Convert a generic list to an array

Question

I have searched for this, but unfortunately, I don\'t get the correct answer.

class Helper {
    public static  T[] toArray(List list) {
           


        

Answer 1

The problem is the component type of the array that is not String.

Also, it would be better to not provide an empty array such as new IClasspathEntry[0]. I think it is better to gives an array with the correct length (otherwise a new one will be created by List#toArray which is a waste of performance).

Because of type erasure, a solution is to give the component type of the array.

Example:

public static  C[] toArray(Class componentType, List list) {
    @SuppressWarnings("unchecked")
    C[] array = (C[])Array.newInstance(componentType, list.size());
    return list.toArray(array);
}

The type C in this implementation is to allow creation of an array with a component type that is a super type of the list element types.

Usage:

public static void main(String[] args) {
    List list = new ArrayList();
    list.add("abc");

    // String[] array = list.toArray(new String[list.size()]); // Usual version
    String[] array = toArray(String.class, list); // Short version
    System.out.println(array);

    CharSequence[] seqArray = toArray(CharSequence.class, list);
    System.out.println(seqArray);

    Integer[] seqArray = toArray(Integer.class, list); // DO NOT COMPILE, NICE !
}

Waiting for reified generics..