Java Iterator remove() Method

The remove() method in Java, part of the java.util.Iterator interface, is used to remove the last element returned by the iterator from the underlying collection.

Table of Contents

1. Introduction
2. remove() Method Syntax
3. Understanding remove()
4. Examples
   • Basic Usage
   • Handling Exceptions
5. Real-World Use Case
6. Conclusion

Introduction

The remove() method removes the last element returned by the iterator from the underlying collection. This method can be called only once per call to next() and will throw an IllegalStateException if it is called under invalid conditions.

remove() Method Syntax

The syntax for the remove() method is as follows:

void remove()

Parameters:

• This method does not take any parameters.

Returns:

• This method does not return a value.

Throws:

• UnsupportedOperationException: If the remove operation is not supported by this iterator.
• IllegalStateException: If the next method has not yet been called, or the remove method has already been called after the last call to the next method.

Understanding remove()

The remove() method allows you to remove the last element returned by the iterator from the underlying collection. This can be useful for modifying collections while iterating over them. It is important to use this method correctly to avoid exceptions.

Examples

Basic Usage

To demonstrate the basic usage of remove(), we will create a list of integers, iterate over it, and remove elements that match a certain condition.

Example

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

public class RemoveExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        numbers.add(5);

        Iterator<Integer> iterator = numbers.iterator();

        while (iterator.hasNext()) {
            Integer number = iterator.next();
            if (number % 2 == 0) {
                iterator.remove();
            }
        }

        System.out.println("Remaining numbers: " + numbers);
    }
}

Output:

Remaining numbers: [1, 3, 5]

Handling Exceptions

This example shows how to handle exceptions when remove() is called under invalid conditions.

Example

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

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

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

        try {
            iterator.remove();
        } catch (IllegalStateException e) {
            System.out.println("Caught IllegalStateException: " + e.getMessage());
        }

        while (iterator.hasNext()) {
            String name = iterator.next();
            if (name.equals("Bob")) {
                iterator.remove();
            }
        }

        try {
            iterator.remove();
        } catch (IllegalStateException e) {
            System.out.println("Caught IllegalStateException: " + e.getMessage());
        }

        System.out.println("Remaining names: " + names);
    }
}

Output:

Caught IllegalStateException: null
Remaining names: [Alice]

Real-World Use Case

Removing Elements During Iteration

In a real-world scenario, you might use the remove() method to remove elements from a collection while iterating over it, such as removing invalid entries or duplicates.

Example

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

public class RemoveInvalidEntries {
    public static void main(String[] args) {
        List<String> emails = new ArrayList<>();
        emails.add("valid@example.com");
        emails.add("invalid-email");
        emails.add("another.valid@example.com");

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

        while (iterator.hasNext()) {
            String email = iterator.next();
            if (!email.contains("@")) {
                iterator.remove();
            }
        }

        System.out.println("Valid emails: " + emails);
    }
}

Output:

Valid emails: [valid@example.com, another.valid@example.com]

Conclusion

The Iterator.remove() method in Java provides a way to remove the last element returned by the iterator from the underlying collection. By using this method, you can modify collections while iterating over them, making it particularly useful for removing elements based on certain conditions. It is important to handle exceptions and use the method correctly to avoid issues during iteration.

Whether you are working with lists, sets, or custom collections, the remove() method offers a reliable way to manage element removal at runtime.