Java Collections Iterator

In Java, the Iterator interface provides a way to traverse (or iterate over) the elements in a collection without exposing the underlying implementation of the collection. It allows you to sequentially access each element in the collection and perform operations like removal, if supported by the underlying collection.

Table of Contents

1. Introduction to Iterator
2. Using Iterator

• Creating an Iterator
• Using Iterator Methods
• Example: Using Iterator to Traverse a List
• Example: Using Iterator to Traverse a Set

1. Conclusion

1. Introduction to Iterator

The Iterator interface is part of the Java Collections Framework (java.util package). It provides methods to iterate over the elements of a collection in a forward direction. The main advantage of using an iterator is that it provides a common way to iterate over different types of collections such as lists, sets, and maps, without knowing their specific implementation.

Key points about Iterator:

• It is a unidirectional cursor, meaning it allows you to move forward through the elements of a collection.
• It provides methods to check if there are more elements (hasNext()) and retrieve the next element (next()).
• It also supports removal of elements using the remove() method, if the underlying collection allows it.

2. Using Iterator

2.1. Creating an Iterator

To obtain an Iterator for a collection, you typically use the iterator() method provided by the Collection interface. Here’s how you create an Iterator:

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

public class IteratorExample {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");

        // Get an iterator for the list
        Iterator<String> iterator = names.iterator();
    }
}

2.2. Using Iterator Methods

Once you have an Iterator, you can use the following methods to iterate over the elements:

• hasNext(): Returns true if there are more elements in the collection.
• next(): Returns the next element in the collection and moves the cursor forward.
• remove(): Removes the last element returned by next() from the collection (optional operation).

2.3. Example: Using Iterator to Traverse a List

Let’s see an example of using an Iterator to traverse a List:

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

public class IteratorExample {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");

        // Get an iterator for the list
        Iterator<String> iterator = names.iterator();

        // Traverse the list using the iterator
        while (iterator.hasNext()) {
            String name = iterator.next();
            System.out.println(name);
        }
    }
}

Output:

Alice
Bob
Charlie

2.4. Example: Using Iterator to Traverse a Set

You can also use an Iterator to traverse a Set:

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

public class IteratorExample {
    public static void main(String[] args) {
        Set<Integer> numbers = new HashSet<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);

        // Get an iterator for the set
        Iterator<Integer> iterator = numbers.iterator();

        // Traverse the set using the iterator
        while (iterator.hasNext()) {
            int number = iterator.next();
            System.out.println(number);
        }
    }
}

Output:

10
20
30

2.5 Example: Iterating over the Entry Set

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("One", 1);
        map.put("Two", 2);
        map.put("Three", 3);

        Iterator<Map.Entry<String, Integer>> iterator = map.entrySet().iterator();

        while (iterator.hasNext()) {
            Map.Entry<String, Integer> entry = iterator.next();
            System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
        }
    }
}

3. Conclusion

The Iterator interface in Java provides a simple and uniform way to iterate over elements in collections such as lists, sets, and other data structures. It allows you to access elements sequentially, check for more elements, and optionally remove elements from the underlying collection. Understanding and using iterators is essential for working with Java collections effectively.

In this tutorial, you learned how to:

• Create an Iterator for a collection using iterator().
• Use methods like hasNext() and next() to traverse the collection.
• Optionally remove elements using remove().

Additional Resources:

• Java Iterator Documentation
• Java Collections Framework