Infinite loop with backward iteration using ListIterator

Question

Here is code using ListIterator. This works properly in the forward direction, but when it iterates backward it goes into an infinite loop.

Why is this happening?

package iteratordemo;
import java.util.*;

public class IteratorDemo {
    public static void main(String[] args) {
        ArrayList<String> al=new ArrayList<String>();
        
        al.add("a");
        al.add("b");
        al.add("c");
        al.add("d");
        al.add("e");
        
        
        System.out.println("original content of al:" +al);
        Iterator<String> itr=al.iterator(); 
        
        while(itr.hasNext()){
           String element=itr.next();
           System.out.println(element + " ");
        }
            
        System.out.println();
        
        // modify the object being iterated
        ListIterator litr=al.listIterator();
        while(litr.hasNext()){
           Object element = litr.next();
           litr.set(element + "+");
           System.out.println("modified content of al: " );
           itr=al.iterator();
           while(itr.hasNext())
           {
              element=itr.next();
              System.out.println(element + " ");
                
           }
           System.out.println();
        
           // now display the list backward
           // while returning in backward direction it goes into infinite loop                 
           System.out.println("modified list backwards:");
           while(litr.hasPrevious()){
               
                 element= litr.previous();
               System.out.println(element + " ");
               
           }
           System.out.println();
        }
    }
}

It seems that for each step you go forward in your list iterator with the "next" call you go back towards the start of the list with the "previous" call. So the list will never be iterated through as the cursor goes forward and backward all the time. You might consider doing the backwards iteration after the forward iteration is complete. — elfwyn
– elfwyn, Commented Jun 26, 2016 at 8:15

Adamovskiy · Accepted Answer · 2016-06-26 08:15:03Z

2

ListIterator is kind of cursor, that points to some element. Methods ListIterator.next() and ListIterator.previous() move this cursor to another element, so your code moves it forward and backward repeatedly, that causes infinite loop.

answered Jun 26, 2016 at 8:15

Adamovskiy

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1 Comment

rashmit chudasama Over a year ago

so what can be the solution, if you have any idea.

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