Question

In Java, how do I convert List<?> to List<T> using a general purpose method so that I can replace patterns like the following with a single method call:

List untypedList = new ArrayList();  // or returned from a legacy method
List<Integer> typedList = new ArrayList<Integer>();
for (Object item: untypedList)
    typedList.add((Integer)item);

Note that the above code does not generate any type-safety warnings and, ideally, your solution shouldn’t generate any such warnings, either.

Will the following solution work provided that list Class<L> has a public default constructor?

public class ListUtil {
    public static <T, L extends List<T>> L typedList(List<?> untypedList, Class<T> itemClass, Class<L> listClass) {
        L list = null;
        try {
            list = listClass.newInstance();
        } catch (InstantiationException e) {
        } catch (IllegalAccessException e) {
        }
        for (Object item: untypedList)
            list.add(itemClass.cast(item));
        return list;
    }
}

(Note that listClass.newInstance() throws InstantiationException or IllegalAccessException if an instance of Class<L> does not have a public default constructor. What problems may arise if the method does not properly handle these exceptions?)

Notes:

• T is the type of each item in the resulting list.
• L is the type of the list that I wish to create (which extends List<T>).
• untypedList is the “untyped” input list, effectively the same as List<Object>.
• itemClass represents the runtime class of T.
• listClass represents the runtime class of L.

Answer 1

Rather than passing in the type of the list you want to instantiate, why not just pass in the empty Collection<T> that you want populated? This gives the users of your api much more flexibility, as using the default constructor is not always ideal. (for example, maybe I want a Set where I provide the expected number of elements, or I want a sorted list where I provide the Comparator).

Also, as a side note, you should always program to the most generic interface possible. In this case, your input need be nothing more specific than an Iterable, and your output a Collection.

Given this, I would write the method this way —

  public static <T, C extends Collection<T>> C typesafeAdd(Iterable<?> from, C to, Class<T> listClass) {
    for (Object item: from) {
      to.add(listClass.cast(item));
    }
    return to;
  }

then the calling code looks like:

  public static void main(String[] args) {
    List<?> untypedStringList = LegacyApi.getStringList();
    List<String> typesafeStringList = typesafeAdd(untypedStringList, new ArrayList<String>(), String.class);
  }

2 Comments here:

• If you can really trust LegacyApi (or whatever provided you the untyped List) to only return to you a collection with the expected type in it, then you can just do an unchecked cast and suppress it. This should be localized to the smallest scope possible. ie: create something like TypesafeLegacyApiWrapper which delegates calls to LegacyApi.
• This method signature still breaks down if you have anything more complicated. For example if you have a List<List<String>> this method does not work.