How to Reverse an Object Collection in Java

This example shows how reverse the contents of a LinkedList by calling the method of the Collections: class: Collections.reverse() . We chose LinkedList as our collection of objects. The same method also applies to other collections that inherit from the java.util.Collection. You have to pass the LinkedList instance as a parameter. This method is used to reverse the order of the items in the list.

Example:

This code handles the following three collections of java objects: ArrayList, LinkedList, and Vector.

import java.util.Collections; 
import java.util.LinkedList; 

public class inverse_collection {

 public static void main(String[] args) {
 
 LinkedList llist = new LinkedList(); 

 llist.add("1"); 
 llist.add("2"); 
 llist.add("3"); 
 llist.add("4"); 

 System.out.println("Before inversion: "+llist); 
 
 Collections.reverse(llist); 

 System.out.println("After inversion: "+llist); 
 }
}

Result:

Before inversion: [1, 2, 3, 4]
After inversion: [4, 3, 2, 1]

Collections that inherit from java.util.Set and java.util.Map are not supported by the Collections.reverse() method. However, a partial solution is possible:

Create an instance of ArrayList with a collection from the Set interface or Map
Apply the Collections.reverse() method on the ArrayList
Items cannot be copied back to the original collection. What for? Because the Set interface does not guarantee the insertion order.

Note: The Map interface is a key/value hash table. This means that we only have the choice to invert either the keys or the values without copying the results back into the collection that implements Map, because it doesn't keep the order of the elements.

Example of TreeSet:

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

public class Intervert {

 public static void main(String[] args) {
 
 // creating TreeSet
 TreeSet  tset = new TreeSet(); 
 
 // add elements to treeset
 tset.add("a"); 
 tset.add("b"); 
 tset.add("c"); 
 
 Iterator iterator = tset.iterator(); 
 
 // display of TreeSet
 System.out.println("Treeset elements in ascending order: "); 
 while (iterator.hasNext()){
 System.out.println(iterator.next()); 
 }

 List list = new ArrayList(tset); 
 
 Collections.reverse(list); 
 
 // display values inversely
 System.out.println("The elements of treeset in reverse order: "); 
 for(String s:list)
 System.out.println(s); 
 }
}

Output:

Treeset elements in ascending order: 
a
b
c
treeset elements in reverse order:  
c
b
a