides something like this?

Answer 1

The simplest way to do this with Guava would be

return Collections.unmodifiableList(Lists.transform(parentList,
  new Function<ParentObject, ChildObject>() {...}));

(Disclosure: I contribute to Guava.)