Sort a List in Java With Comparable and Comparator

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1. Hi,

   If we have generic list with different object, how can we sort it
   Below is the example…

   List list=new ArrayList();

   Laptop l1 =new Laptop(1,”Mac”,800)
   Laptop l2=new Laptop(2,”dell”,500)
   Laptop l3 =new Laptop(3,”acer”,200)
   list.add(l1);
   list.add(l2);
   list.add(l3);

   Book b1=new Book(1,”science”,500);
   Book b2=new Book(2″science”,600);
   Book b3=new Book(3,”science”,700);
   list.add(b1);
   list.add(b2);
   list.add(b3);

   I tried implementing compareTo() method in both the entity classes but it does work

   Both Laptop and Book has a common variable of price
   Can we sort this based on price? if so please help me doing it

   Thanks in advance

   • I think you need to abstract out the price in a superclass and inherit the Laptop and Book from that. The superclass say Item should inherit from comparable and implement the interface compareTo() method based on the price. Hope this helps.

     public class MyItem implements Comparable<MyItem>  {
             double price;
     
     	@Override
     	public int compareTo(MyItem o) {
     		return Double.compare(this.getPrice(), o.getPrice());
     	}
     }
     List<MyItem> items = new ArrayList<MyItem>();
     ...
     ...
     Collections.sort(items );
     //This will give output 200, 500, 500, 600, 700, 800
     
     

2. Hi Lokesh,

   I have one Doubt that i have gone through java docs and it has mentioned that “Lists (and arrays) of objects that implement this interface can be sorted automatically by Collections.sort (and Arrays.sort). “.Then let me know is it true that sort() method internally calls compareTo() method or not.How it call internally

   Ex:

   public class Country implements Comparable{
   int countryId;
   String countryName;

   public Country(int countryId, String countryName) {
   super();
   this.countryId = countryId;
   this.countryName = countryName;
   }

   @Override
   public int compareTo(Country country) {
   return (this.countryId country.countryId ) ? 1:0 ;
   }

   public int getCountryId() {
   return countryId;
   }

   public void setCountryId(int countryId) {
   this.countryId = countryId;
   }

   public String getCountryName() {
   return countryName;
   }

   public void setCountryName(String countryName) {
   this.countryName = countryName;
   }

   }

   ComparableMain.java

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

   public class ComparableMain {

   public static void main(String[] args) {
   Country indiaCountry=new Country(1, “India”);
   Country chinaCountry=new Country(4, “USA”);
   Country nepalCountry=new Country(3, “Russia”);
   Country bhutanCountry=new Country(2, “Japan”);

   List listOfCountries = new ArrayList();
   listOfCountries.add(indiaCountry);
   listOfCountries.add(usaCountry);
   listOfCountries.add(russiaCountry);
   listOfCountries.add(japanCountry);

   System.out.println(“Before Sort : “);
   for (int i = 0; i < listOfCountries.size(); i++) {
   Country country=(Country) listOfCountries.get(i);
   System.out.println("Country Id: "+country.getCountryId()+"||"+"Country name:"+country.getCountryName());
   }
   Collections.sort(listOfCountries);

   //Here Hoe it call compareTo() internally.

   • Collections.sort() internally call Arrays.sort(), which in turn use Arrays.mergeSort(). This mergeSort() method uses compareTo() for comparing elements.

3. Hi lokesh,
   I am getting the following error when I want to override the method of Comparable interface.Can you please help me to know the reason for the error.The below is the source code which I have written.

   Error : The method compareTo(Employee) of type Employee must override a superclass method.

   public class Employee implements Comparable {
   private int empId;
   private String empName;
   private int age;
   private int salary;
   public int getEmpId() {
   return empId;
   }
   public void setEmpId(int empId) {
   this.empId = empId;
   }
   public String getEmpName() {
   return empName;
   }
   public void setEmpName(String empName) {
   this.empName = empName;
   }
   public int getAge() {
   return age;
   }
   public void setAge(int age) {
   this.age = age;
   }
   public int getSalary() {
   return salary;
   }
   public void setSalary(int salary) {
   this.salary = salary;
   }
   @Override
   public int compareTo(Employee e) {
   // TODO Auto-generated method stub
   return this.empId-e.empId;
   }
   @Override
   public String toString(){
   return “Employee Details: Emp Id: “+empId +” Name: “+empName+” Age: “+age+” Salary: “+salary;
   }

   }

   Thanks and Regards,
   Chandra.

   • Method is “public int compareTo(Object e)”. Inside method cast the object from object to employee.
     OR, declare the class like this: Employee implements Comparable <Employee>

4. Hi Lokesh,

   Can you explain more on how to make compare() and compareTo() null safe and its contract with override equal() method.

   Regards,
   Himansu

   • For null handling. refer to this guide.

     As per java docs of Comparable interface :

     “The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C. Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.”

5. Hi Lokesh,

   This is really a gud and lucid explanation but how can we write more null safe compare and compareto method. Entities with null reference or with null fields can slip to this method breaking the code.

   Regards,
   Himansu

6. in your example for comparable, how Collections class know that it need to sort on the basis of id only

   • Here I have mentioned it.
     @Override
public int compareTo(Employee o) {
return this.id - o.id;
}

7. Thanks for your nice text, but I have seen one good article on same topic. If anyone interested on same can read that here…

8. Hi Lokesh,
   I know Comparable & Comparator interface, it is using for sorting objects. But i have some confusion in these 2 Interface when i use comparable & Comparator…?

   • Use Comparable if you want to implement default sorting. e.g. Employees will be sort based on their employee ids. Comparator should be used when you are adding multiple ways of sorting or sorting on uncommon fields. e.g. sorting based on Lastname or emaployee age.

     • Thx Lokesh. Really it’s great job…..

     • Suppose I need to sort based on both fields Lastname AND employee age then? for example flight search Time and Price then how I would proceed?

       • Before java 8 solution will be like this if you want to sort first by lastName and then firstName filed:

         import java.util.*;
         
         public class Name implements Comparable<Name> {
             private final String firstName, lastName;
         
            //More code
             public String toString() {
         	return firstName + " " + lastName;
             }
         
             public int compareTo(Name n) {
                 int lastCmp = lastName.compareTo(n.lastName); //First compare by last name
                 return (lastCmp != 0 ? lastCmp : firstName.compareTo(n.firstName)); //If lat name matches then compare by first name
             }
         }
         

         For java 8 solution, use something like this:

         List<Employee> employees  = getEmployees();
          
         //Sorting on multiple fields; Group by.
         Comparator<Employee> groupByComparator = Comparator.comparing(Employee::getFirstName)
                                                             .thenComparing(Employee::getLastName);
         employees.sort(groupByComparator);
         

         https://howtodoinjava.com/java8/comparator-example-lambda/

9. If you want to sort by any field of an object no need to write different comparators for each of the field, instead create the constructor for comparator and pass the sort field and sort direction and compare method to have code for each of the cases.

   • Yes, this is also possible. But it would need to put all such cases/logic in single class. I do not prefer this.

10. Please correct the following sentence in your blog :)

    Comparable interface provides one method compare(Object o) to implement in any class so that two instances of that class can be compared.

    from compare(Object o) to int compareTo(T o)

    May be that should be a typo :)

    • Good catch :)

      • Hi Lokesh,
        Please correct the typo asap. It wil really create confusions.

        • Done

11. Is there any way to sort the objects with the Id’s in middle out fashion ,say objects with ids 1,2,3,4,5 are in order.I want to sort the order in the following manner 3 ,4,2,5,1

    Thank you.

    • Raj, Its really interesting logic but I don’t have any specific logic in mind right now. May be I will try to find something. If you get it working, please share.

    • Hi Raj

      It’s really nice to heard your question.But in normal we are having two types of sorting i.e. Asc./Desc. But per your requirement You are looking something extra than these two.So if You find any solution Please share with us.

12. // Sorting user-defined object in TreeSet based on id, name
    import java.util.*;

    class Stud
    {
    int id;
    String name;

    Stud(int id, String name)
    {
    this.id=id;
    this.name=name;
    }

    public String toString()
    {
    return id + ” ” + name;
    }
    }

    class Studnames implements Comparator
    {
    public int compare(Stud s1, Stud s2)
    {

    return s1.name.compareTo(s2.name);
    }
    }

    class Studid implements Comparator
    {
    public int compare(Stud s1, Stud s2)
    {

    if(s1.id>s2.id)
    {
    return 0;
    }
    else if(s1.id>s2.id)
    {
    return -1;
    }
    else
    {
    return -1;
    }
    }
    }

    public class Al
    {

    public static void main(String args[])
    {

    SortedSet st = new TreeSet( new Studnames());
    st.add(new Stud(100,”amritsar”));
    st.add(new Stud(21,”bangalore”));
    st.add(new Stud(212,”orrisa”));
    st.add(new Stud(213,”mumbai”));

    System.out.println(“Sorting by names”);
    System.out.println(st);

    System.out.println(” “);

    System.out.println(“Sorting by id”);
    SortedSet st1 = new TreeSet( new Studid());
    st1.add(new Stud(101,”amritsar”));
    st1.add(new Stud(221,”bangalore”));
    st1.add(new Stud(124,”orrisa”));
    st1.add(new Stud(121,”mumbai”));

    System.out.println(st1);

    }

    }

    1. Have look at the above code
    2. Have used TreeSet to sort user defined objects based on id and names.
    3. OUTPUT:

    Sorting by names
    [100 amritsar, 21 bangalore, 213 o, 212 orrisa]

    Sorting by id
    [101 amritsar]

    4. why id’s not getting sorted only one id is getting displayed?

    • Correct compare method for class Studid implements Comparator is:

      public int compare(Stud s1, Stud s2)
      {
      if (s1.id > s2.id) {
      return 1;
      } else if (s1.id < s2.id) { return -1; } else { return 0; } } Use above method and you will get correct output: Sorting by names [100 amritsar, 21 bangalore, 213 mumbai, 212 orrisa] Sorting by id [101 amritsar, 121 mumbai, 124 orrisa, 221 bangalore]

      • Is there any way to sort the objects with the Id’s and Names ,say objects with ids 1,1,2,2 and Names with A,B,A,B.I want to sort the order in the following manner 1,1,A,A,2,2,B,B.

        • If I will be in your place, then I will d this kind of sorting at database later while executing SELECT query. No need to put complex things in application, if desired result is easily achievable through one group by and one sort column in SELECT.

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