Java: How can I dynamically create an array of a specified type based on the type of an object?

Question

I would like to take a passed List that I know is homogeneous and from it create an array of the same type as the elements within it.

Something like...

List<Object> lst = new ArrayList<Object>;

lst.add(new Integer(3));

/// somewhere else ...

assert(my_array instanceof Integer[]);

Answer 1

The conversion would happen runtime, while the type is lost at compile time. So you should do something like:

public <T> T[] toArray(List<T> list) {
    Class clazz = list.get(0).getClass(); // check for size and null before
    T[] array = (T[]) java.lang.reflect.Array.newInstance(clazz, list.size());
    return list.toArray(array);
}

But beware that the 3rd line above may throw an exception - it's not typesafe.

Answer 2

This method is type safe, and handles some nulls (at least one element must be non-null).

public static Object[] toArray(Collection<?> c)
{
  Iterator<?> i = c.iterator();
  for (int idx = 0; i.hasNext(); ++idx) {
    Object o = i.next();
    if (o != null) {
      /* Create an array of the type of the first non-null element. */
      Class<?> type = o.getClass();
      Object[] arr = (Object[]) Array.newInstance(type, c.size());
      arr[idx++] = o;
      while (i.hasNext()) {
        /* Make sure collection is really homogenous with cast() */
        arr[idx++] = type.cast(i.next());
      }
      return arr;
    }
  }
  /* Collection is empty or holds only nulls. */
  throw new IllegalArgumentException("Unspecified type.");
}