泛型不执行类型擦除

Question

Following code is throwing java.lang.ArrayStoreException exception. To my understanding compiler should generate error or warning as I am using generics .

Please check the code below and find the commented line.

   public class Test1 {
        public static void main(String[] args) {
            Collection<Number> nums = new ArrayList<Number>();
            nums.add(new Integer(1));
            nums.add(new Long(-1));
            //every thing is fine here
            Number[] aa = nums.toArray(new Number[0]);
            for (Number num : aa) {
                System.out.println(num);
            }
            // Even if generics are there,no warning or error is reported by complier
            Integer[] bb= nums.toArray(new Integer[nums.size()]);
            for (Integer integer : bb) {
                System.out.println(integer);
            }

        }

    }

Kindly help me understand this behavior. Can you please help me understand how to fix this as well?

Answer 1

The Collection.toArray() method is defined as:

<T> T[] toArray(T[] a);

meaning that the type parameter T only binds the parameter type to the return value's type. It s independent of any type parameters of the enclosing class ( E for element type in the case of Collection ).