Iterator and ListIterator in Java

In Java, Iterator and ListIterator are used to traverse elements in a collection. They are part of the java.util package and help access elements sequentially without exposing the underlying data structure.

Iterator in Java

Iterator is an interface used to traverse collections like ArrayList, HashSet, LinkedList, etc.

Package: java.util

Iterator<Type> itr = collection.iterator();
while(itr.hasNext()) {
    Type element = itr.next();
    System.out.println(element);
}

Iterator Methods

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boolean hasNext() Checks if there are more elements remaining in the collection to iterate over.

Example:

Iterator<String> itr = list.iterator();
while(itr.hasNext()) {
    System.out.println(itr.next());
}

E next() Returns the next element in the iteration and moves the cursor forward.

Example:

Iterator<String> itr = list.iterator();
String element = itr.next();
System.out.println(element);

void remove() Removes the last element returned by the next() method from the underlying collection.

Example:

Iterator<String> itr = list.iterator();
while(itr.hasNext()) {
    String element = itr.next();
    if(element.equals("Python")) {
        itr.remove();  // removes "Python" from the list
    }
}
System.out.println(list);

Key Points about Iterator

Works for any Collection (List, Set, etc.)
Can traverse forward only
Can remove elements during iteration
Does not have index-based access

ListIterator in Java

ListIterator is a sub-interface of Iterator used specifically for List collections like ArrayList and LinkedList.

Package: java.util

ListIterator<Type> listItr = list.listIterator();

// Forward iteration
while(listItr.hasNext()) {
    System.out.println(listItr.next());
}

// Backward iteration
while(listItr.hasPrevious()) {
    System.out.println(listItr.previous());
}

ListIterator Methods

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hasNext() Checks if there is a next element in the list.

Example:

ListIterator<String> listItr = list.listIterator();
while(listItr.hasNext()) {
    System.out.println(listItr.next());
}

next() Returns the next element and moves the cursor forward.

Example:

String element = listItr.next();
System.out.println(element);

hasPrevious() Checks if there is a previous element in the list.

Example:

while(listItr.hasPrevious()) {
    System.out.println(listItr.previous());
}

previous() Returns the previous element and moves the cursor backward.

Example:

String element = listItr.previous();
System.out.println(element);

nextIndex() Returns the index of the element that would be returned by a subsequent call to next().

Example:

System.out.println(listItr.nextIndex());

previousIndex() Returns the index of the element that would be returned by a subsequent call to previous().

Example:

System.out.println(listItr.previousIndex());

remove() Removes the last element returned by next() or previous().

Example:

if(listItr.next().equals("Python")) {
    listItr.remove();  // removes "Python" from list
}

add(E e) Inserts the specified element at the current cursor position.

Example:

listItr.add("C#");
System.out.println(list);

set(E e) Replaces the last element returned by next() or previous() with the specified element.

Example:

listItr.next();
listItr.set("JavaScript");
System.out.println(list);

Key Points about ListIterator

Works only with List collections
Can traverse both forward and backward
Can add, remove, or modify elements during iteration
Maintains cursor position

Iterator vs ListIterator

Feature	Iterator	ListIterator
Collection Type	Any Collection	Only List
Direction	Forward only	Forward & backward
Remove	Yes	Yes
Add	No	Yes
Modify	No	Yes (`set()`)
Index Access	No	Yes (`nextIndex()`, `previousIndex()`)
Interface	java.util.Iterator	java.util.ListIterator

Example Programs

Example 1: Using Iterator

import java.util.*;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Java");
        list.add("Python");
        list.add("C++");

        Iterator<String> itr = list.iterator();
        while (itr.hasNext()) {
            String lang = itr.next();
            System.out.println(lang);
        }
    }
}

Output:

Java
Python
C++

Example 2: Using ListIterator

import java.util.*;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Java");
        list.add("Python");
        list.add("C++");

        ListIterator<String> listItr = list.listIterator();

        // Forward iteration
        System.out.println("Forward:");
        while(listItr.hasNext()) {
            System.out.println(listItr.next());
        }

        // Backward iteration
        System.out.println("Backward:");
        while(listItr.hasPrevious()) {
            System.out.println(listItr.previous());
        }
    }
}

Output:

Forward:
Java
Python
C++
Backward:
C++
Python
Java

Points to Remember

Iterator: forward-only traversal, works for all collections, can remove elements.
ListIterator: bi-directional traversal, works for lists, can add/remove/modify elements, provides index info.
Use Iterator when you only need forward iteration.
Use ListIterator when you need more control or backward traversal in a list.

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