Generics: SupressWarnings("unchecked"). Why, and why not in java.util.ArrayList

Hello

I have a problem with generics, which I can solve adding
'@SupressWarnings("unchecked")' before
the methods. I have seen, that the java.util.ArrayList uses a
similar code and does not use this
warning supression.

What could be the problem in the following selfmade List?

Using
 SUNs jdk1.6.0_16,
 Eclipse Platform 3.5.0.v20090611a-9gEeG1HFtQcmRThO4O3aR_fqSMvJR2sJ
 Ubuntu 9.04

Thanks in advance

phi

Here the two compilation Units in the same Package "generics":
**********
package generics;

public interface List <T> extends Iterable<T>{

    T getFirst();
    void add(T obj);

    void removeFirst();

} // end of class List
*********
package generics;

import java.util.Iterator;

/**
 * @author Philipp Gressly (phi@gressly.ch)
 */
public class ListImplementation<T> implements generics.List<T> {

   Object [] data;
   int size;

   public ListImplementation () {
       this.data = new Object[20]; }

    public void add(T obj) {
        data[size++] = obj; }

    public T getFirst() {
        if (isEmpty()) {
            return null; }
        return (T) data[0]; } // <<---- WARNING

    private boolean isEmpty() {
        return 0 == size; }

    public void removeFirst() {
        if(isEmpty()) { return ; }

        int toPos = 0;
        while(toPos < size - 1) {
            data[toPos] = data[toPos + 1];
            toPos = toPos + 1; }
        size = size - 1; }

    @Override
    public Iterator<T> iterator() {
       return new ListIterator<T>(this.data, this.size); }

} // end of class ListImplementation

class ListIterator<T> implements Iterator<T> {

    Object [] objectArray;
    int size;
    int pos;

    public ListIterator(Object [] oarr, int size) {
        this.objectArray = oarr.clone();
        this.size = size; }

    @Override
    public boolean hasNext() {
        return pos < size ; }

    @Override
    public T next() {
        return (T) objectArray[pos++]; } // <<---- WARNING

    @Override
    public void remove() { }
}
*********