Java Iterator

Introduction

The Iterator interface in Java is a part of the java.util package.

It provides a way to traverse a collection of elements one by one.

It is a fundamental part of the Java Collections Framework and is used to retrieve elements sequentially from a collection.

Table of Contents

1. What is the Iterator Interface?
2. Common Methods
3. Examples of Using the Iterator Interface
4. Conclusion

1. What is the Iterator Interface?

The Iterator interface provides methods to iterate over a collection. It helps to access elements of a collection sequentially without exposing the underlying representation. Iterators are used to remove elements from the underlying collection during the iteration.

2. Common Methods

• boolean hasNext(): Returns true if the iteration has more elements.
• E next(): Returns the next element in the iteration.
• void remove(): Removes the last element returned by the iterator.

3. Examples of Using the Iterator Interface

Example 1: Basic Usage of Iterator

This example demonstrates how to use 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> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Cherry");

        Iterator<String> iterator = list.iterator();
        while (iterator.hasNext()) {
            String fruit = iterator.next();
            System.out.println(fruit);
        }
    }
}

Output:

Apple
Banana
Cherry

Example 2: Removing Elements Using Iterator

This example shows how to remove elements from a collection using an Iterator.

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

public class RemoveExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Cherry");

        Iterator<String> iterator = list.iterator();
        while (iterator.hasNext()) {
            String fruit = iterator.next();
            if ("Banana".equals(fruit)) {
                iterator.remove();
            }
        }

        System.out.println("List after removal: " + list);
    }
}

Output:

List after removal: [Apple, Cherry]

Example 3: Using Iterator with a Set

This example demonstrates how to use an Iterator to traverse a Set.

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

public class SetIteratorExample {
    public static void main(String[] args) {
        Set<String> set = new HashSet<>();
        set.add("Red");
        set.add("Green");
        set.add("Blue");

        Iterator<String> iterator = set.iterator();
        while (iterator.hasNext()) {
            String color = iterator.next();
            System.out.println(color);
        }
    }
}

Output:

Red
Blue
Green

Example 4: Using Iterator with a Map

This example shows how to use an Iterator to traverse a Map.

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

public class MapIteratorExample {
    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("Apple", 1);
        map.put("Banana", 2);
        map.put("Cherry", 3);

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

Output:

Apple: 1
Cherry: 3
Banana: 2

Example 5: Using ListIterator

This example demonstrates how to use a ListIterator to traverse a List in both directions.

import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;

public class ListIteratorExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("One");
        list.add("Two");
        list.add("Three");

        ListIterator<String> listIterator = list.listIterator();

        System.out.println("Forward traversal:");
        while (listIterator.hasNext()) {
            System.out.println(listIterator.next());
        }

        System.out.println("Backward traversal:");
        while (listIterator.hasPrevious()) {
            System.out.println(listIterator.previous());
        }
    }
}

Output:

Forward traversal:
One
Two
Three
Backward traversal:
Three
Two
One

4. Conclusion

The Iterator interface in Java provides a convenient way to traverse collections sequentially. It allows for safe removal of elements during iteration and ensures that the underlying collection’s structure is not exposed. The examples provided demonstrate common usage patterns and highlight the capabilities of the Iterator interface.