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[]);
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.
List<String>. Completely useless for a List<Shape>List<Shape> the first element could be a Rectangle while the second was an Ellipse2D.Double (which will not fit in a Rectangle[].)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.");
}
java.lang.reflect.Array.newInstance(Class<?> componentType, int length)
If you need to dynamically create an array based on a type known only at runtime (say you're doing reflection or generics) you'll probably want Array.newInstance
