Getting a list of elements from an ArrayList

Question

Let's say I have a bean like below.

class Customer{
  private String code;
  private String name;
  private Integer value;
  //getters setters omitted for brevity
}

Then from a method I get a List<Customer> back. Now let's say I want to get a list of all member "name" from the List. Obviously I can traverse and build a List<String> of element "name" myself.

However, I was wondering if there is a short cut or more effiecient way to this technique that anyone knows . For instance, if I want to get a list of all keys in a Map object I get do map.keySet(). Something along that line is what I am trying to find out.

Answer 1

Guava has Lists.transform that can transform a List<F> to a List<T>, using a provided Function<F,T> (or rather, Function<? super F,? extends T>).

From the documentation:

public static <F,T>
   List<T> transform(
               List<F> fromList,
               Function<? super F,? extends T> function
           )

Returns a list that applies function to each element of fromList. The returned list is a transformed view of fromList; changes to fromList will be reflected in the returned list and vice versa.

The function is applied lazily, invoked when needed.

Similar live-view transforms are also provided as follows:

• Iterables.transform (Iterable<F> to Iterable<T>)
• Iterators.transform (Iterator<F> to Iterator<T>)
• Collections2.transform (Collection<F> to Collection<T>)
• Maps.transformValues (Map<K,V1> to Map<K,V2>)

Answer 2

Looks like you're looking for the Java equivalent of Perl's map function. This kind of thing might be added to the collections library once (if) Java receives closures. Until then, I think this is the best you can do:

List<String> list = new ArrayList<String>(customers.size());
for ( Customer c : customers ) {
    list.add(c.getName());
}

You could also write a map function that uses a simple interface to provide the mapping function. Something like this:

public interface Transform<I, O> {
    O transform(I in);
}
public <I, O> List<O> map(Collection<I> coll, Transform<? super I, ? extends O> xfrm) {
    List<O> list = new ArrayList<O>(coll.size());
    for ( I in : coll ) {
        list.add(xfrm.transform(in));
    }
    return list;
}