public static  Comparator reverseOrder()

// Java Program to Demonstrate Working of reverseOrder()
// method of Collections class
// To sort a list in descending order

// Importing required utility classes
import java.util.*;

// Main class
// Collectionsorting
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {
        // Creating a list of integers for which we
        // create an empty ArrayList by
        // declaring object of ArrayList class
        ArrayList<Integer> al = new ArrayList<Integer>();

        // Custom input integer elements
        al.add(30);
        al.add(20);
        al.add(10);
        al.add(40);
        al.add(50);

        // Using sort() method of Collections class to
        // sort the elements and passing list and using
        // reverseOrder() method to sort in descending order
        Collections.sort(al, Collections.reverseOrder());

        // Lastly printing the descending sorted list on
        // console
        System.out.println(
            "List after the use of Collection.reverseOrder()"
            + " and Collections.sort() :\n" + al);
    }
}

List after the use of Collection.reverseOrder() and Collections.sort() :
[50, 40, 30, 20, 10]
```** 

> ****注意:**极客们现在你们一定在想我们能用[*arrays . sort()*](https://www.geeksforgeeks.org/arrays-sort-in-java-with-examples/)*吗？***
> 
> **Arrays.sort()不能直接用于按降序对基元数组进行排序。如果我们试图通过传递 Collections.reverseOrder()定义的反向比较器来调用 Arrays.sort()方法，它将抛出如下所示的错误:**

**![](img/a6887655370f6935d0747b73afa74648.png)**

> ****提示**:但是这对于像**整数**数组这样的“对象数组”来说很好，但是对于像 int 数组这样的原始数组就不行了。**

****情况 2:** 按降序排列数组**

****例****

## **Java 语言(一种计算机语言，尤用于创建网站)**

```java
// Java Program to Demonstrate Working of reverseOrder()
// method of Collections class
// To Sort an Array in Descending Order

// Importing required utility classes
import java.util.*;

// Main class
// CollectionSorting
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an array to be sorted in descending
        // order
        Integer[] arr = { 30, 20, 40, 10 };

        // Collections.sort method is sorting the
        // elements of arr[] in descending order
        // later on Arrays.sort() is applied to sort array
        Arrays.sort(arr, Collections.reverseOrder());

        // Printing the sorted array on console
        System.out.println(
            "Array after the use of Collection.reverseOrder()"
            + " and Arrays.sort() :\n"
            + Arrays.toString(arr));
    }
}

Array after the use of Collection.reverseOrder() and Arrays.sort() :
[40, 30, 20, 10]
```** 

****案例 3:** 当有用户自定义的比较器做反向比较时，按卷号降序对学生进行排序。**

```java
public static Comparator reverseOrder(Comparator c) 

// Java Program to Demonstrate Working of
// reverseOrder(Comparator c)
// To sort students in descending order of roll numbers
// when there is a user defined comparator to do reverse

// Importing required classes
import java.io.*;
import java.lang.*;
import java.util.*;

// Class 1
// Helper student class
// to represent a student
class Student {
    int rollno;
    String name, address;

    // Constructor
    public Student(int rollno, String name, String address)
    {

        // This keyword refers to current instance itself
        this.rollno = rollno;
        this.name = name;
        this.address = address;
    }

    // Method of Student class
    // To print student details inside main() method
    public String toString()
    {

        return this.rollno + " " + this.name + " "
            + this.address;
    }
}

// Class 2
// Helper class implementing interface
class Sortbyroll implements Comparator<Student> {

    // Method
    // Used for sorting in ascending order of
    // roll number
    public int compare(Student a, Student b)
    {
        return a.rollno - b.rollno;
    }
}

// Class 3
// Main class
class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating an empty ArrayList
        ArrayList<Student> ar = new ArrayList<Student>();

        // Adding custom attributes defined in Student class
        // using add() method
        ar.add(new Student(111, "bbbb", "london"));
        ar.add(new Student(131, "aaaa", "nyc"));
        ar.add(new Student(121, "cccc", "jaipur"));

        // Display message for better readability
        System.out.println("Unsorted");

        // Printing list of students
        for (int i = 0; i < ar.size(); i++)
            System.out.println(ar.get(i));

        // Sorting a list of students in descending order of
        // roll numbers using a Comparator
        // that is reverse of Sortbyroll()
        Comparator c
            = Collections.reverseOrder(new Sortbyroll());
        Collections.sort(ar, c);

        // Display message for better readability
        System.out.println("\nSorted by rollno");

        // Printing sorted students in descending order
        for (int i = 0; i < ar.size(); i++)
            System.out.println(ar.get(i));
    }
}

Unsorted
111 bbbb london
131 aaaa nyc
121 cccc jaipur

Sorted by rollno
131 aaaa nyc
121 cccc jaipur
111 bbbb london