Converting List<Integer> to List<String>

Question

Using Google Collections from Guava-Project, you could use the transform method in the Lists class

import com.google.common.collect.Lists;
import com.google.common.base.Functions

List<Integer> integers = Arrays.asList(1, 2, 3, 4);

List<String> strings = Lists.transform(integers, Functions.toStringFunction());

The List returned by transform is a view on the backing list - the transformation will be applied on each access to the transformed list.

Be aware that Functions.toStringFunction() will throw a NullPointerException when applied to null, so only use it if you are sure your list will not contain null.

Solution for Java 8. A bit longer than the Guava one, but at least you don't have to install a library.

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

//...

List<Integer> integers = Arrays.asList(1, 2, 3, 4);
List<String> strings = integers.stream().map(Object::toString)
                                        .collect(Collectors.toList());

As far as I know, iterate and instantiate is the only way to do this. Something like (for others potential help, since I'm sure you know how to do this):

List<Integer> oldList = ...
/* Specify the size of the list up front to prevent resizing. */
List<String> newList = new ArrayList<>(oldList.size());
for (Integer myInt : oldList) { 
  newList.add(String.valueOf(myInt)); 
}

What you're doing is fine, but if you feel the need to 'Java-it-up' you could use a Transformer and the collect method from Apache Commons, e.g.:

public class IntegerToStringTransformer implements Transformer<Integer, String> {
   public String transform(final Integer i) {
      return (i == null ? null : i.toString());
   }
}

..and then..

CollectionUtils.collect(
   collectionOfIntegers, 
   new IntegerToStringTransformer(), 
   newCollectionOfStrings);

The source for String.valueOf shows this:

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

Not that it matters much, but I would use toString.