Iterator interface in java.

Here we are learning about the Iterator interface in java.

Iterator interface :-

An iterator interface is used to iterate or traverse the collection of elements one by one.
It is iterating the data only forward direction.
After calling iterate method if we want to try add elements in ArrayList(), then we cannot add elements before while loop, we will get concurrent modification exception.
itr.hasNext() method is checking that there are any elements right to cursor is available or not.
It is containing three methods.

Syntax :- public boolean hasNext();

public object next();

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

public class IteratorDemo {
public static void main(String[] args) {
	List<String>list=new ArrayList<String>();
	list.add("abc");
	list.add("xyz");
	list.add("test");
	System.out.println("Iterate list of data :- ");
	Iterator<String> itr=list.iterator();
	while(itr.hasNext()) {
		System.out.println(itr.next());
	}
}
}

Output :-
Iterate list of data :-
abc
xyz
test

Remove Elements: –

We can modify collection during iteration using remove() method.
Here inside if condition within while loop we are comparing the given element with iterate element, if
it is equals then from remove() method, we are remove elements.

Syntax :- public default void remove();

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

public class RemoveIterateDemo {
	public static void main(String[] args) {
		List<String>list=new ArrayList<String>();
		list.add("abc");
		list.add("xyz");
		list.add("test");
		System.out.println("remove elements from list of data :- ");
		Iterator<String> itr=list.iterator();
		while(itr.hasNext()) {
			Object obj=itr.next();
			if(obj.equals("xyz")) {
				itr.remove();
			}
			
		}
		System.out.println(list);
	}
}

Output :-
remove elements from list of data :-
[abc, test]

ListIterator() method:-

This class is extending the iterator interface.
From this we can iterate the data in forward and backward direction.
In order to iterate the data in reverse order we use hasPrevious() and previous()
To use ListIterator, we first iterate the data from hashNext and next() method.

Syntas :- public interface ListIterator<E> extends Iterator<E>

				
					import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;

public class ListIteratorDemo {

	public static void main(String[] args) {

		List<String> list = new ArrayList<String>();
		list.add("abc");
		list.add("xyz");
		list.add("test");

		// Now use list iterator
		ListIterator<String> itr = list.listIterator();

		System.out.println("Traversing the list in forward direction:- ");
		while (itr.hasNext()) {
			System.out.println(itr.next());
		}
		System.out.println("Traversing the list in backward direction:- ");
		while (itr.hasPrevious()) {
			System.out.println(itr.previous());
		}
	}

}

Output :-
Traversing the list in forward direction:-
abc
xyz
test
Traversing the list in backward direction:-
test
xyz
abc