Java Iterator interface example

Java Iterator interface used to iterate over the elements in a collection (list, set or map). It helps to retrieve the specified collection elements one by one and perform operations over each element.

1. Java Iterator interface

All Java collection classes provide iterator() method which return the instance of Iterator to walk over the elements in that collection. For example, arraylist class iterator() method return an iterator over the elements in this list in proper sequence.

ArrayList<String> list = new ArrayList<>();
        
list.add("A");
list.add("B");
list.add("C");
list.add("D");

Iterator<String> iterator = list.iterator();

while(iterator.hasNext()) {
    System.out.println( iterator.next() );
}

Program Output.

A
B
C
D

2. Java Iterator methods

2.1. Iterator hasNext()

• This method returns true if the iteration has more elements remaining in the collection.
• If iterator has gone over all elements then this method will return false.

2.2. Iterator next()

• This method returns the next element in the iteration.
• It throws NoSuchElementException if the iteration has no more elements.

2.3. Iterator remove()

• It removes from the underlying collection the last element returned by the iterator (optional operation).
• This method can be called only once per call to next().
• If the underlying collection is modified while the iteration is in progress in any way other than by calling remove() method, iterator will throw an ConcurrentModificationException.
• Iterators that do this are known as fail-fast iterators, as they fail quickly and cleanly, rather that risking arbitrary, non-deterministic behavior at an undetermined time in the future.

2.4. Iterator forEachRemaining()

• This method performs the given action for each remaining element until all elements have been processed or the action throws an exception.
• Actions are performed in the order of iteration, if that order is specified.
• It throws NullPointerException if the specified action is null.

3. Java Iterator example

3.1. ArrayList Iterate Example

Java example to iterate over ArrayList elements.

ArrayList<String> list = new ArrayList<>();
        
list.add("A");
list.add("B");
list.add("C");
list.add("D");

System.out.println(list);

//Get iterator
Iterator<String> iterator = list.iterator();

//Iterate over all elements
while(iterator.hasNext()) 
{
    //Get current element
    String value = iterator.next();
    
    System.out.println( value );
    
    //Remove element
    if(value.equals("B")) {
        iterator.remove();
    }
}

System.out.println(list);

Program Output.

[A, B, C, D]
A
B
C
D
[A, C, D]

3.2. HashSet Iterate Example

Iterating over a HashSet is very similar to iterate over a list. No visible differences.

HashSet<String> hashSet = new HashSet<>();
        
hashSet.add("A");
hashSet.add("B");
hashSet.add("C");
hashSet.add("D");

System.out.println(hashSet);

//Get iterator
Iterator<String> iterator = hashSet.iterator();

//Iterate over all elements
while(iterator.hasNext()) 
{
    //Get current element
    String value = iterator.next();
    
    System.out.println( value );
    
    //Remove element
    if(value.equals("B")) {
        iterator.remove();
    }
}

System.out.println(list);

Program Output.

[A, B, C, D]
A
B
C
D
[A, C, D]

3.3. HashMap Keys Iterator Example

Java example to iterate over keys of a HashMap.

HashMap<Integer, String> map = new HashMap<>();
        
map.put(1, "A");
map.put(2, "B");
map.put(3, "C");
map.put(4, "D");

System.out.println(map);

//Get iterator
Iterator<String> iterator = map.keys().iterator();

//Iterate over all keys
while(iterator.hasNext()) 
{
    String key = iterator.next();
    System.out.println( "Key : " + key + ", Value : " + map.get(key) );
}

Program Output.

{1=A, 2=B, 3=C, 4=D}
Key : 1, Value : A
Key : 2, Value : B
Key : 3, Value : C
Key : 4, Value : D

3.4. HashMap Values Iterator Example

Java example to iterate over values of a HashMap.

HashMap<Integer, String> map = new HashMap<>();
        
map.put(1, "A");
map.put(2, "B");
map.put(3, "C");
map.put(4, "D");

System.out.println(map);

//Get iterator
Iterator<String> iterator = map.values().iterator();

//Iterate over all values
while(iterator.hasNext()) 
{
    System.out.println( "Value : " + iterator.next() );
}

Program Output.

{1=A, 2=B, 3=C, 4=D}
Value : A
Value : B
Value : C
Value : D

3.5. Iterator forEachRemaining() Example

Java example to iterate over ArrayList elements and perform an action on them.

ArrayList<String> list = new ArrayList<>();

list.add("A");
list.add("B");
list.add("C");
list.add("D");

list.iterator().forEachRemaining( System.out::println );

Program Output.

A
B
C
D

4. Convert Iterator to Stream

Convert iterator to stream by first converting iterator to Spliterator and then using StreamSupport to get stream from Spliterator.

import java.util.Arrays;
import java.util.Iterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

public class IteratorToStream 
{
	public static void main(String[] args) 
	{
		// Iterator
		Iterator<String> iterator = Arrays.asList("a", "b", "c")
											.listIterator();
		//Extra step to get Spliterator
		Spliterator<String> splitItr = Spliterators
				.spliteratorUnknownSize(iterator, Spliterator.ORDERED);

		// Iterator -> Stream
		Stream<String> stream = StreamSupport.stream(splitItr, false);

		// Apply stream operations
		stream.forEach(System.out::println);
	}
}

Program output:

a
b
c

5. Conclusion

In this tutorial, we learned the Java Iterator interface. We learned the iterator methods and simple examples to iterate over different collections such as List, Set and Map.

Drop me your questions in the comments section.

Happy Learning !!

References:

Iterator Interface Java Docs