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.

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

2. 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

Solution 1

I think you want a descendingIterator.

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

Solution 2

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());

Comments

[Sonny]

This is a compile error (when the code is used as-is).

[ZhekaKozlov]

for (int i : (Iterable<Integer>) obj::descendingIterator) { ... }