How to iterate a LinkedList elements in reverse order?

Question

I m new to Java Collections and my doubt is why can't i traverse a element in linkedlist in backward directions.Below I'll explain what i did and please clarify my doubts.

I've created interface iterator for forward iterations and listiterator for backward iterations. Why backward iterations are not working ?

Can't i use iterator and listiterator interface in the same program to traverse a set of elements in forward and backward iterations ?

Code Snippet :

import java.util.*;
class NewClass{
public static void main(String args[]){
  LinkedList<String> obj = new LinkedList<String>();

   obj.add("vino");
   obj.add("ajith");
   obj.add("praveen");
   obj.add("naveen");

   System.out.println(obj);

   System.out.println("For loop ");
   //using for loop
   for(int count=0; count < obj.size(); count++){
     System.out.println(obj.get(count));
   }
  System.out.println("For each loop ");

  //using foreach loop
  for(String s:obj){
     System.out.println(s);
  }
  System.out.println("Whileloop ");

  //using whileloop
  int count=0;
  while(obj.size() > count){
      System.out.println(obj.get(count));
    count++;
  }
  System.out.println("Forward Iterations ");
  //using iterator 
  Iterator it = obj.iterator();
  while(it.hasNext()){
     System.out.println(it.next());
  }
  ListIterator lit = obj.listIterator();
  System.out.println("Backward Iterations");
   while(lit.hasPrevious()){
    System.out.println(lit.previous());
      }
    }
 }

Output

[vino, ajith, praveen, naveen]
 For loop 
 vino
 ajith
 praveen
 naveen
For each loop 
 vino
 ajith
 praveen
 naveen
Whileloop 
 vino
 ajith
 praveen
 naveen
Forward Iterations 
 vino
 ajith
 praveen
 naveen
 Backward Iterations

Where is the output for Backward Iterations? Please anyone help me.Thanks in advance

Are you sure you start iterating from the last list element? — Renaud Pacalet, Sep 13 '15 at 06:01

score 31 · Accepted Answer · edited Nov 08 '18 at 09:24

31

I think you want a descendingIterator.

Iterator lit = obj.descendingIterator();
System.out.println("Backward Iterations");
while(lit.hasNext()){
  System.out.println(lit.next());
}

edited Nov 08 '18 at 09:24

frogatto

28,539
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answered Sep 13 '15 at 05:52

tph

326
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This is a compile error (when the code is used as-is). – Sonny Oct 03 '17 at 17:39
`for (int i : (Iterable) obj::descendingIterator) { ... }` – ZhekaKozlov Nov 08 '18 at 09:37

score 7 · Answer 2 · answered Sep 13 '15 at 05:52

You can do it, but you need to use the method listIterator(int index) to specify that you want to start at the end of the List.

LinkedList<String> obj = new LinkedList<String>();

obj.add("vino");
obj.add("ajith");
obj.add("praveen");
obj.add("naveen");

ListIterator<String> it = obj.listIterator(obj.size());
while (it.hasPrevious())
    System.out.println(it.previous());

score 0 · Answer 3 · answered Sep 01 '23 at 22:53

Starting from Java 21, simply using LinkedList.reversed:

LinkedList<String> list = new LinkedList<>(List.of("A", "B", "C"));
for (String s : list.reversed()) {
  System.out.println(s);
}

Note: It returns a reverse-ordered view of this collection.

Additional details in JEP 431: Sequenced Collections.

How to iterate a LinkedList elements in reverse order?

3 Answers3