Re: Generics in Java 1.5 ( or is it java 5.0 ?... I always have confusion)

Roland de Ruiter <>
Thu, 12 Jun 2008 14:23:12 +0200
On 12-6-2008 10:37, Vikram wrote:

      Looking at the signature of the following method in

public interface List<E> extends Collection<E> {

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

I wrote a small program as below:

    ArrayList<String> c = new ArrayList<String>();
    Integer[] i = new Integer[20];

This did not give me a compilation error, though it fails at runtime
giving java.lang.ArrayStoreException, which is perfect.

My question is , why did the above mentioned method be declared as
<E> E[] toArray(E[] a);

You probably mean
    E[] toArray(E[] a);
without the formal type parameter. Now E refers to the type parameter E
of class List.

which will force the method to take only the array of formal type ( in
this case a String[] ) at the compile time

In that case you could only convert the list to an array of E, and not
to an array of a superclass / interface of E.

There may be cases where it's useful to convert a list to an array of
Objects. With your proposal that's not possible (it would generating a
compile time error).

Modified example:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class ToArray {
    public static void main(String[] args) {
       List<String> list = new ArrayList<String>();

       // The following is OK
       Object[] arrO = new Object[list.size()];
       Object[] resArrO = list.toArray(arrO);

       // The following is OK; String implements Serializable
       Serializable[] arrS = new Serializable[list.size()];
       Serializable[] resArrS = list.toArray(arrS);

       // The following is OK at compile time, but toArray causes an
       // ArrayStoreException at runtime
       Integer[] arrI = new Integer[list.size()];
       Integer[] resArrI = list.toArray(arrI);



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