Java基础之集合框架--Collections.reverse()方法

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package newFeatures8;import java.util.ArrayList;import java.util.Collections;import java.util.List;public class CollectionsDemo {public static void main(String[] args) {reverseDemo();}public static void reverseDemo(){List<String> list=new ArrayList<>();list.add("abcd");//String类本身就是实现了Comparable接口list.add("kkkkk");list.add("qq1");list.add("z");list.add("zz1");list.add("zz");list.add("qq1");list.add("qq1");list.add("qq2");list.sort(null);//Collections.sort(list);System.out.println("原集合是："+list);//Collections.reverse(list);swap(list);System.out.println("交换后的集合是："+list);}/** * @author ljh * @param nothing * @return void * @since 1.2 * @description 在List中前后两两交换元素，相当于reverse反转</br> * list.set(int index,E element)</br> * public E set(int index,E element)//返回：  以前位于该指定位置上的元素  *  */public static void swap(List<String> list){int size=list.size();//折半交换，只需要循环一半即可int mid=size>>1;//相当于/2for (int i = 0; i < mid; i++) {   //分解步骤：  //String endElement=list.set(size-1-i, list.get(i));  //list.set(i, endElement);list.set(i, list.set(size-1-i,list.get(i)));}} }

reverse方法的源码：

 @SuppressWarnings({"rawtypes", "unchecked"})    public static void reverse(List<?> list) {        int size = list.size();        if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {            for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)                swap(list, i, j);//请看下面swap方法        } else {            // instead of using a raw type here, it's possible to capture            // the wildcard but it will require a call to a supplementary            // private method            ListIterator fwd = list.listIterator();            ListIterator rev = list.listIterator(size);            for (int i=0, mid=list.size()>>1; i<mid; i++) {                Object tmp = fwd.next();                fwd.set(rev.previous());                rev.set(tmp);            }        }    }

 @SuppressWarnings({"rawtypes", "unchecked"})    public static void swap(List<?> list, int i, int j) {        // instead of using a raw type here, it's possible to capture        // the wildcard but it will require a call to a supplementary        // private method        final List l = list;        l.set(i, l.set(j, l.get(i)));    }

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