Learn about collections in Java such as LinkedLists, HashSet, HashMap and more.

Java for Programmers

Collections is one of the most important topics for Java Programming. Without the proper knowledge of every collection type in Java, it becomes difficult for you to decide which option to pick in a particular scenario.

This course will give you a great understanding of all collection types available in the Collections Framework like lists, linked lists, sets, and hashmaps.

After completing this course, you will know how data can be modified within a collection, how to sort a collection, and how it can be made thread-safe. You will also cover the internal working of each Java collection so you can work with them more efficiently.

Java Collections

## Iterating an ArrayList

Below are the different methods to iterate over an ArrayList.

### Using `for` loop
An ArrayList can be iterated easily using a simple `for` loop or an enhanced `for` loop as shown below.

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

public class ArrayListDemo {

	public static void main(String args[]) {
		List <Integer> list = new ArrayList<Integer>();
		list.add(10);
		list.add(20);
		list.add(30);
		list.add(40);
		 
		for (int i = 0; i < list.size(); i++) { //Simple for loop
			System.out.println(list.get(i));
		}

		for (Integer i : list) {   //Enhanced for loop
			System.out.println(i);
		}
	}
}


### Using `Iterator`
The `iterator()` method in ArrayList returns an `Iterator` type object. The `Iterator` interface declares the below methods that help with iterating an ArrayList.

1. `hasNext()` — This method returns `true` if there are more elements in the list; otherwise, it returns `false`.

2. `next()` — This method returns the next element in the list. Before calling `next()`, we should always call `hasNext()` to verify that there is an element; otherwise, `NoSuchElementException` will be thrown.

3. `remove()` — This method removes the last element returned by the iterator. It can be called only once per call to the `next()`.

4. `forEachRemaining(Consumer<? super E> action)` — This method was introduced in Java 8. It performs the given action for each remaining element until all elements have been processed or the action throws an exception. This method’s benefit is that we do not need to check if there is a next element every time.

> To understand the working of the `forEachRemaining()` method, you should be familiar with basic concepts of functional programming that were introduced in Java 8. 

Below is an example of iterating an ArrayList using the iterator.


# Iterating an ArrayList

Below are the different methods to iterate over an ArrayList.

## Using `for` loop
An ArrayList can be iterated easily using a simple `for` loop or an enhanced `for` loop as shown below.

## Using `Iterator`
The `iterator()` method in ArrayList returns an `Iterator` type object. The `Iterator` interface declares the below methods that help with iterating an ArrayList.

1. `hasNext()` — This method returns `true` if there are more elements in the list; otherwise, it returns `false`.

2. `next()` — This method returns the next element in the list. Before calling `next()`, we should always call `hasNext()` to verify that there is an element; otherwise, `NoSuchElementException` will be thrown.

3. `remove()` — This method removes the last element returned by the iterator. It can be called only once per call to the `next()`.

4. `forEachRemaining(Consumer<? super E> action)` — This method was introduced in Java 8. It performs the given action for each remaining element until all elements have been processed or the action throws an exception. This method’s benefit is that we do not need to check if there is a next element every time.

> To understand the working of the `forEachRemaining()` method, you should be familiar with basic concepts of functional programming that were introduced in Java 8. 

Below is an example of iterating an ArrayList using the iterator.


Let's discuss how an ArrayList can be iterated.

Introduction

ArrayList

LinkedList

CopyOnWriteArrayList

HashSet

TreeSet

HashMap

TreeMap

LinkedHashMap

Additional Map Implmentations

Arrays

Collections class

ArrayList: Iteration

Iterating an ArrayList

Using `for` loop

Using `Iterator`