How sort list of object with multiple data sort in java

Question

I got lot of sort example in single value inside pojo class.

but i want to sort with multiple value look like below example

before sort

Name          City           Roll
--------------------------------------

    sekhar ---------bbsr------------23
    himanshu -------agra------------23
    nitin ----------delhi------------23
    nitin ----------delhi------------22
    nitin ----------bbsr------------23
    arun -----------patna------------23
    arun -----------kendrapara-------23

after sort

Name          City           Roll
--------------------------------------

    arun -------kendrapara------------23
    arun -------patna------------23
    himanshu ----agra------------23
    nitin -------bbsr------------23
    nitin -------delhi------------22
    nitin -------delhi------------23
    sekhar -------bbsr------------23

My code is here

package example;

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

public class Demo {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Demo demo = new Demo();
        List<Student> list = new ArrayList<Student>(); // your Car list
        Student st = demo.new Student();
        st.setName("sekhar");
        st.setCity("bbsr");
        st.setRoll(23);
        list.add(st);
        st = demo.new Student();
        st.setName("himanshu");
        st.setCity("agra");
        st.setRoll(23);
        list.add(st);
        st = demo.new Student();
        st.setName("nitin");
        st.setCity("delhi");
        st.setRoll(23);
        list.add(st);
        st = demo.new Student();
        st.setName("nitin");
        st.setCity("delhi");
        st.setRoll(22);
        list.add(st);
        st = demo.new Student();
        st.setName("nitin");
        st.setCity("bbsr");
        st.setRoll(23);
        list.add(st);
        st = demo.new Student();
        st.setName("arun");
        st.setCity("patna");
        st.setRoll(23);
        list.add(st);
        st = demo.new Student();
        st.setName("arun");
        st.setCity("kendrapara");
        st.setRoll(23);
        list.add(st);
        System.out.println("before sort");
        System.out.println("Name:" + "-------------------" + "City...........roll");
        for (Student st1 : list) {
            System.out.println(st1.getName() + " --------------" + st1.getCity() + "------------" + st1.getRoll());
        }

        Collections.sort(list, demo.new CarHorsePowerComparator());
        System.out.println("after sort");
        System.out.println("Name:" + "-------------------" + "City...............roll");
        for (Student st1 : list) {
            System.out.println(st1.getName() + " --------------" + st1.getCity() + "------------" + st1.getRoll());
        }
    }

    public class Student {

        String name;
        String city;
        int roll;

        public int getRoll() {
            return roll;
        }

        public void setRoll(int roll) {
            this.roll = roll;
        }

        public String getCity() {
            return city;
        }

        public void setCity(String city) {
            this.city = city;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }
    }

    class CarHorsePowerComparator implements Comparator<Student> {
        public int compare(Student stu, Student stu1) {
            int sort = (stu.getName() + stu.getCity() + stu.getRoll()).compareTo((stu1.getName() + stu1.getCity() + stu1.getRoll()));
            return sort;
        }
    }
}

but i'am using custom comparator

class CarHorsePowerComparator implements Comparator<Student> {
            public int compare(Student stu, Student stu1) {
                int sort = (stu.getName() + stu.getCity() + stu.getRoll()).compareTo((stu1.getName() + stu1.getCity() + stu1.getRoll()));
                return sort;
            }

it's working for me but it's not right way. how to write this code in correct way Please suggest me.

int sort = (stu.getName() + stu.getCity() + stu.getRoll()).compareTo((stu1.getName() + stu1.getCity() + stu1.getRoll()));

Answer 1

(stu.getName() + stu.getCity() + stu.getRoll()).compareTo((stu1.getName() + stu1.getCity() + stu1.getRoll())) 

means you are contcatinating Name, City and Roll number and sorting on the resulting string.

You should decide the sorting priorities first and sort on the fields in that order. Looking at the expected result seems like you want to sort on name first then on city and lastly on roll.

class CarHorsePowerComparator implements Comparator<Student> {
            public int compare(Student stu, Student stu1) {
                int sort;
                // Sort on Name first 
                sort = stu.getName().compareTo(stu1.getName);
                if(sort == 0){
                  // If name is equal, sort on city
                  sort = stu.getCity().compareTo(stu1.getCity);
                }   
                if(sort == 0){
                  // If name and city is equal, sort on Roll
                  sort = Integer.compare(stu.getRoll(),stu1.getRoll());
                } 
                return sort;
            }
}

Answer 2

When you are using the plus operator you create lots of new String objects. String should be used to store some data not for logic purpose.

You should not use the parameters names as stu and stu1, because is error prone. You can use me and other or left and 'right'.

You do not need a class declaration as you will be reusing it. The annonymous is fine

private final static Comparator<Student> STUDENT_NAME_CITY_ROLL_COMPARATOR = new Comparator<Student> {

  private final Collator collator = Collator.getInstance(); //Default local is taken. 

  public int compare(Student left, Student right) {

    int result = collator.compare(left.getName(),right.getName()); 

    if(result == 0) {
        result = collator.compare(left.getCity(),right.getCity());
    } 

    if(result == 0) {
        result = Integer.compare(left.getRoll(), right.getRoll()); //JDK 7
       //result = Double.compare(left.getRoll(), right.getRoll());
    }

    return result;
}

Note: That the implementation will throw an NPE when some of the elements are null.

Collator

Integer.compare(int,int)

Answer 3

int sort = stu.getName().compareTo(stu1.getName());
if (sort == 0)
    sort = stu.getCity().compareTo(stu1.getCity())
if (sort == 0)
...
return res;

The point is that you continue comparison only if current components are equal