java.util.Collections
/**
* Randomly permutes the specified list using a default source of
* randomness. All permutations occur with approximately equal
* likelihood.<p>
*
* The hedge "approximately" is used in the foregoing description because
* default source of randomness is only approximately an unbiased source
* of independently chosen bits. If it were a perfect source of randomly
* chosen bits, then the algorithm would choose permutations with perfect
* uniformity.<p>
*
* This implementation traverses the list backwards, from the last element
* up to the second, repeatedly swapping a randomly selected element into
* the "current position". Elements are randomly selected from the
* portion of the list that runs from the first element to the current
* position, inclusive.<p>
*
* This method runs in linear time. If the specified list does not
* implement the {@link RandomAccess} interface and is large, this
* implementation dumps the specified list into an array before shuffling
* it, and dumps the shuffled array back into the list. This avoids the
* quadratic behavior that would result from shuffling a "sequential
* access" list in place.
*
* @param list the list to be shuffled.
* @throws UnsupportedOperationException if the specified list or
* its list-iterator does not support the <tt>set</tt> operation.
*/
public static void shuffle(List<?> list) {
if (r == null) {
r = new Random();
}
shuffle(list, r);
}
private static Random r;
java.util.Random
/**
* Randomly permute the specified list using the specified source of
* randomness. All permutations occur with equal likelihood
* assuming that the source of randomness is fair.<p>
*
* This implementation traverses the list backwards, from the last element
* up to the second, repeatedly swapping a randomly selected element into
* the "current position". Elements are randomly selected from the
* portion of the list that runs from the first element to the current
* position, inclusive.<p>
*
* This method runs in linear time. If the specified list does not
* implement the {@link RandomAccess} interface and is large, this
* implementation dumps the specified list into an array before shuffling
* it, and dumps the shuffled array back into the list. This avoids the
* quadratic behavior that would result from shuffling a "sequential
* access" list in place.
*
* @param list the list to be shuffled.
* @param rnd the source of randomness to use to shuffle the list.
* @throws UnsupportedOperationException if the specified list or its
* list-iterator does not support the <tt>set</tt> operation.
*/
public static void shuffle(List<?> list, Random rnd) {
int size = list.size();
if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
for (int i=size; i>1; i--)
swap(list, i-1, rnd.nextInt(i));
} else {
Object arr[] = list.toArray(); // Shuffle array
for (int i=size; i>1; i--)
swap(arr, i-1, rnd.nextInt(i)); // Dump array back into list
ListIterator it = list.listIterator();
for (int i=0; i<arr.length; i++) {
it.next();
it.set(arr[i]);
}
}
}
private static final int SHUFFLE_THRESHOLD = 5;
/**
* Swaps the elements at the specified positions in the specified list.
* (If the specified positions are equal, invoking this method leaves
* the list unchanged.)
*
* @param list The list in which to swap elements.
* @param i the index of one element to be swapped.
* @param j the index of the other element to be swapped.
* @throws IndexOutOfBoundsException if either <tt>i</tt> or <tt>j</tt>
* is out of range (i < 0 || i >= list.size()
* || j < 0 || j >= list.size()).
* @since 1.4
*/
public static void swap(List<?> list, int i, int j) {
final List l = list; //这一步有什么用
l.set(i, l.set(j, l.get(i)));
}
java.util.List
@org.intellij.lang.annotations.Flow(sourceIsContainer=true)
public abstract E set(int index,
@org.intellij.lang.annotations.Flow(targetIsContainer=true) E element)
Replaces the element at the specified position in this list with the specified element (optional operation).
Parameters:
index - index of the element to replace
element - element to be stored at the specified position
Returns:
the element previously at the specified position
/**
* Swaps the two specified elements in the specified array.
*/
private static void swap(Object[] arr, int i, int j) {
Object tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
}
这里假设集合List由四个元素List1、List2、List3和List4组成，当使用语句Iterator it = List.Iterator()时，迭代器it指向的位置是上图中Iterator1指向的位置，当执行语句it.next()之后，迭代器指向的位置后移到上图Iterator2所指向的位置。
首先看一下Iterator和ListIterator迭代器的方法有哪些。
Iterator迭代器包含的方法有：
hasNext()：如果迭代器指向位置后面还有元素，则返回 true，否则返回false
next()：返回集合中Iterator指向位置后面的元素
remove()：删除集合中Iterator指向位置后面的元素
ListIterator迭代器包含的方法有：
add(E e): 将指定的元素插入列表，插入位置为迭代器当前位置之前
hasNext()：以正向遍历列表时，如果列表迭代器后面还有元素，则返回 true，否则返回false
hasPrevious():如果以逆向遍历列表，列表迭代器前面还有元素，则返回 true，否则返回false
next()：返回列表中ListIterator指向位置后面的元素
nextIndex():返回列表中ListIterator所需位置后面元素的索引
previous():返回列表中ListIterator指向位置前面的元素
previousIndex()：返回列表中ListIterator所需位置前面元素的索引
remove():从列表中删除next()或previous()返回的最后一个元素（有点拗口，意思就是对迭代器使用hasNext()方法时，删除ListIterator指向位置后面的元素；当对迭代器使用hasPrevious()方法时，删除ListIterator指向位置前面的元素）
set(E e)：从列表中将next()或previous()返回的最后一个元素返回的最后一个元素更改为指定元素e
一．相同点
都是迭代器，当需要对集合中元素进行遍历不需要干涉其遍历过程时，这两种迭代器都可以使用。
二．不同点
1.使用范围不同，Iterator可以应用于所有的集合，Set、List和Map和这些集合的子类型。而ListIterator只能用于List及其子类型。
2.ListIterator有add方法，可以向List中添加对象，而Iterator不能。
3.ListIterator和Iterator都有hasNext()和next()方法，可以实现顺序向后遍历，但是ListIterator有hasPrevious()和previous()方法，可以实现逆向（顺序向前）遍历。Iterator不可以。
4.ListIterator可以定位当前索引的位置，nextIndex()和previousIndex()可以实现。Iterator没有此功能。
5.都可实现删除操作，但是ListIterator可以实现对象的修改，set()方法可以实现。Iterator仅能遍历，不能修改。
三：Iterator和ListIterator用法示例
ListIterator用法：
package com.collection; import java.util.LinkedList;
import java.util.List;
import java.util.ListIterator; public class ListIteratorTest { public static void main(String[] args) { List<String> staff = new LinkedList<>();
staff.add("zhuwei");
staff.add("xuezhangbin");
staff.add("taozhiwei");
ListIterator<String> iter = staff.listIterator();
String first = iter.next(); //删除zhuwei
iter.remove(); //把zhuwei改为simei
//iter.set("simei");
System.out.println("first:"+first); iter.add("xiaobai"); //遍历List元素
System.out.println("遍历List中元素，方法一：");
for(String str : staff)
System.out.println(str+" "); iter = staff.listIterator();
System.out.println("遍历List中元素，方法二：");
while(iter.hasNext())
{
System.out.println(iter.next());
}
} }
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