Iterator Interface

Posted by Adrian Zinko on 22 February, 2021 / 1 min read

Table of contents

Iterator Interface

Each collection implements the Iterable interface, so we can use it in for-each loops. We will take a closer look at this issue.

The Iterable interface imposes that any class that implements it exposes a class that implements the Iterator interface. For example, the ArrayList class has a method called iterator () that returns an object that implements the Iterator interface.

What is an iterator?

The easiest way to imagine a collection iterator is as a cursor in an editor on a text document. Initially, when the document is opened, the cursor is positioned in front of the first character of the text. Imagine the cursor can execute two commands:

• answer the question whether there is any sign on the right side of it or whether it has already reached the end of the text,
• take the character to the right of the cursor and move one place to the right. We can imagine that with these two commands, we are able to move the cursor through the entire text. Each time we check if there is any character left to the right of the cursor. If so, we read this character and move the cursor one place to the right.

This is exactly how the Iterator interface works - of course, it does not operate on text, but on objects contained in the collection. The Iterator interface actually has several methods, but two of them are the most important:

• boolean hasNext () - returns true when the collection contains more elements to be read,
• E next () - takes the value of the next object of type E from the collection and moves the iterator "cursor" one place to the right.

Following snippet shows example of the iterator:

import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

public class ExternalIteratorDemo 
{
  public static void main(String[] args) 
  {
    List<String> names = new LinkedList<>();
    names.add("Agata");
    names.add("Grzegorz");
    names.add("Zbigniew");
    
    // Getting Iterator
    Iterator<String> namesIterator = names.iterator();
        
    // Traversing elements
    while(namesIterator.hasNext()){
        System.out.println(namesIterator.next());           
    }   
    
  }
}

The same operation with for-each loop:

import java.util.LinkedList;
import java.util.List;

public class InternalIteratorDemo 
{
  public static void main(String[] args) 
  {
    List<String> names = new LinkedList<>();
    names.add("Agata");
    names.add("Grzegorz");
    names.add("Zbigniew");
        
    for(String name: names){
        System.out.println(name);           
    }   
    
  }
}

The for-each loop uses this mechanism internally for collections that implement the Iterable interface, and thus the Iterator interface.

Example with the use of while loop

/* Class Employee represents an employee in the company */
class Employee {

   String firstName;
   String lastName;
   LocalDate birthDate;

   // Constructor
   public Employee(String firstName, String lastName, int yearOfBirth,
               int monthOfBirth, int dayOfBirth) {
      this.firstName = firstName;
      this.lastName = lastName;
      // Class LocalDate provided by java.time library is used
      // to store dates. Constructor of(int year, int month, int day) could be
      // used to initialize LocalDate variable value
      this.birthDate = LocalDate.of(yearOfBirth, monthOfBirth, dayOfBirth);
   }

   @Override
   public int hashCode() {
      // methods getYear() and getMonthValue() of LocalDate class could be used
      // to retrieve a year and month of stored date
      return birthDate.getYear() * 100 + birthDate.getMonthValue();
   }

   @Override
   public boolean equals(Object o) {
      Employee e = (Employee) o;
      return (firstName.equals(e.getFirstName())) &&
         (lastName.equals(e.getLastName())) &&
         (birthDate.getYear() == e.getBirthDate().getYear()) &&
         (birthDate.getMonthValue() == e.getBirthDate().getMonthValue()) &&
         (birthDate.getDayOfMonth() == e.getBirthDate().getDayOfMonth());
   }

   @Override
   public String toString() {
      return firstName + ", " + lastName + ", born: " + birthDate;
   }

   // getters
   public String getFirstName() {
      return firstName;
   }

   public String getLastName() {
      return lastName;
   }

   public LocalDate getBirthDate() {
      return birthDate;
   }
}

/* Name of the class has to be "Main" only if the class is public. */
class HashSetExample {

   public static void main (String[] args) throws java.lang.Exception {
      // Creating 5 example Employee objects
      Employee worker1 = new Employee("Steven", "Newmack", 1987, 6, 30);
      Employee worker2 = new Employee("Jennifer", "Nowak", 1988, 2, 28);
      Employee worker3 = new Employee("Robert", "Greenfield", 1987, 6, 15);
      Employee worker4 = new Employee("Monica", "Smith", 1984, 10, 18);
      Employee worker5 = new Employee("Stephanie", "Jackson", 1985, 4, 3);

      // Creating a HashSet and filling it with objects
      Set<Employee> workersSet = new HashSet<>();
      workersSet.add(worker1);
      workersSet.add(worker2);
      workersSet.add(worker3);
      workersSet.add(worker4);
      workersSet.add(worker5);

      // Print out every worker from the set using while loop
      Iterator<Employee> iteratorWorkersSet = workersSet.iterator();
      while (iteratorWorkersSet.hasNext()) { 

         Employee currentWorker = iteratorWorkersSet.next();

         System.out.println(currentWorker);
      }
      System.out.println("Loop has finished its work");
   }
}

The while loop executes as long as the loop condition returns true. The while loop has many uses - it is used almost as often as the for loop. Its typical use (apart from iterating through collection objects) is waiting for an event to occur.

~ Adrian Zinko