Java基础之集合框架--Collections.reverse()方法

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/2
		for (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)));
    }

    /**
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

        E oldValue = (E) elementData[index];
        elementData[index] = element;
        return oldValue;
    }

本文转自
Java基础之集合框架--Collections.reverse()方法