Collections提供了两个填充容器的静态方法fill,nCopies
用法
// 创建一个存放int型的数组，同时填充四个整型对象1
List<Integer> intArr = new ArrayList<Integer>(Collections.nCopies(4, 1));
System.out.println(intArr.toString());
//将intArr容器填充为2
Collections.fill(intArr, 2);
System.out.println(intArr.toString());
# out
[1, 1, 1, 1]
[2, 2, 2, 2]
源码介绍
fill源码
/**
* Replaces all of the elements of the specified list with the specified
* element. <p>
*
* This method runs in linear time.
*
* @param <T> the class of the objects in the list
* @param list the list to be filled with the specified element.
* @param obj The element with which to fill the specified list.
* @throws UnsupportedOperationException if the specified list or its
* list-iterator does not support the <tt>set</tt> operation.
*/
/**
* list - 使用指定元素填充的列表。
* obj - 用来填充指定列表的对象。
*/
public static <T> void fill(List<? super T> list, T obj) {
int size = list.size();
// 这里判断容器大小是否超过了默认设置的阀值25或者该容器是实现RandomAccess接口的
if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
for (int i=0; i<size; i++)
//直接填充
list.set(i, obj);
} else {
//如果不是则调用List中的方法listIterator，返回迭代器进行设置,使用迭代器的目的是为了提高执行效率
ListIterator<? super T> itr = list.listIterator();
for (int i=0; i<size; i++) {
itr.next();
itr.set(obj);
}
}
}
nCopies源码
/**
* Returns an immutable list consisting of <tt>n</tt> copies of the
* specified object. The newly allocated data object is tiny (it contains
* a single reference to the data object). This method is useful in
* combination with the <tt>List.addAll</tt> method to grow lists.
* The returned list is serializable.
*
* @param <T> the class of the object to copy and of the objects
* in the returned list.
* @param n the number of elements in the returned list.
* @param o the element to appear repeatedly in the returned list.
* @return an immutable list consisting of <tt>n</tt> copies of the
* specified object.
* @throws IllegalArgumentException if {@code n < 0}
* @see List#addAll(Collection)
* @see List#addAll(int, Collection)
*/
/**
* n - 返回列表中的元素数, 即需要填充的数量。
* o - 填充的对象。
* 返回值：由指定对象 n 个副本组成的不可变列表。
*/
public static <T> List<T> nCopies(int n, T o) {
if (n < 0)
throw new IllegalArgumentException("List length = " + n);
return new CopiesList<>(n, o);
}
