Java List sort() Example
February 07, 2020
sort is default method of java.util.List interface. The sort method of List has been introduced in Java 8.
1. The
sort method accepts the Comparator as an argument and sorts this List according to specified Comparator.
default void sort(Comparator<? super E> c)
List are comparable i.e. the element class is implementing Comparable interface, then we can pass null to sort method and sorting will take place according to natural ordering.
3. The
List must not be resizable though it can be modifiable.
sort() with Comparator
Example-1: We will create aComparator object using Comparator.comparing method.
Comparator<Student> nameComparator = Comparator.comparing(Student::getName);
sort method of the List.
students.sort(nameComparator);
students will sort all the students object according to the order induced by specified Comparator.
We can reverse the soring order as following.
students.sort(nameComparator.reversed());
ListSort1.java
package com.concretepage;
import java.util.Comparator;
import java.util.List;
public class ListSort1 {
public static void main(String[] args) {
List<Student> students = Student.getStudents();
System.out.println("--- Sort by name in ascending order ---");
Comparator<Student> nameComparator = Comparator.comparing(Student::getName);
students.sort(nameComparator);
students.forEach(s -> System.out.println(s));
System.out.println("--- Sort by name in descending order ---");
students.sort(nameComparator.reversed());
students.forEach(s -> System.out.println(s));
System.out.println("--- Sort by age in ascending order ---");
Comparator<Student> ageComparator = Comparator.comparing(Student::getAge);
students.sort(ageComparator);
students.forEach(s -> System.out.println(s));
System.out.println("--- Sort by age in descending order ---");
students.sort(ageComparator.reversed());
students.forEach(s -> System.out.println(s));
}
}
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class Student {
private String name;
private int age;
public Student(String name, int age) {
this.name = name;
this.age = age;
}
//Setters and Getters
public static List<Student> getStudents() {
Student s1 = new Student("Lakshman", 25);
Student s2 = new Student("Ram", 30);
Student s3 = new Student("Shatrudhan", 23);
Student s4 = new Student("Bharat", 28);
return Arrays.asList(s1, s2, s3, s4);
}
@Override
public String toString() {
return name + " - " + age;
}
}
--- Sort by name in ascending order --- Bharat - 28 Lakshman - 25 Ram - 30 Shatrudhan - 23 --- Sort by name in descending order --- Shatrudhan - 23 Ram - 30 Lakshman - 25 Bharat - 28 --- Sort by age in ascending order --- Shatrudhan - 23 Lakshman - 25 Bharat - 28 Ram - 30 --- Sort by age in descending order --- Ram - 30 Bharat - 28 Lakshman - 25 Shatrudhan - 23
Example-2: Here we will create comparator classes by implementing
Comparator interfaces and use them with sort method.
ListSort2.java
package com.concretepage;
import java.util.Comparator;
import java.util.List;
class NameComparator implements Comparator<Student> {
@Override
public int compare(Student s1, Student s2) {
return s1.getName().compareTo(s2.getName());
}
}
class AgeComparator implements Comparator<Student> {
@Override
public int compare(Student s1, Student s2) {
return s1.getAge() - s2.getAge();
}
}
public class ListSort2 {
public static void main(String[] args) {
List<Student> students = Student.getStudents();
System.out.println("--- Sort by name in ascending order ---");
students.sort(new NameComparator());
students.forEach(s -> System.out.println(s));
System.out.println("--- Sort by age in Ascending order ---");
students.sort(new AgeComparator());
students.forEach(s -> System.out.println(s));
}
}
--- Sort by name in ascending order --- Bharat - 28 Lakshman - 25 Ram - 30 Shatrudhan - 23 --- Sort by age in Ascending order --- Shatrudhan - 23 Lakshman - 25 Bharat - 28 Ram - 30
sort() with Comparable
If the element class ofList is implementing Comparable interface, we can sort it using sort method of List i.e. natural ordering. To sort the elements of List according to its natural order using sort method, we can pass null or Comparator.naturalOrder() to sort method.
employees.sort(null); employees.sort(Comparator.naturalOrder());
ListSort3.java
package com.concretepage;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
public class ListSort3 {
public static void main(String[] args) {
Employee e1 = new Employee(101, "Mohit");
Employee e2 = new Employee(102, "Mahesh");
Employee e3 = new Employee(103, "Jitendra");
Employee e4 = new Employee(104, "Keshav");
List<Employee> employees = Arrays.asList(e1, e2, e3, e4);
System.out.println("--- Sort by employee name in ascending order ---");
employees.sort(null);
//employees.sort(Comparator.naturalOrder());
employees.forEach(s -> System.out.println(s));
System.out.println("--- Sort by employee name in descending order ---");
employees.sort(Comparator.reverseOrder());
employees.forEach(s -> System.out.println(s));
}
}
class Employee implements Comparable<Employee> {
private int empId;
private String empName;
public Employee(int empId, String empName) {
this.empId = empId;
this.empName = empName;
}
//Setters and Getters
@Override
public int compareTo(Employee e) {
return empName.compareTo(e.getEmpName());
}
@Override
public String toString(){
return empId + "-" + empName;
}
}
--- Sort by employee name in ascending order --- 103-Jitendra 104-Keshav 102-Mahesh 101-Mohit --- Sort by employee name in descending order --- 101-Mohit 102-Mahesh 104-Keshav 103-Jitendra
