Java Comparator.reversed
March 07, 2019
reversed is the default method of Java Comparator functional interface. This method is introduced in Java 8. reversed returns a Comparator that imposes the reverse ordering of this Comparator. It has been declared as following.
default Comparator<T> reversed()
reversed method, we need to instantiate our Comparator and call this method. reversed will return new instance of Comparator that will impose the reverse ordering of this comparator. Find the sample code to use this method.
Comparator<Student> nameComparator = (s1, s2) -> s1.getName().compareTo(s2.getName()); Collections.sort(list, nameComparator.reversed());
Comparator instance can be passed as an argument in Stream.sorted, List.sort and Collections.sort to sort the elements. Now lets us discuss using Comparator.reversed step-by-step.
Contents
Comparator.reversed with Stream.sorted
Stream.sorted returns a stream consisting of the elements of this stream, sorted according to the provided comparator.
StreamSortedDemo.java
package com.concretepage;
import java.util.Comparator;
import java.util.List;
public class StreamSortedDemo {
public static void main(String[] args) {
List<Student> list = Student.getStudentList();
Comparator<Student> ageComparator = (s1, s2) -> s1.getAge() - s2.getAge();
list.stream().sorted(ageComparator.reversed()).forEach(s -> System.out.print(s.getAge() + " "));
System.out.println("\n-----------");
Comparator<Student> nameComparator = (s1, s2) -> s1.getName().compareTo(s2.getName());
list.stream().sorted(nameComparator.reversed()).forEach(s -> System.out.print(s.getName() + " "));
System.out.println("\n-----------");
list.stream().sorted(Comparator.comparing(Student::getAge).reversed()).forEach(s -> System.out.print(s.getAge() + " "));
System.out.println("\n-----------");
list.stream().sorted(Comparator.comparing(Student::getName).reversed()).forEach(s -> System.out.print(s.getName() + " "));
}
}
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;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
public static List<Student> getStudentList(){
Student s1 = new Student("Ram", 18);
Student s2 = new Student("Shyam",22);
Student s3 = new Student("Mohan",19);
Student s4 = new Student("Mahesh",20);
Student s5 = new Student("Krishna",21);
List<Student> list = Arrays.asList(s1,s2,s3,s4,s5);
return list;
}
}
22 21 20 19 18 ----------- Shyam Ram Mohan Mahesh Krishna ----------- 22 21 20 19 18 ----------- Shyam Ram Mohan Mahesh Krishna
Comparator.reversed with List.sort
List.sort sorts this list according to the given Comparator instance.
ListSortDemo.java
package com.concretepage;
import java.util.Comparator;
import java.util.List;
public class ListSortDemo {
public static void main(String[] args) {
List<Student> list = Student.getStudentList();
Comparator<Student> ageComparator = (s1, s2) -> s1.getAge() - s2.getAge();
list.sort(ageComparator.reversed());
list.forEach(s -> System.out.print(s.getAge() + " "));
System.out.println("\n-----------");
Comparator<Student> nameComparator = (s1, s2) -> s1.getName().compareTo(s2.getName());
list.sort(nameComparator.reversed());
list.forEach(s -> System.out.print(s.getName() + " "));
System.out.println("\n-----------");
list.sort(Comparator.comparing(Student::getAge).reversed());
list.forEach(s -> System.out.print(s.getAge() + " "));
System.out.println("\n-----------");
list.sort(Comparator.comparing(Student::getName).reversed());
list.forEach(s -> System.out.print(s.getName() + " "));
}
}
Comparator.reversed with Collections.sort
Collections.sort sorts the specified list according to the given Comparator instance.
CollectionsSortDemo.java
package com.concretepage;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class CollectionsSortDemo {
public static void main(String[] args) {
List<Student> list = Student.getStudentList();
Comparator<Student> ageComparator = (s1, s2) -> s1.getAge() - s2.getAge();
Collections.sort(list, ageComparator.reversed());
list.forEach(s -> System.out.print(s.getAge() + " "));
System.out.println("\n-----------");
Comparator<Student> nameComparator = (s1, s2) -> s1.getName().compareTo(s2.getName());
Collections.sort(list, nameComparator.reversed());
list.forEach(s -> System.out.print(s.getName() + " "));
System.out.println("\n-----------");
Collections.sort(list, Comparator.comparing(Student::getAge).reversed());
list.forEach(s -> System.out.print(s.getAge() + " "));
System.out.println("\n-----------");
Collections.sort(list, Comparator.comparing(Student::getName).reversed());
list.forEach(s -> System.out.print(s.getName() + " "));
}
}
References
Interface ComparatorJava 8 Stream sorted() Example
