10 difference between an ArrayList and Array in Java [List vs Array]

Hello there, if you are wondering what is the difference between a list and an array, or particularly an ArrayList and an Array in Java then you have come to the right place. Earlier, I have shared the best data structure and algorithm courses and in this article, I will explain to you the differences and similarities between a list and an array in Java.  Both array and ArrayList are two important data structures in Java and frequently used in Java programs. Even though ArrayList is internally backed by an array, knowing the difference between an array and an ArrayList in Java is critical for becoming a good Java developer. If you know the similarity and differences, you can judiciously decide when to use an array over an ArrayList or vice-versa. 

In this article, I'll help you understand the difference between ArrayList and an array in Java. If you are coming from C or C++ background then you already know that array is one of the most useful data structures in the programming world. It offers O(1) performance for index-based search and one of the fundamental ways to store data.

The ArrayList on the other hand is a class in the Java Collection framework which was introduced as a dynamic array. Since an array is static in nature, I mean you cannot change the size of an array once created, So, if you need an array that can resize itself then you should use the ArrayList. 

It's like a dynamic array that can grow itself whenever needed. This is the fundamental difference between an array and an ArrayList.

Btw, I am expecting that you are familiar with basic Java Programing and Java API in general. If you are a complete beginner then I suggest you first go through a comprehensive course like  The Complete Java MasterClass on Udemy to learn more about core Java basics as well as such gems from Java API.




10 Difference between Array vs ArrayList in Java

If you want to understand array and ArrayList better then It's best to compare them on some points, this will make the differences easy to understand. So let's see what are the points on which you can compare an array with the ArrayList in Java

1. Implementation

The array is a native programming component or a data structure but ArrayList is a class from the Java Collections framework, an API. In fact, ArrayList is internally implemented using an array in Java. Since ArrayList is a class it holds all properties of a class like you can create objects and call methods but even though the array is an object in Java it doesn't provide any method. It just exposes a length attribute to give you the length of the array, which is constant.

2. Performance

Since ArrayList is based upon an array you would assume that it provides the same performance as an array. This is true to some extent but because of the extra functionality ArrayList provides there is some difference in the performance of ArrayList and array, mainly in terms of memory usage and CPU time.

For index-based access, both ArrayList and array provide O(1) performance but add can be O(logN) in ArrayList if adding a new element triggers resize, as it involves creating a new array in the background and copying elements from the old array to new array.

The memory requirement for ArrayList is also more than an array for storing the same number of objects like an int[] will take less memory to store 20 int variables than an ArrayList because of object metadata overhead on both ArrayList and wrapper class.

By the way, if you want to learn more about Java Performance or interested in writing high-performance Java applications then I highly recommend Java Multithreading, Concurrency & Performance Optimization Udemy course by Michael Pogrebinsky. It's a great course for experienced Java developers. 

10 difference between an ArrayList and Array in Java [List vs Array]


3. Type Safety

Now, the difference between ArrayList and array is around type safety, which means whether you can store integers on a String array or not. ArrayList is type-safe because it supports generics which allows the compiler to check if all objects stored in ArrayList are of the correct type. 

On the other hand, the array doesn't support Generics in Java. This means compile-time checking is not possible but array provides runtime type checking by throwing ArrayStoreException if you try to store an incorrect object into an array, like storing a String into an int array.

4. Flexibility

Flexibility is the single most important thing which separates array and ArrayList. In short, ArrayList is more flexible than a plain native array because it's dynamic. It can grow itself when needed, which is not possible with the native array.

ArrayList also allows you to remove elements that are not possible with native arrays. By remove, we mean not just assigning null to the corresponding index but also copying the rest of the elements one index down, which ArrayList automatically does for you. You can learn more about removing objects from ArrayList in my article difference between clear() and removeAll().


5. Primitives

If you first start using ArrayList then you will realize that you cannot store primitives on ArrayList. This is a key difference between array and ArrayList because array allows storing both primitives and objects.

For example, int[] numbers are valid but ArrayList of int is not valid. how do you deal with this problem? Suppose you want to store int primitives into ArrayList then how do you that? Well, you can use the wrapper class.

This is one of the reasons why the wrapper class was introduced in Java. So if you want to store int 2 into ArrayList just put it, autoboxing will do the rest. Btw, this difference is not so obvious from Java 5 onwards because of auto-boxing as you will see that ArrayList.add(21) is perfectly valid and works.


6. Generics

One more significant difference between an ArrayList and an array is that the former supports Generics but later doesn't. Since an array is of covariant type, you can use Generics with them. This means it's not possible for a compiler to check the type-safety of an array at compile time but they can verify the type-safety of Array.

So how do you deal with this problem while writing a type-safe class in Java? Well, you can use the technique shown in Effective Java, where you can declare an array, like E[] and later use typecasting.


7. Iteration

ArrayList provides more ways for iteration i.e. accessing all elements one by one than an array. You can only use loops like for loop, while loop, enhanced for loop and do-while to iterate over an array but you can also use the Iterator and ListIterator class to iterate over ArrayList. See here to learn different ways to iterate over ArrayList in Java.


8. Supported Operations

Since ArrayList is backed by an array internally, it exposes the operation which is possible with an array but given its dynamic nature, it also added an operation that is not possible with native array like you can store elements in both array and ArrayList, but the only ArrayList allows you to remove an element.

Though you can simulate that with an array by assigning null to the respective index, it won't be like remove unless you also move all elements above that index in the array to one level down.

Both ArrayList and array also provide ways to retrieve elements like get() method of ArrayList uses an index to get an element from an array, like version[0] will return the first element. ArrayList also provides an operation to clear and reuse like clear() and removeAll(), the array doesn't provide that but you can loop over Array and assign each index null to simulate that.

9. Size() vs length

Array only provides a length attribute that tells you the number of slots in the array i.e. how many elements it can store, it doesn't provide you any method to find out how many are filled and how many slots are empty i.e. the current number of elements.

While ArrayList does provide a size() method which tells the number of objects stored in ArrayList at a given point of time. The size() is always different than the length, which is also the capacity of ArrayList. If you want to know more, I suggest you read the difference between size() and length in the ArrayList article.


10. Dimension

Another significant difference between an array and an ArrayList is that array can be multi-dimensional like you can have a two-dimensional array or a three-dimensional array, which makes it a really special data structure to represent matrices and 2D terrains. 

On the other hand, ArrayList doesn't allow you to specify dimensions. See this tutorial to learn more about how to use a multi-dimensional array in Java.

By the way, It's also one of the frequently asked Java interviews, and if you are preparing for your next job, then knowing these details could be really useful. You can also take benefit from the variety of questions from the Java Interview Guide: 200+ Interview Questions and Answers course, one of the best courses to prepare for Java programmer job interviews.

Here is a nice slide highlighting the all-important difference between Array and ArrayList in Java. 

Difference between an Array vs ArrayList in Java




The similarity between Array and ArrayList

So far you have seen the difference between an ArrayList and an array, now let's concentrate on some of the similarities. Since ArrayList internally uses an array, it's bound to have a lot of similarities as seen below:

1. Data Structure

Both allow you to store objects in Java and both are an index-based data structure that provides O(1) performance to retrieve an element, but search without an index is still log(N) if your array is sorted and you use a binary search algorithm.

2. Order

Both array and ArrayList maintain order on which elements are added into them.

3. Search

You can search for an element using an index, that's O(1) otherwise you can use linear search if your array is not sorted, which takes around O(n) time or you can use binary search after sorting an array in Java, this is sorting + O(logN).

4. Null values

Both array and ArrayList allow null values but remember only object array allows null primitive array don't they store the default value of primitive type like zero for int and false for boolean.

5. Duplicates

Both array and ArrayList allow duplicates. It's also one of the common array-based coding questions to write a program to find out duplicates from an array in place.

6. Performance

ArrayList mimics the array's performance like O(1) access if you know the index but it has additional memory overhead because it's an object and also holds additional data to automatically resize the ArrayList.


7. Zero-based Index

Both array and ArrayList have zero-based index i.e. first element starts at zeroth index.


That's all about the real difference between an array and an ArrayList in Java. The most important difference you should remember is that the array is static in nature i.e. you cannot change their size once created but ArrayList is a dynamic array, which can resize itself if a number of elements in the ArrayList are more than the resize threshold.

Based upon this difference, you should use an array as a data structure to store objects if you know the size in advance and sure it's not going to change, if you are unsure then just use the ArrayList.

Further Learning
The Complete Java Masterclass
Java Fundamentals: Collections
Data Structures and Algorithms: Deep Dive Using Java


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Thanks for reading this article so far. If you like this List vs array comparison then, please share it with your friends and colleagues. If you have any questions or feedback then please drop a note.

P.S. - If you are a beginner Java developer or someone looking to self-teach Java to yourself, I suggest you checkout Java Tutorial for Complete Beginners(FREE) course from Udemy. More than 1 million Java programmers have joined this course to learn Java online, and it's completely free. 

7 comments :

Unknown said...

> add can be O(logN) in ArrayList if adding a new element triggers resize, as it involves creating a new array in background and copying elements from the old array to new array.

Is it really O(logN)? Copying should take O(n)

javin paul said...

@Unknown, Yes, array copy is theoretically O(n) but ArrayList.add() internally uses System.arrayCopy() to copy data. Since System.arraycopy() is a native call which does copy operation directly at memory, it's much faster than O(n). It also depends where you are adding element e.g. if you append it's constant or logarithmic but if insert it at the middle than it could be linear or (nlogn) if growing, but remove is always liner because it always perform complete copy.

Unknown said...

Type Safety statement is incorrect: You will get compilation error if you try to store the String in int array.

Anonymous said...

Even if you are using System.arrayCopy, it is still O(N), it doesn't matter if it is Java/Ruby/Aseembly or you are using RAM, HD or CPU Cache, what matter is the algorithm used to copy

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Somanath K. said...

Really Nice Explanation.
Thank you!!

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