要对 ArrayList 进行降序排序，可以使用 Collections 类的 reverseOrder 方法。参考以下示例：

import java.util.ArrayList;  
import java.util.Collection;  
import java.util.Collections;  
import java.util.Comparator;  
import java.util.Iterator;  
import java.util.List;  

public class ReverseArrayList {  
    public static void main(String[] args) {  
         List list = new ArrayList<>();  
         list.add(10);  
         list.add(50);  
         list.add(30);  
         list.add(60);  
         list.add(20);  
         list.add(90);  

         Iterator i = list.iterator();  
         System.out.println("printing the list....");  
         while(i.hasNext())  
         {  
             System.out.println(i.next());  
         }  

        Comparator cmp = Collections.reverseOrder();  
        Collections.sort(list,cmp);  
         System.out.println("printing list in descending order....");  
         Iterator i2 = list.iterator();  
         while(i2.hasNext())  
         {  
             System.out.println(i2.next());  
         }  

    }  
}

运行结果：

printing the list....
10
50
30
60
20
90
printing list in descending order....
90
60
50
30
20
10