Iterator Interface In Java

Java Iterator Interface of java collections allows us to access elements of the collection and is used to iterate over the elements in the collection(Map, List or Set). It helps to easily retrieve the elements of a collection and perform operations on each element. Iterator is a universal iterator as it can be applied to any Collection object. We can traverse only in the forward direction using iterator. Using ListIterator which extends Iterator, can traverse in both directions. Both read and remove operations can be performed by the iterator interface. This is included in Java JDK 1.2. The only Enumeration is the first iterator to be included in JDK 1.0. To use an iterator, we must import java.util package.

Limitations of Enumeration Interface:

An Iterator interface is used in place of Enumeration in Java Collection. 

• Enumeration is not a universal iterator and is used for legacy classes like Vector, Hashtable only.
• Iterator allows the caller to remove elements from the given collection during iterating over the elements.
• Only forward direction iteration is possible in an Enumeration.

Declaration of iterator interface

public interface Iterator<E>

E – the type of elements returned by this iterator

Subinterfaces of Iterator

EventIterator: 

public interface EventIterator extends Iterator<Event>

EventIterators are unmodifiable.

Methods: nextEvent() which returns the next Event in an EventSet.

ListIterator<E>:

public interface ListIterator<E> extends Iterator<E>

An Iterator for lists which allows to traverse the list in either of the forward or backward direction or modify the list during the iteration and to obtain the current position of the iterator. ListIterator has no current element.

PrimitiveIterator<T,T_CONS>, PrimitiveIterator.OfInt, PrimitiveIterator.OfLong

Implementing Classes:

• BeanContextSupport.BCSIterator
• EventReaderDelegate
• Scanner

Example: Implementation of Iterator

All classes in the Collection Framework provide iterator() method which returns the instance of Iterator to iterate over the elements in that collection.

Java

// Java program to show the usage of Iterator()
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
public class JavaIteratorExample1 {
    public static void main(String[] args)
    {
          // create a list
        List<String> list = new LinkedList<>();
        list.add("Welcome");
        list.add("to");
        list.add("GFG");

        System.out.println("The list is given as : "
                           + list);

          // get the iterator on the list
        Iterator<String> itr = list.iterator();

        // Returns true if there are more number of
        // elements.
        while (itr.hasNext()) {
            // Returns the next element.
            System.out.println(itr.next());
        }

        // Removes the last element.
        itr.remove();
        System.out.println(
            "After the remove() method is called : "
            + list);
    }
}

Output

The list is given as : [Welcome, to, GFG]
Welcome
to
GFG
After the remove() method is called : [Welcome, to]

ArrayList Iterator Example

Java

// Java program to iterate over an arraylist
// using Iterator
import java.util.*;
class GFG {
    public static void main(String[] args)
    {
        // initializing ArrayList
        List<Integer> numbers
            = Arrays.asList(10, 20, 30, 40, 50, 60, 70, 80);

        // Looping ArrayList using Iterator
        Iterator it = numbers.iterator();
        while (it.hasNext())
            System.out.print(it.next() + " ");
    }
}

Output

10 20 30 40 50 60 70 80 

Develop Custom Class Iterator

To provide similar functionality for user-defined /custom class, we should follow the below steps:

• Define a custom class.
• Define the collection class to this custom class.
• The collection class should import java.util package and implement iterable interface.
• This collection class should now provide implementation to Iterable interface’s method iterator().

Example code of developing custom class:

Java

// java program to show the creation of
// custom class that implements iterable interface
import java.util.*;
import java.io.*;
class Employees implements Iterable {

    List<String> str = null;
    public Employees()
    {
        str = new ArrayList<String>();
        str.add("practice");
        str.add("geeks");
        str.add("for");
        str.add("geeks");
        str.add("to");
        str.add("learn");
        str.add("coding");
    }

    // if we are implementing Iterable interface, the we
    // need to define the iterator() method of Iterable
    // interface
    @Override public Iterator<String> iterator()
    {
        return str.iterator();
    }
}

public class EmployeesTester {
    public static void main(String[] args)
    {
        Employees emps = new Employees();
        for (String st : emps.str) {
            System.out.println(st);
        }
    }
}

Output

practice
geeks
for
geeks
to
learn
coding

Using remove() method to remove items from a collection

• It removes the last element of the collection returned by the iterator.
• If the iteration is in progress and meanwhile underlying collection is modified then by calling remove() method, an Iterator will throw a ConcurrentModificationException.

Java

// java program to remove()
// elements from a collection

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

public class MyClass {
    public static void main(String[] args)
    {
          // create a list of Integers
        ArrayList<Integer> numbers
            = new ArrayList<Integer>();
        numbers.add(12);
        numbers.add(8);
        numbers.add(2);
        numbers.add(23);

          // get the iterator on the list
        Iterator<Integer> it = numbers.iterator();
        while (it.hasNext()) {

              // gives the next element
              // and iterator moves to next
              // element
            Integer i = it.next();

            if (i < 10) {

                  // removes the current element
                it.remove();
            }
        }
        System.out.println(numbers);
    }
}

Output

[12, 23]

Iterator forEachRemaining() Example

Java

// Java program to show the usage of
// Iterator forEachRemaining()
import java.util.*;
class GFG {
    public static void main(String[] args)
    {
        //  initializing ArrayList
        List<Integer> numbers
            = Arrays.asList(10, 20, 30, 40, 50, 60, 70, 80);

        numbers.iterator().forEachRemaining(
            System.out::println);
    }
}

Output

10
20
30
40
50
60
70
80

Advantages of Java Iterator:

1. It is not a legacy interface and can traverse overall collections like ArrayList, HashMap, TreeSet, HashSet etc.
2. It can be used for any java collection and therefore known as Universal Cursor for Collection API.
3. Read and Remove operations are supported.
4. Simple and easily usable method names.

Limitations of Java Iterator:

1. It does not support Create and Update operations in CRUD (Create, Read, Update, Delete) operations.
2. It supports only uni-directional traversal i.e in forwarding direction.
3. It does not support a better iteration on a large volume of data in comparison to Spliterator.
4. It supports only sequential iteration i.e it does not support iterating elements parallel.

Difference between Iterator and Enumeration:

Iterator	Enumeration
It was introduced in JDK 1.2.	It was introduced in JDK 1.0.
It is a universal Cursor.i.e can be used in any java collections.	It is not a universal cursor.i.e we can use it only for some legacy classes.
It supports both Read and Remove operations.	It supports only Read operation.
It has simple method names.	It has lengthy method names.
One can do any modification while traversing over the elements.	We cannot do any modifications while traversing.
Not a legacy interface. Can traverse overall collections like ArrayList, HashMap, TreeSet, Vector etc.collections	Legacy interface. Traverse only Vector, Hashtable.

Methods:

Methods	Type	Explanation
hasNext()	boolean	If iteration has more elements, then it returns true. If the iterator has gone through all the elements, it returns false
next()	E	It returns the next element of iteration. It throws NoSuchElementException if the iterator has no more elements.
remove()	void	It removes the last element of the collection returned by the iterator. If the iteration is in progress and meanwhile underlying collection is modified then by calling remove() method, iterator will throw an ConcurrentModificationException.
forEachRemaining()	E	It performs the given action for each remaining element until all elements have been processed. If the order is specified, the actions are performed in the order of iteration. It throws NullPointerException if the action is null.