java iterator with index parameter

Question

hi a normal iterator for a LinkedList would be the following, however, how do we build an iterator that returns an iterator starting at a specified index? How do we build:

public Iterator<E>iterator(int index)??? 

thanks! normal Iterator:

 public Iterator<E> iterator( )
 {
 return new ListIterator();
 }
private class ListIterator implements Iterator<E>
 {
 private Node current;
 public ListIterator()
 {
 current = head; // head in the enclosing list
 }
 public boolean hasNext()
 {
 return current != null;
 }
 public E next()
 {
 E ret = current.item;
 current = current.next;
 return ret;
 }
 public void remove() { /* omitted because optional */ }
 }

• Chetan Kinger: If you don't plan to remove elements from your Colleciton while iterating, you rather just use a traditional for loop.
• Anton: LinkedList has a method listIterator(int) it does exactly what you want. Here is link to docs

Answer 1

Well you could just call the normal iterator() method, then call next() that many times:

public Iterator<E> iterator(int index) {
 Iterator<E> iterator = iterator();
 for (int i = 0; i < index && iterator.hasNext(); i++) {
 iterator.next();
 }
 return iterator;
}

• Chetan Kinger: Wouldn't you rather just iterate through the collection using a traditional for loop and skip the indexes you are not interested in?
• Jon Skeet: @ChetanKinger: I'm not sure what you mean - this isn't a method I'd usually write, but it answers the question that was asked...
• Chetan Kinger: I'll try to elaborate through code : for(int i=0;i<collection.size();++i) { if(i>index) { break; } else { //dosomething } }. Why create an iterator in the first place? Only reason this would make sense is if you plan to remove elements while iterating.
• Katherine: @JonSkeet thank u Jon! great help, you are a legend!

Answer 2

This is kick-off example how to implement such iterator, but it's advised also to create or extend appropriate interface and make this object implementing this interface for convention.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
public class IterableObject {
 private List<String> values = new ArrayList<String>();
 public Iterator<String> getIterator(final int index) {
 Iterator<String> it = new Iterator<String>() {
 private int current = index;
 @Override
 public void remove() {
 // TODO Auto-generated method stub
 }
 @Override
 public String next() {
 String value = values.get(current);
 current++;
 return value;
 }
 @Override
 public boolean hasNext() {
 if(values.size() > current){
 return true;
 }else{
 return false;
 }
 }
 };
 return it;
 }
}

UPDATE

According to comments I've written an Iterator for LinkedList

public Iterator<String> getIterator(final int index) {
 Iterator<String> it = new Iterator<String>() {
 private Object currentObject = null;
 {
 /*initialize block where we traverse linked list 
 that it will pointed to object at place index*/
 System.out.println("initialize" + currentWord);
 for(int i = 0; currentObject.next != null && i < index; i++, currentObject = currentObject.next)
 ;
 } 
 @Override
 public void remove() {
 // TODO Auto-generated method stub
 }
 @Override
 public String next() {
 Object obj = currentObject.next;
 currentObject = currentObject.next;
 return obj;
 }
 @Override
 public boolean hasNext() {
 return currentObject.next != null;
 }
 };
 return it;
 }

Because Iterator is object of Anonymous class we can't use constructor but can initialise it in initialise block look at this answer: https://stackoverflow.com/a/362463/947111 We traverse it once at the beginning (sorry for C style) so it will point to currentObject. All remain code is self explained.

• Anatoly: @ChetanKinger, yes I've made mistake and fixed it already. Thank you for pointing me to that.
• Chetan Kinger: Just tried out your code and it works as advertised. This is a better way to do it than the accepted answer IMO.
• Anatoly: @ChetanKinger, ups I accidentally deleted your comment :( Sorry
• Lii: This would be extremely inefficient for a linked list since values.get(index) needs to traverse the list from the start every time it is executed to get the specified element.
• Lii: The hasNext method could be written more succinctly with a body like this: return values.get(current) != null;.