Array和Collection的相互转化

• Array Collection的区别
  数组：子元素是相同类型，且数组大小有界
  集合：子元素可以是不同类型，可自动扩容

• 相互转化方法

1. 数组转List集合：
   Array类中的asList(T…array)方法

/**
* Returns a fixed-size list backed by the specified array. (Changes to
* the returned list "write through" to the array.) This method acts
* as bridge between array-based and collection-based APIs, in
* combination with {@link Collection#toArray}. The returned list is
* serializable and implements {@link RandomAccess}.
*
* <p>This method also provides a convenient way to create a fixed-size
* list initialized to contain several elements:
* <pre>
* List&lt;String&gt; stooges = Arrays.asList("Larry", "Moe", "Curly");
* </pre>
*
* @param a the array by which the list will be backed
* @return a list view of the specified array
*/
@SafeVarargs
public static <T> List<T> asList(T... a) {
return new ArrayList<>(a);
}

2. 集合转数组
   集合类中的 toArray(Object[] objs)

/**
 * Returns an array containing all of the elements in this list in proper
 * sequence (from first to last element); the runtime type of the returned
 * array is that of the specified array.  If the list fits in the
 * specified array, it is returned therein.  Otherwise, a new array is
 * allocated with the runtime type of the specified array and the size of
 * this list.
 *
 * <p>If the list fits in the specified array with room to spare
 * (i.e., the array has more elements than the list), the element in
 * the array immediately following the end of the collection is set to
 * <tt>null</tt>.  (This is useful in determining the length of the
 * list <i>only</i> if the caller knows that the list does not contain
 * any null elements.)
 *
 * @param a the array into which the elements of the list are to
 *          be stored, if it is big enough; otherwise, a new array of the
 *          same runtime type is allocated for this purpose.
 * @return an array containing the elements of the list
 * @throws ArrayStoreException if the runtime type of the specified array
 *         is not a supertype of the runtime type of every element in
 *         this list
 * @throws NullPointerException if the specified array is null
 */
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
    if (a.length < size)
        // Make a new array of a's runtime type, but my contents:
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}