ArrayList源码,这点看不懂,该如何解决

public class ArrayList<E> extends AbstractList<E>        implements List<E>, RandomAccess, Cloneable, java.io.Serializable{    private static final long serialVersionUID = 8683452581122892189L;    /**     * The array buffer into which the elements of the ArrayList are stored.     * The capacity of the ArrayList is the length of this array buffer.     */    private transient E[] elementData;    /**     * The size of the ArrayList (the number of elements it contains).     *     * @serial     */    private int size;    /**     * Constructs an empty list with the specified initial capacity.     *     * @param   initialCapacity   the initial capacity of the list.     * @exception IllegalArgumentException if the specified initial capacity     *            is negative     */    public ArrayList(int initialCapacity) {    super();        if (initialCapacity < 0)            throw new IllegalArgumentException("Illegal Capacity: "+                                               initialCapacity);    this.elementData = (E[])new Object[initialCapacity];  //这里为什么可以这样,E类型肯定是Object的                                   //子类,父类怎么可以转换子类,要可以转换,得父类最开始的类型就是它要转换                                      //成子类的类                型才行啊    }

public class TestSet {    /**     * @param args     */    public static void main(String[] args) {        String [] strs = null;                strs =(String[]) new Object[5];//一运行报java.lang.ClassCastException错,这里肯定不可以这样转换        System.out.println(strs.length);    }}