基于多个属性的排序列表Collection.sort（）编译错误

Question

I need to sort a list using multiple properties, I tried this code but I am getting a compilation error

package com.demo;

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

import lombok.Data;
import lombok.extern.log4j.Log4j;

@Log4j
public class TestSort {

    public static void main(String[] args) {

        List<Kot> kots = new ArrayList<Kot>(){{

            add(new Kot("aa",1));
            add(new Kot("vv",1));
            add(new Kot("zz",2));
            add(new Kot("bb",3));
            add(new Kot("cc",1));
        }};

        log.info(kots);
        Collections.sort(kots);
        log.info(kots);
    }
}

@Data
class Kot implements Comparator<Kot> {

    private String productName;
    private Integer kotNo;

    public Kot(){}

    public Kot(String productName,Integer kotNo){
        this.productName = productName;
        this.kotNo = kotNo;
    }

    @Override
    public int compare(Kot kot1, Kot kot2) {
        int kotNoCompare = kot1.kotNo.compareTo(kot2.kotNo);
        if (kotNoCompare == 0) {
            int productNameCompare = kot1.productName.compareTo(kot2.productName);
            return productNameCompare;
        }
        return kotNoCompare;
    }
}

Error showing in following line

Collections.sort(kots);

Error Saying that,

Bound mismatch: The generic method sort(List) of type Collections is not applicable for the arguments (List). The inferred type Kot is not a valid substitute for the bounded parameter >

What I'm doing wrong?

Answer 1

you need to implement Comparable Interface for this. Eg:

class Kot implements Comparable<Kot> {

    private String productName;
    private Integer kotNo;

    public Kot() {
    }

    public Kot(String productName, Integer kotNo) {
        this.productName = productName;
        this.kotNo = kotNo;
    }

    @Override
    public int compareTo(Kot kot1) {
        int kotNoCompare = kot1.kotNo.compareTo(this.kotNo);
        if (kotNoCompare == 0) {
            int productNameCompare = kot1.productName.compareTo(this.productName);
            return productNameCompare;
        }
        return kotNoCompare;
    }
}

If you want to use Comparator only, then you have to move that comparison logic into a separate class. Kot will remain a simple POJO class, containing only getter and setters.

Eg of Comparataor only logic:

class Comp implements Comparator<Kot>{
     @Override
        public int compare(Kot kot1, Kot kot2) {
            int kotNoCompare = kot1.getKotNo().compareTo(kot2.getKotNo());
            if (kotNoCompare == 0) {
                int productNameCompare = kot1.getProductName().compareTo(kot2.getProductName());
                return productNameCompare;
            }
            return kotNoCompare;
        }
}

Now to sort your list, you can use other method of sorting:

Collections.sort(kots, new Comp());

Answer 2

You made your Kot a Comparator of Kot s. However, sort() method expects Kot to be Comparable to other Kot s:

class Kot implements Comparable<Kot> {
    ...
    public int compareTo(Kot otherKot) {
        // Comparison logic needs to be transformed
        // to compare otherKot to this, rather than kot1 to kot2
        ...
    }
}

You could also split your class into Kot and KotComparator , move comparator logic into KotComparator , and use sort() overload that takes a custom comparator to do sorting.

Answer 3

I assume you need to sort

• First with kotNo
• Then with productName

Actually this Comparable thing is an age old one. With Java 8 all you need is only one line,

List<Kot> kots = new ArrayList<Kot>(){{
        add(new Kot("aa",1));
        add(new Kot("vv",1));
        add(new Kot("zz",2));
        add(new Kot("bb",3));
        add(new Kot("cc",1));
}};

kots = kots.stream().sorted(
                comparing(Kot::getKotNo)
                .thenComparing(comparing(Kot::getProductName)))
                .collect(toList());