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Java Iterator Tutorial with Examples

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1- Iterator

Iterator is one of the ways to traverse elements of a Collection. Here are the characteristics of the Iterator:
  • Iterator does not guarantee iteration order of elements.
  • Iterator can allow to remove elements from collection during iteration, which depends on the Collection type.
Hierarchy of Iterator sub-interfaces:
The reason why you can traverse elements of a Collection by Iterator is because Collection extends from Iterable interface.
Collection Interface

// Definition of the Collection interface:
public interface Collection<E> extends Iterable<E>

// Definition of the Iterable interface:
public interface Iterable<T> {
    Iterator<T> iterator();

    default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }

    default Spliterator<T> spliterator() {
        return Spliterators.spliteratorUnknownSize(iterator(), 0);
    }
}
Iterator Methods

boolean hasNext()

E next();

// Optional operation.
default void remove()  

default void forEachRemaining(Consumer<? super E> action)

2- Examples

With a Collection object, you can create an Iterator by Collection.iterator() method, then iterate over elements of Iterator using next() method.
Example:
IteratorEx1.java

package org.o7planning.iterator.ex;

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

public class IteratorEx1 {

    public static void main(String[] args) {
        // List is a subinterface of Collection.
        List<String> flowers = new ArrayList<String>();

        flowers.add("Tulip");
        flowers.add("Daffodil");
        flowers.add("Poppy");
        flowers.add("Sunflower");
        flowers.add("Bluebell");
        
        Iterator<String> iterator = flowers.iterator();
        
        while(iterator.hasNext()) {
            String flower = iterator.next();
            System.out.println(flower);
        }
    }
}
Output:

Tulip
Daffodil
Poppy
Sunflower
Bluebell

3- remove()

While traversing elements of a Collection using an Iterator, you can remove the current element from CollectionIterator.remove() method allows you to do that. However, not all Iterator(s) support this operation, it depends on the Collection type. If unsupported, UnsupportedOperationException will be thrown.

// Optional Operation
public default void remove()
Example: An ArrayList<Integer> contains numbers. We will iterate over its elements and remove the current one if it is even.
Iterator_remove_ex1.java

package org.o7planning.iterator.ex;

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

public class Iterator_remove_ex1 {

    public static void main(String[] args) {
        // List is a subinterface of Collection.
        List<Integer> years = new ArrayList<Integer>();

        years.add(1998);
        years.add(1995);
        years.add(2000);
        years.add(2006);
        years.add(2021);
        
        Iterator<Integer> iterator = years.iterator();
        
        while(iterator.hasNext()) {
            Integer current = iterator.next();
            
            if(current % 2 ==0) {
                iterator.remove(); // Remove current element.
            }
        }
        // After remove all even numbers:
        for(Integer year: years)  {
            System.out.println(year);
        }
    }
}
Output:

1995
2021
Example of a Collection whose Iterator does not support Iterator.remove() operation:
Iterator_remove_ex2.java

package org.o7planning.iterator.ex;

import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

public class Iterator_remove_ex2 {

    public static void main(String[] args) {
        // Fixed-size List.
        // Its Iterator does not support remove() operation.
        List<Integer> years = Arrays.asList(1998, 1995, 2000, 2006, 2021);
        
        Iterator<Integer> iterator = years.iterator();
        
        while(iterator.hasNext()) {
            Integer current = iterator.next();
            
            if(current % 2 ==0) {
                iterator.remove(); // UnsupportedOperationException!!
            }
        }
        // After remove all even numbers:
        for(Integer year: years)  {
            System.out.println(year);
        }
    }
}
Output:

Exception in thread "main" java.lang.UnsupportedOperationException: remove
    at java.base/java.util.Iterator.remove(Iterator.java:102)
    at org.o7planning.iterator.ex.Iterator_remove_ex2.main(Iterator_remove_ex2.java:20)

4- forEachRemaining(Consumer)

Performs the given action for each remaining element until all elements have been processed or the action throws an exception.

public default void forEachRemaining(Consumer<? super E> action)
Example:
Iterator_forEachRemaining.java

package org.o7planning.iterator.ex;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class Iterator_forEachRemaining {

    public static void main(String[] args) {
        // Set is a subinterface of Collection.
        Set<String> flowers = new HashSet<String>();

        flowers.add("Tulip");
        flowers.add("Daffodil");
        flowers.add("Poppy");
        flowers.add("Sunflower");
        flowers.add("Bluebell");
        
        // Note: Iterator doesn't guarantee iteration order
        Iterator<String> iterator = flowers.iterator();
        
        String flower1 = iterator.next();
        String flower2 = iterator.next();
        
        System.out.println("Flower 1: " + flower1);
        System.out.println("Flower 2: " + flower2);
        
        System.out.println();
        
        iterator.forEachRemaining(flower -> System.out.println(flower));
    }
}
Output:

Flower 1: Poppy
Flower 2: Tulip

Daffodil
Sunflower
Bluebell
  • TODO Link?

5- ListIterator

ListIterator is a sub interface of Iterator. It is one of the ways to traverse the elements of a List. Unlike Iterator, ListIterator supports traversing elements in both forward and backward directions. ListIterator also supports removing, updating or inserting an element during iteration.
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