使用Java中的比较器按降序对 ArrayList 进行排序

Comparator 是一个接口，用于以排序的方式重新排列 ArrayList。比较器用于对用户定义的对象的 ArrayList 进行排序。在Java中，在Java.util 包中提供了一个 Comparator。在多个变量的基础上使用Comparator对ArrayList进行排序，或者干脆实现Comparator而不影响原来的User-defined类。要使用 Comparator 对 ArrayList 进行排序，请覆盖比较器接口提供的 compare() 方法。

以这样一种方式覆盖compare() 方法，它将按降序而不是升序对 ArrayList 重新排序。仅在比较部分更改为降序。查看两者 compare()函数之间的区别。

Ascending Order
public int compare(Item i1, Item i2)
    {
        if (i1.Property== i2.Property)
            return 0;
        else if (s1.Property > s2.Property)
            return 1;
        else
            return -1;
    }
    
Descending Order
public int compare(Item i1, Item i2)
    {
        if (i1.Property== i2.Property)
            return 0;
        else if (s1.Property < s2.Property)
            return 1;
        else
            return -1;
    }

如果需要根据名称等字符串类型的变量对 ArrayList 重新排序，请重写 compare()函数，如下所示。

Ascending Order
public int compare(Shop s1, Shop s2)
    {
        return s1.name.compareTo(s2.name);
    }
    
Descending Order
public int compare(Shop s1, Shop s2)
    {
        return s2.name.compareTo(s1.name);
    }

例子：

Java

// Java Program to sort the ArrayList 
// in descending order using comparator
import java.util.*;
  
// create the Laptop class
class Laptop {
    int ModalNo;
    String name;
    int ram;
    Laptop(int ModalNo, String name, int ram)
    {
        this.ModalNo = ModalNo;
        this.name = name;
        this.ram = ram;
    }
}
  
// creates the comparator for comparing RAM
class RamComparator implements Comparator {
    // override the compare() method
    public int compare(Laptop l1, Laptop l2)
    {
        if (l1.ram == l2.ram) {
            return 0;
        }
        else if (l1.ram < l2.ram) {
            return 1;
        }
        else {
            return -1;
        }
    }
}
  
class GFG {
    public static void main(String[] args)
    {
        // create the ArrayList object
        ArrayList l = new ArrayList();
        l.add(new Laptop(322, "Dell", 2));
        l.add(new Laptop(342, "Asus", 8));
        l.add(new Laptop(821, "HP", 16));
        l.add(new Laptop(251, "Lenovo", 6));
        l.add(new Laptop(572, "Acer", 4));
  
        System.out.println("before sorting");
        System.out.println("Ram"
                           + " "
                           + "Name"
                           + " "
                           + "ModalNo");
        for (Laptop laptop : l) {
            System.out.println(laptop.ram + " "
                               + laptop.name + " "
                               + laptop.ModalNo);
        }
        System.out.println();
  
        System.out.println("After sorting(sorted by Ram)");
        System.out.println("Ram"
                           + " "
                           + "Name"
                           + " "
                           + "ModalNo");
  
        // call the sort function
        Collections.sort(l, new RamComparator());
        for (Laptop laptop : l) {
            System.out.println(laptop.ram + " "
                               + laptop.name + " "
                               + laptop.ModalNo);
        }
    }
}

输出

before sorting
Ram Name ModalNo
2 Dell 322
8 Asus 342
16 HP 821
6 Lenovo 251
4 Acer 572

After sorting(sorted by Ram)
Ram Name ModalNo
16 HP 821
8 Asus 342
6 Lenovo 251
4 Acer 572
2 Dell 322