In this article, we will discuss how to reverse order of java.util.Comparator using Collections class’ utility reverseOrder() method
This is used to get reverse comparator, which can be used to reverse the elements of List as against original order of comparator
Reversing order of Comparator
Method signature:
public static Collection reverseOrder(Comparator c);
For example,
Comparator revOrder = Collections.reverseOrder(orginialComparatorOrder);
Where,
- revOrder = descending order
- orginialComparatorOrder = ascending order
Customer.java
- Customer POJO with 2 member variables of Integer and String type
- 2-arg constructor
- Overriding toString() method
package in.bench.resources.java.collection;
public class Customer {
// member variables
int customerId;
String customerName;
// 2-arg parameterized constructor
public Customer(int customerId, String customerName) {
super();
this.customerId = customerId;
this.customerName = customerName;
}
// override toString() method
@Override
public String toString() {
return "Customer ["
+ "customerId=" + customerId
+ ", customerName=" + customerName
+ "]";
}
}
CustomerIdComparator.java
- This is separate class which implements Comparator interface providing customized sorting logic
- compare() method provides sorting logic, according to customer Id
package in.bench.resources.java.collection;
import java.util.Comparator;
public class CustomerIdComparator implements Comparator<Customer> {
@Override
public int compare(Customer o1, Customer o2) {
return o1.customerId - o2.customerId;
}
}
Main class
- This class uses above customer POJO and customized sorting logic class
- implementing comparator interface, to store objects inside ArrayList
- sorting according to comparator (i.e.; reverse ordering of customer Id)
- prints customer objects in descending order of customer Id
ReverseOrderOfComparator.java
package in.bench.resources.java.collection;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
public class ReverseOrderOfComparator {
public static void main(String[] args) {
// creating ArrayList object of type Customer
ArrayList<Customer> al = new ArrayList<Customer>();
// adding elements to TreeSet object
al.add(new Customer(101, "Narayan Murthy"));
al.add(new Customer(107, "Dinesh"));
al.add(new Customer(103, "Nandan Nilekeni"));
al.add(new Customer(102, "Ashok Arora"));
al.add(new Customer(104, "Shibulal"));
al.add(new Customer(106, "Kris Gopalakrishnan"));
al.add(new Customer(105, "Raghavan"));
System.out.println("Before Sorting:"
+ " Insertion Order\n");
// insertion order
for(Customer cust : al){
System.out.println(cust.customerId
+ " "
+ cust.customerName);
}
// original order of comparator
CustomerIdComparator originalComparatorOrder =
new CustomerIdComparator();
// sorting using
// Collections.sort(al, originalComparatorOrder);
Collections.sort(al, originalComparatorOrder);
System.out.println("\n\nAfter Sorting"
+ " using Original Comparator order\n");
// reverse ordering of customer Id using Comparator
for(Customer cust : al){
System.out.println(cust.customerId
+ " "
+ cust.customerName);
}
// original order of comparator
Comparator<Customer> reverseComparatorOrder =
Collections.reverseOrder(originalComparatorOrder);
// sorting using Collections.sort(al, reverseComparatorOrder);
Collections.sort(al, reverseComparatorOrder);
System.out.println("\n\nAfter Sorting"
+ " using reverse Comparator order\n");
// reverse ordering of customer Id using Comparator
for(Customer cust : al){
System.out.println(cust.customerId
+ " "
+ cust.customerName);
}
}
}
Output:
Before Sorting: Insertion Order 101 Narayan Murthy 107 Dinesh 103 Nandan Nilekeni 102 Ashok Arora 104 Shibulal 106 Kris Gopalakrishnan 105 Raghavan After Sorting using Original Comparator order 101 Narayan Murthy 102 Ashok Arora 103 Nandan Nilekeni 104 Shibulal 105 Raghavan 106 Kris Gopalakrishnan 107 Dinesh After Sorting using reverse Comparator order 107 Dinesh 106 Kris Gopalakrishnan 105 Raghavan 104 Shibulal 103 Nandan Nilekeni 102 Ashok Arora 101 Narayan Murthy
Difference between reverse() and reverseOrder() method ?
- reverse() method is used to reverse the order of elements present in List
- reverseOrder() method is used to reverse the order of Comparator (as seen in the above example)
References:
- https://docs.oracle.com/javase/tutorial/collections/intro/
- https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
- https://docs.oracle.com/javase/tutorial/collections/interfaces/list.html
- https://docs.oracle.com/javase/7/docs/api/java/util/List.html
- https://docs.oracle.com/javase/tutorial/collections/interfaces/order.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html
- https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html
Happy Coding !!
Happy Learning !!