使用ArrayList.subList()和Arrays.asList()方法要注意的地方

一. ArrayList.subList()方法

1. ArrayList.subList(int fromIndex, int toIndex)方法返回一个List: 此List的值包含fromIndex所在的值, 但不包含toIndex所在的值. 如果fromIndex等于toIndex, 那么会返回一个没有任何元素的空List. 返回的List支持java.util.List接口的所有操作.

2. **ArrayList.subList() 方法返回的List不能强转为ArrayList. **
   ArrayList.subList() 方法返回的List实际类型是ArrayList的一个内部类SubList, 即java.util.ArrayList$SubList

3. ArrayList.subList() 方法返回的List是ArrayList中某段数据的一个视图. 因此, 在操作此方法返回的List时, 同样会改变ArrayList的数据. 我们来看看ArrayList.SubList这个类的几个方法就知道了:

public E set(int index, E e) {
    rangeCheck(index);
    checkForComodification();
    E oldValue = ArrayList.this.elementData(offset + index);
    ArrayList.this.elementData[offset + index] = e;
    return oldValue;
}
public E get(int index) {
    rangeCheck(index);
    checkForComodification();
    return ArrayList.this.elementData(offset + index);
}

我们知道ArrayList的内部是用对象数组(成员: elementData)存储数据的, 可以看到ArrayList的内部类SubList的add和get方法操作的就是ArrayList内部的对象数组. 因此对ArrayList.subList()方法的返回值的任何操作都会反映到ArrayList对象本身.

4. 如果原ArrayList对象有改动, 那么ArrayList.subList()方法返回的视图将会无效. 也就是说在操作ArrayList.subList()方法的返回值前不能对原ArrayList做任何修改, 否则你将得到一个java.util.ConcurrentModificationException异常.

5. ArrayList的subList方法定义如下:

/**
     * Returns a view of the portion of this list between the specified
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  (If
     * {@code fromIndex} and {@code toIndex} are equal, the returned list is
     * empty.)  The returned list is backed by this list, so non-structural
     * changes in the returned list are reflected in this list, and vice-versa.
     * The returned list supports all of the optional list operations.
     *
     * 
This method eliminates the need for explicit range operations (of
     * the sort that commonly exist for arrays).  Any operation that expects
     * a list can be used as a range operation by passing a subList view
     * instead of a whole list.  For example, the following idiom
     * removes a range of elements from a list:
     * 
     *      list.subList(from, to).clear();
     * 
     * Similar idioms may be constructed for {@link #indexOf(Object)} and
     * {@link #lastIndexOf(Object)}, and all of the algorithms in the
     * {@link Collections} class can be applied to a subList.
     *
     * 
The semantics of the list returned by this method become undefined if
     * the backing list (i.e., this list) is structurally modified in
     * any way other than via the returned list.  (Structural modifications are
     * those that change the size of this list, or otherwise perturb it in such
     * a fashion that iterations in progress may yield incorrect results.)
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public List subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }

更多细节可参考JDK中java.util.ArrayList类的源码

二. Arrays.asList()方法

1. Arrays.asList(T ... args)方法将一个数组转换成一个List. Arrays.asList()方法返回的对象是Arrays类的一个内部类, 并没有实现修改集合的相关方法 (看源码可以知道java.util.Arrasy$ArrayList类支持set()方法, 所以还是可以修改的). Arrays.asList()体现的是适配器模式, 只是转换接口, 后台数据仍是被转换的数组. 因此: 对原数组的改动会影响返回的List, 反之亦然!!
2. 看源码可以知道Arrays.asList()的返回值是ArrayList, 但是请注意, 此ArrayList是Arrays类的一个内部类(java.util.Arrays$ArrayList), 而不是java.util.ArrayList. 因此不要把返回值强转成java.util.ArrayList
3. Arrays.asList()方法的返回值不支持add/remove/clear方法 (返回的List没有实现修改操作), 如果调用, 你会得到一个UnsupportedOperationException异常.
4. Arrays.asList()方法定义如下:

/**
 * 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}.
 *
 * 
This method also provides a convenient way to create a fixed-size
 * list initialized to contain several elements:
 * 
 *     List stooges = Arrays.asList("Larry", "Moe", "Curly");
 * 
 *
 * @param  the class of the objects in the array
 * @param a the array by which the list will be backed
 * @return a list view of the specified array
 */
@SafeVarargs
@SuppressWarnings("varargs")
public static  List asList(T... a) {
    return new ArrayList<>(a);
}

此方法主要作用是: 用已存在的数组初始化一个固定大小的List.

三. 总结

1. 对API使用有疑问的话, 最好是去看源码(如果有源码) 或 API文档.
2. 定期阅读JDK源码 (把其中常用的API吃透, 也就不会出现由于对API理解有误而导致的错误!!)