报错原因分析
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：
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/**
* 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. <p>
*
* 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 <tt>set</tt> operation.
*/
public static <T> void copy(List<? super T> dest, List<? extends T> 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<srcSize; i++)
dest.set(i, src.get(i));
} else {
ListIterator<? super T> di=dest.listIterator();
ListIterator<? extends T> si=src.listIterator();
for (int i=0; i<srcSize; i++) {
di.next();
di.set(si.next());
}
}
}
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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？
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List dest = new ArrayList(src.size());
Collections.copy(dest, src);
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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？
解决
当你百度如何深拷贝List时，你可能会看到以下两种写法
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List dest = Arrays.asList(new String[src.size()]);
CollectionUtils.addAll(dest, new Object[src.size()]);
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其本质都是把dest撑起来，此时再执行copy就没有问题了。
