Sort List of objects on multiple fields

This is one of my favorite interview questions on java collections. Let’s say there is Employee object with fields like firstName, lastName and salary. Now I want to sort list of employees based on firstName and Salary.

firstName	lastName	salary
Hari	Krishna	25000
Hari	Kishan	50000
Piyush	Chawla	65000
Shreyas	Desai	150000
Maruthi	Kumar	75000
Maruthi	Krishna	35000

We can solve this problem by writing custom comparator. First we have to compare employee objects using their firstName. If their firstNames are equal, then we have to compare again using salaries and return the value. If firstNames are different, then no need to compare employee objects further.

Compare function code looks like below.

public int compare(Employee o1, Employee o2) {
        String firstName1 = o1.getFirstName();
        String firstName2 = o2.getFirstName();

        /* If names are equal compare by using salaries */
        if(firstName1.compareTo(firstName2) == 0){
            Double salary1 = o1.getSalary();
            Double salary2 = o2.getSalary();
            return salary1.compareTo(salary2);
        }

        return firstName1.compareTo(firstName2);
}

Employee.java

public class Employee{
    private String firstName;
    private String lastName;
    private double salary;

    public Employee(String firstName, String lastName, double salary){
        this.firstName = firstName;
        this.lastName = lastName;
        this.salary = salary;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public double getSalary() {
        return salary;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }
}

EmployeeComparator.java

import java.util.*;

public class EmployeeComparator implements Comparator<Employee>{

    public int compare(Employee o1, Employee o2) {
        String firstName1 = o1.getFirstName();
        String firstName2 = o2.getFirstName();

        /* If names are equal compare by using salaries */
        if(firstName1.compareTo(firstName2) == 0){
            Double salary1 = o1.getSalary();
            Double salary2 = o2.getSalary();
            return salary1.compareTo(salary2);
        }

        return firstName1.compareTo(firstName2);
    }

}

TestApplication.java

import java.util.*;

public class TestApplication {
    public static void main(String args[]){
        Employee e1 = new Employee("Hari", "Krishna", 25000);
        Employee e2 = new Employee("Hari", "Kishan", 50000);
        Employee e3 = new Employee("Piyush", "Chawla", 65000);
        Employee e4 = new Employee("Shreyas", "Desai", 150000);
        Employee e5 = new Employee("Maruthi", "Kumar", 75000);
        Employee e6 = new Employee("Maruthi", "Krishna", 35000);
        Employee emp;

        List<Employee> empList = new ArrayList<Employee> ();
        empList.add(e1);
        empList.add(e2);
        empList.add(e3);
        empList.add(e4);
        empList.add(e5);
        empList.add(e6);

        Collections.sort(empList, new EmployeeComparator());
        Iterator<Employee> iter = empList.iterator();

        while(iter.hasNext()){
            emp = iter.next();
            System.out.println(emp.getFirstName() +" " + emp.getLastName() +" " +emp.getSalary());
        }
        
    }
}

Output

Hari Krishna 25000.0
Hari Kishan 50000.0
Maruthi Krishna 35000.0
Maruthi Kumar 75000.0
Piyush Chawla 65000.0
Shreyas Desai 150000.0

Related Links

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