明明设置了目的List的大小，为什么执行Collections.copy()还是报错：Source does not fit in dest

首先说一下，报错Source does not fit in dest。

在复制List时，使用Collections.copy(dest, src)方法，首先会检查src的大小是否大于dest的大小，如果大于，则报错。

这一点，源码写的很清楚：

    /**
     * Copies all of the elements from one list into another.  After the
     * operation, the index of each copied element in the destination list
     * will be identical to its index in the source list.  The destination
     * list must be at least as long as the source list.  If it is longer, the
     * remaining elements in the destination list are unaffected. 

     *
     * This method runs in linear time.
     *
     * @param  dest The destination list.
     * @param  src The source list.
     * @throws IndexOutOfBoundsException if the destination list is too small
     *         to contain the entire source List.
     * @throws UnsupportedOperationException if the destination list's
     *         list-iterator does not support the set operation.
     */
    public static  void copy(List dest, List src) {
        int srcSize = src.size();  
        if (srcSize > dest.size())   //在这里判断大小
            throw new IndexOutOfBoundsException("Source does not fit in dest");

        if (srcSize < COPY_THRESHOLD ||
            (src instanceof RandomAccess && dest instanceof RandomAccess)) {
            for (int i=0; i di=dest.listIterator();
            ListIterator si=src.listIterator();
            for (int i=0; i

但是，如果给dest的List设置了大小，比如下面这样，为什么还是报错？

List dest = new ArrayList(src.size());
Collections.copy(dest, src);

实际上，这样传入的size，只分配了内存，却没有定义元素。

如果这时候打印dest的size，得到的是0。

 

怎么办？

当你百度如何深拷贝List时，你可能会看到以下两种写法

List dest = Arrays.asList(new String[src.size()]);
CollectionUtils.addAll(dest, new Object[src.size()]);

其本质都是把dest撑起来，此时再执行copy就没有问题了。