Cursors in Java

Cursors in Java are used to traverse through collection of objects. Java provides two types of cursors.

1. Iterator - Unidirectional cursor
2. Listiterator - Bidirectional cursor

1. Iterator

A Iterator is a cursor taht allows traversing elements only in a forward direction.

Features

The iterator can traverse the objects only in a forward direction hence it is called as unidirectional cursor.
The iterator is used to traverse all types of collections.
By using an Iterator cursor we can read and remove the objects.

Important Method in Iterator

1. hasNext() - Used to return true(returns Boolean result) if there exists an element in the collection.
2. next() - It is used to return the next element from the collection.
3. remove() - This method removes the element returned by next() from the collection.

Example:

package com.quipoin;

import java.util.ArrayList;
import java.util.Iterator;

public class Test {
	public static void main(String[] args) {
		ArrayList aList=new ArrayList<>();
		aList.add(10);
		aList.add("Praveen");
		aList.add(15.50);
		aList.add(21);

		Iterator i=aList.iterator();
		while(i.hasNext()) {
			System.out.println(i.next());
		}
		System.out.println("-------------------------------");
		System.out.println("List before remove:\n"+aList);
		System.out.println("-------------------------------");
		aList.remove(2);//Index 2 value is 12.20
		System.out.println("List after remove:\n"+aList);
	}
}

Output:

10
Praveen
15.5
21
-------------------------------
List before remove:
[10, Praveen, 15.5, 21]
-------------------------------
List after remove:
[10, Praveen, 21]

2. ListIterator

ListIterator is an advanced version of Iterator that allows both forward and backward traversal.

Features

List Iterator can traverse the objects in both forward and reverse directions hence it is called as bidirectional cursor.
List Iterator is used to traverse only the List type of collection. It cannot be used to traverse other than the List type of collection.
By using List Iterator we can read, remove, replace, and add the elements.

Important Method of List Iterator

1. hasNext() - returns true if there exists an element in the list.
2. next() - This method returns the next element present in the list.
3. previous() - returns the previous elements of the list.
4. hasPrevious() - returns true if there exists a previous element in the list.
5. remove() - This method removes the element returned by previous() from the collection.

Example:

package com.quipoin;

import java.util.Iterator;
import java.util.LinkedList;
import java.util.ListIterator;

public class DemoListIterator {
	public static void main(String[] args) {
		LinkedList list=new LinkedList<>();

		list.add("Java");
		list.add("Sql");
		list.add("Spring_Boot");
		list.add("Python");

		System.out.println("Traversing elements in forward direction!!");
		ListIterator itr=list.listIterator();
		while(itr.hasNext()) {
			System.out.println(itr.next());
		}
		System.out.println("------------------------------------------");
		System.out.println("Traversing elements in reverse direction!!");
		while(itr.hasPrevious()) {
			System.out.println(itr.previous());
		}
	}
}

Output:

Traversing elements in forward direction!!
Java
Sql
Spring_Boot
Python
------------------------------------------
Traversing elements in reverse direction!!
Python
Spring_Boot
Sql
Java

Difference Between Iterator and List Iterator

Feature	Iterator	List Iterator
Traversal Direction	Forward only	Both forward and backward
Applicable to	All Collections	Only List (Eg. - ArrayList, LinkedList, etc.)
Add Elements	Not Possible	Possible
Modify Elements	Not Possible	Possible
Remove Elements	Possible	Possible

Two Minute Drill

Iterator

Works with all Java collections (List, Set, Queue, etc.).
Traverses elements only in the forward direction.
Can remove elements but cannot modify or add them.

List Iterator

Works only with List implementations (ArrayList, LinkedList).
Supports both forward and backward traversal.
Allows adding, modifying, and removing elements while iterating.