How to sort List in Java

You can sort list in java using Comparator and Collections class. I am going to explain with following example. Following example Uses List of Employees and sort them by using firstName, lastName and id.

public class Employee {

    private int id;
    private String firstName;
    private String lastName;

    Employee(int id, String firstName, String lastName) {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

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

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getLastName() {
        return lastName;
    }

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

    @Override
    public String toString() {
        return "Employee{" + "id=" + id + "firstName=" + firstName + "lastName=" + lastName + '}';
    }
}

import java.util.Comparator;

public class EmployeeIdComparator implements Comparator<Employee> {

    public int compare(Employee emp1, Employee emp2) {
        return emp1.getId() - emp2.getId();
    }
}

import java.util.Comparator;

public class EmployeeFirstNameComparator implements Comparator<Employee> {

    public int compare(Employee emp1, Employee emp2) {
        return emp1.getFirstName().compareTo(emp2.getFirstName());
    }
}

import java.util.Comparator;

public class EmployeeLastNameComparator implements Comparator<Employee> {

    public int compare(Employee emp1, Employee emp2) {
        return emp1.getLastName().compareTo(emp2.getLastName());
    }
}

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

public class TestEmployee {

    static void printList(List<Employee> empList, String msg) {
        System.out.println(msg);
        System.out.println("----------------");
        for (Employee emp : empList) {
            System.out.println(emp);
        }
    }

    public static void main(String args[]) {
        Employee emp1 = new Employee(1, "Shreyas", "Desai");
        Employee emp2 = new Employee(3, "Piyush", "Rai");
        Employee emp3 = new Employee(2, "Shanmugam", "Chinnappaiyan");
        Employee emp4 = new Employee(5, "Ankit", "Suyal");
        Employee emp5 = new Employee(4, "Hari Krishna", "Gurrm");

        List<Employee> empList = new ArrayList<Employee>();

        empList.add(emp1);
        empList.add(emp2);
        empList.add(emp3);
        empList.add(emp4);
        empList.add(emp5);

        printList(empList, "Before sorting");

        Collections.sort(empList, new EmployeeIdComparator());
        printList(empList, "After sorting by Employee Id");

        Collections.sort(empList, new EmployeeFirstNameComparator());
        printList(empList, "After sorting by Employee first name");

        Collections.sort(empList, new EmployeeLastNameComparator());
        printList(empList, "After sorting by Employee last name");
    }
}

Output

Before sorting
----------------
Employee{id=1firstName=ShreyaslastName=Desai}
Employee{id=3firstName=PiyushlastName=Rai}
Employee{id=2firstName=ShanmugamlastName=Chinnappaiyan}
Employee{id=5firstName=AnkitlastName=Suyal}
Employee{id=4firstName=Hari KrishnalastName=Gurrm}
After sorting by Employee Id
----------------
Employee{id=1firstName=ShreyaslastName=Desai}
Employee{id=2firstName=ShanmugamlastName=Chinnappaiyan}
Employee{id=3firstName=PiyushlastName=Rai}
Employee{id=4firstName=Hari KrishnalastName=Gurrm}
Employee{id=5firstName=AnkitlastName=Suyal}
After sorting by Employee first name
----------------
Employee{id=5firstName=AnkitlastName=Suyal}
Employee{id=4firstName=Hari KrishnalastName=Gurrm}
Employee{id=3firstName=PiyushlastName=Rai}
Employee{id=2firstName=ShanmugamlastName=Chinnappaiyan}
Employee{id=1firstName=ShreyaslastName=Desai}
After sorting by Employee last name
----------------
Employee{id=2firstName=ShanmugamlastName=Chinnappaiyan}
Employee{id=1firstName=ShreyaslastName=Desai}
Employee{id=4firstName=Hari KrishnalastName=Gurrm}
Employee{id=3firstName=PiyushlastName=Rai}
Employee{id=5firstName=AnkitlastName=Suyal}