Java Iterator
What is a Java Iterator?
In Java, an Iterator is an object used to traverse elements in a collection such as ArrayList, HashSet, or LinkedList. It provides a standard way to access elements sequentially without exposing the underlying structure of the collection.
The term iteration refers to the process of looping through elements one by one.
To use an Iterator, you must import it from the java.util package:
import java.util.Iterator;
How to Get an Iterator in Java
You can obtain an iterator from any collection by calling the iterator() method.
Example: Accessing Elements Using Iterator
import java.util.ArrayList;
import java.util.Iterator;
public class IteratorExample {
public static void main(String[] args) {
ArrayList<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Mango");
fruits.add("Banana");
fruits.add("Orange");
Iterator<String> iterator = fruits.iterator();
// Access first element
if (iterator.hasNext()) {
System.out.println("First item: " + iterator.next());
}
}
}
Iterating Through a Collection
The Iterator interface provides two key methods:
hasNext() → checks if more elements are available
next() → returns the next element
Example: Looping Through All Elements
import java.util.ArrayList;
import java.util.Iterator;
public class IterateCollection {
public static void main(String[] args) {
ArrayList<String> cities = new ArrayList<>();
cities.add("Chennai");
cities.add("Delhi");
cities.add("Mumbai");
cities.add("Bangalore");
Iterator<String> iterator = cities.iterator();
while (iterator.hasNext()) {
String city = iterator.next();
System.out.println(city);
}
}
}
Removing Elements Using Iterator
One of the major advantages of using an Iterator is that it allows safe removal of elements during iteration.
The remove() method deletes the current element returned by next().
Example: Remove Even Numbers
import java.util.ArrayList;
import java.util.Iterator;
public class RemoveExample {
public static void main(String[] args) {
ArrayList<Integer> values = new ArrayList<>();
values.add(11);
values.add(20);
values.add(35);
values.add(40);
values.add(55);
Iterator<Integer> iterator = values.iterator();
while (iterator.hasNext()) {
int num = iterator.next();
if (num % 2 == 0) {
iterator.remove(); // Safe removal
}
}
System.out.println("After removal: " + values);
}
}
Using var with Iterator (Java 10+)
From Java 10 onwards, you can use the var keyword to simplify iterator declarations.
Example: Using var
import java.util.ArrayList;
public class VarIteratorExample {
public static void main(String[] args) {
ArrayList<String> languages = new ArrayList<>();
languages.add("Java");
languages.add("Python");
languages.add("C++");
var iterator = languages.iterator();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
}
Why Use var?
- Reduces verbosity
- Improves readability in simple cases
- Compiler still infers the correct type (Iterator<String>)
However, many developers prefer explicit types for better clarity in large codebases.
Key Benefits of Using Iterator
- Provides a uniform way to traverse different collections
- Supports safe removal of elements
- Works with all classes in the Java Collection Framework
- Helps avoid runtime errors when modifying collections
