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ArrayList Vs. LinkedList: Key Differences Explained in Detail

The essence of data structures in the programming culture is that they act as tools, incorporating respective techniques for efficient data management. Some data structures in Java exist under the Collection Framework, with List being the most used to store ordered collections. With most List interface implementations, confusion arises between the two most prominent, the ArrayList and the LinkedList, among new programmers and even experienced people.

In this article, we will explore the difference between ArrayList and LinkedList to understand how they work, where each excels, and which one to choose based on different use cases. We’ll begin by introducing both classes individually, then delve into a detailed comparison, followed by some similarities and practical applications. Let's go in!

Overview of ArrayList and LinkedList

An ArrayList is a resizable array implementation of the List Interface in Java. Random access of elements is allowed in the ArrayList, so it performs very well where read operations are frequent or index-based access is required; otherwise, the performance is very poor. A dynamic array is used in this case to store the element array.

LinkedList is an implementation of the List and Deque interfaces in Java that is modeled after a doubly linked list. It is typically used for applications requiring many insertions and deletions in the middle of the list. Each element is stored in an element node that contains links to its previous and next link nodes.

Difference Between ArrayList and LinkedList

To begin, let us understand the key differences between ArrayList and LinkedList:

Feature	ArrayList	LinkedList
Underlying Data Structure	Backed by a resizable array, where elements are stored in contiguous memory locations.	Backed by a doubly linked list, where each node contains data and references to the next and previous nodes.
Access Time (Search)	Provides fast random access using indices. get(index) and set(index, value) have time complexity of O(1).	Slower access as it requires traversal from the beginning or end. Index access like get(index) takes O(n) time.
Insertion/Deletion	Inserting or deleting elements, especially in the middle, requires shifting elements, making it O(n) in worst cases.	Insertions and deletions (at head or tail or in the middle) are more efficient as no shifting is needed. Time complexity is typically O(1) at the ends.
Memory Consumption	More memory-efficient as it stores only the data.	Consumes more memory due to the storage of extra pointers (next and previous) in each node.
Performance with Iterators	Iteration is faster because of contiguous memory, which enhances cache performance.	Iteration is slower compared to ArrayList due to pointer dereferencing and non-contiguous memory allocation.
Resizing Cost	When capacity exceeds, a new, larger array is created, and existing elements are copied, causing overhead.	No need for resizing, as nodes are added as needed, but node creation incurs its own overhead.
Use Case	Preferred when there are frequent reads and rare insertions/deletions, especially from the middle of the list.	Ideal when there are frequent insertions and deletions, especially at the start or middle of the list.
Interfaces Implemented	Implements only the List interface.	Implements List, Deque, and Queue interfaces, making it more versatile for use cases like stacks and queues.
Thread Safety	Not thread-safe by default. Requires external synchronization or use of Collections.synchronizedList() or CopyOnWriteArrayList for thread safety.	Also not thread-safe. Synchronization must be handled manually or using concurrent collections like ConcurrentLinkedQueue.
Null Elements Support	Allows any number of null elements.	Allows any number of null elements as well.
Traversal Direction	Supports forward traversal only using an iterator	Supports both forward and backward traversal through ListIterator thanks to the doubly linked nature.
Insertion Order	Maintains the order in which elements are inserted.	Also maintains insertion order.
Initial Capacity	Can be initialized with a specific capacity. If not, defaults to 10.	No concept of initial capacity as memory is allocated dynamically with each node.
Fail-Fast Behavior	Iterator is fail-fast; it throws ConcurrentModificationException if the list is structurally modified during iteration.	Also fail-fast under similar concurrent modification scenarios.
Implementation Complexity	Simpler to implement and use for general-purpose lists.	More complex internally due to node structure and pointer management.
Garbage Collection Impact	Easier garbage collection due to direct memory references and contiguous storage.	Slightly harder for GC to optimize due to non-contiguous memory and many intermediate node objects.

ArrayList and Its Features

An ArrayList is part of Java's Collection Framework, representing an array whose size can be modified. Therefore, it will grow or shrink dynamically as elements are added or removed. It is among the most widely used classes for storing and manipulating lists of data when the size of the collection is not definite.

Here are some of the core features that make ArrayList a popular choice among Java developers:

Dynamic Array: Unlike regular arrays, the ArrayList dynamically resizes in size when elements are added or removed; hence, there is no need to give it an initial fixed size.
Random Access: ArrayList gives fast and direct access to elements via indices using the get(index) and set(index, element) operations with O(1) time complexity.
Ordered Collection: It preserves the order of the elements; hence, it retrieves elements in the order they were inserted.
Enables Duplicates and Nulls: The ArrayList allows duplicate values and multiple null entries, giving flexibility to developers relative to a data structure.
List Interface: The ArrayList implements List and inherits all methods, such as add(), remove(), contains(), etc. Hence, it is a fully functional ordered list.
Synchronized: The ArrayList is not synchronized, which means that it ought to be externally synchronized while being used in concurrent surroundings.
Internal Implementation: Internally, elements are stored in arrays. When capacity is exceeded, a larger new array is created, and all elements are transferred into this new array.

Working Of ArrayList

Let us now consider the working mechanism of an ArrayList:

Inserting Data in the ArrayList

To insert elements into an ArrayList, you can use the add() method. This method appends the specified element to the end of the list.

ArrayList<String> list = new ArrayList<>();
list.add(“Apple”);
list.add(“Banana”);
list.add(“Mango”);

You can also insert an element at a specific position using:

list.add(1, “Orange”); // Inserts “Orange” at in dex 1

Accessing and Searching Values in the ArrayList

To access a value at a particular index, use the get() method. To search for a value, you can use contains() or indexOf().

String fruit = list.get(0); // Accesses first element
Boolean hasMango = list.contains(“Mango”); // Checks if “Mango” is in the list
Int index = list.index0f(“Banana’); // Finds index of “Banana”

Modifying the ArrayList

To modify (update) an element, use the set() method by specifying the index and the new value.

list.set(2, “Pineapple”); // Replaces element at index 2 with “Pineapple”

Deleting from the ArrayList

To remove elements, use remove() by index or by value. You can also clear all elements using clear().

list.remove(“Orange”); // Removes the element “Orange”
list.remove(0); // Removes the element at index 0
list.clear(); // Removes all elements from the list

LinkedList and Its Features

A linked list in Java is a type of linear data structure that uses nodes to store elements. Each node has data and a reference (or 'link') to the next node in the sequence. It is part of the Java Collections Framework and implements both List and Deque interfaces, thus making it a versatile data structure for dynamic data operations.

Key Features of LinkedList

Dynamic: LinkedList does not need an initial capacity; it increases or decreases automatically when data is added or removed. Thus, it is useful for applications where the number of actual elements is unpredictable.
Efficient Insertions and Deletions: Inserting or deleting an element from a LinkedList is more efficient than with an ArrayList when it comes to doing so from its beginning or middle, as no shifting is required.
Doubly LinkedList: LinkedList in Java is implemented as a doubly-linked list. Each node contains references to both the previous and next nodes, thus enabling the efficient traversal of the list in both forward and backward directions.
Implements List and Deque: It acts as both a List (for indexed access) and a Deque (for queue and stack operations), through addFirst(), addLast(), poll(), and peek() methods.
Null and Duplicate Entries: As any other collection in Java, LinkedList allows null values and duplicates.
Not Synchronized: By default, LinkedList is not thread-safe. If used in a multithreaded environment, it must be synchronized externally or wrapped using Collections.synchronizedList().
Slower Random Access: Unlike ArrayList, accessing elements by index is slower (O(n)) as it requires sequential traversal from the head or tail.
Standard Collection Support: It supports all standard collection operations like add(), remove(), contains(), size(), and iterator().

Working Of LinkedList

Let us now study the working mechanism of LinkedList:

Inserting Data in the LinkedList

To insert data, we use methods like add(), addFirst(), and addLast():

LinkedList<String> list = new LinkedList<>();
list.add(“Apple”); // Adds at the end
list.addFirst(Mango); // Adds at the beginning
list.addLast(“Banana”) // Adds at the end
list.add(1, “Orange”) // Inserts at index 1

Accessing and Searching Values in the LinkedList

To access elements, we use get(index). To search, we can use contains() or indexOf():

String firstElement = list.get(0); // Access by index
boolean hasBanana = list.contains(“Banana”); // Check if “Banana” exists
int position = list.index0f(“Orange”); // Get index of “ Orange”

Modifying the LinkedList

We can modify a value at a specific index using set(index, value):

list.set(2, “Pineaple”); // Replaces element at index 2 with “Pineapple”

Deleting from the LinkedList

Elements can be removed using remove(), removeFirst(), removeLast() or by index/value:

list.remove(“Apple”); // Removes first occurrence of “Apple”
list.remove(0); // Removes element at index 0
list.removeFirst(); // Removes the first element
list.removesLast(); // Removes the Last element

Difference Between ArrayList and LinkedList

Let us now look at some of the key differences between ArrayList and LinkedList:

Feature / Criteria	ArrayList	LinkedList
Usage	Best suited for scenarios where random access to elements is needed frequently, as it provides fast access via indexes.	Ideal for applications that require frequent insertions and deletions (especially at the beginning or middle). These operations are more efficient in LinkedLists.
Underlying Data Structure	ArrayList uses a dynamic array to store elements. When the array gets full, it resizes itself by allocating a larger array and copying the elements over.	LinkedList uses a doubly linked list, where each element (node) contains pointers to both the next and previous elements, allowing for efficient bidirectional traversal.
Memory	ArrayList has a lower memory overhead per element, as it only stores the elements in the array. There is no need to store additional pointers.	LinkedList requires more memory because each node in the list stores the actual data and two references (pointers): one to the next node and one to the previous node.
Resizability	When the ArrayList reaches its capacity, it automatically doubles its size to accommodate more elements, ensuring that the list can grow dynamically.	LinkedList doesn't require resizing; elements are added one by one, and the list expands as needed, with no reallocating of large contiguous blocks of memory.
Capacity Management	ArrayList has an initial capacity and increases its capacity by 50% when it runs out of space. This resizing process can be expensive in terms of time and memory	LinkedList dynamically allocates new nodes and doesn't require a resizing mechanism. Nodes are added or removed without having to reallocate memory blocks.
Insertion Time (Middle)	Inserting in the middle of an ArrayList is relatively slow because all elements after the insertion point need to be shifted to make room for the new element. This operation is O(n).	In a LinkedList, inserting in the middle is more efficient. Once the node reference is found, the insertion involves only adjusting pointers (next/previous), which takes O(1) time. However, finding the middle element may still take O(n)
Insertion Time (End)	Appending elements to the end of an ArrayList is fast (amortized O(1)), unless the array needs to be resized. The operation is usually quick because it simply involves adding the element to the next available index.	Inserting at the end of a LinkedList is also O(1) if a tail pointer is maintained. However, if a tail pointer is absent, traversing the list to the last element takes O(n).
Insertion Time (Start)	Inserting at the beginning of an ArrayList requires shifting all existing elements to the right to make room for the new element. This makes it O(n) in terms of time complexity.	In LinkedList, inserting at the start is fast (O(1)), as the new node can be directly linked to the head of the list, and no shifting is required.
Deletion Time (Middle)	Deleting an element from the middle of an ArrayList is O(n) due to the shifting of elements after the deleted element.	Deletion in the middle of a LinkedList is O(1) (if the node reference is known), because it involves adjusting only a few pointers (previous and next)
Deletion Time (End)	Deletion from the end of an ArrayList is O(1), as no elements need to be shifted. However, if you delete from the middle or start, shifting is still required.	Deleting from the end of a LinkedList takes O(1) time if the tail pointer is available. If not, it requires traversing the list, making the time complexity O(n).
Access Time	ArrayList provides fast (O(1)) random access, making it ideal when you need to retrieve elements frequently by index.	LinkedList has O(n) access time for retrieving an element, as it requires sequential traversal of the list from either the head or the tail.
Iteration Performance	ArrayList tends to have better iteration performance compared to LinkedList because of its contiguous memory allocation. This is more cache-friendly, making it faster during traversal.	LinkedList has slower iteration performance because it is not contiguous in memory and requires following pointer references from node to node, which is less cache-friendly
Search Time	ArrayList allows fast (O(1)) access by index, but searching for an element (e.g., using contains()) takes O(n) time as it has to scan each element sequentially.	LinkedList requires O(n) time for searching, as it does not allow random access and has to traverse from either the head or tail to find the desired element.
Thread Safety	ArrayList is not thread-safe by default. If concurrent modifications are made to the list, it must be externally synchronized or use a thread-safe variant like CopyOnWriteArrayList.	LinkedList is also not thread-safe by default. Similar to ArrayList, it requires external synchronization for safe concurrent access.
Use Case Scenario	ArrayList is best suited for frequent access to elements by index, such as in scenarios where you need to perform random access or need to store large amounts of data.	LinkedList is better when your application involves frequent insertions or deletions of elements, particularly in situations where memory management and efficient resizing are a concern.

When Should We Use ArrayList?

ArrayList is backed by a dynamic array, making it an excellent choice when:

Need quick random access to elements: ArrayList can get any element directly in constant time (O(1)), making it a natural choice for those cases where you are frequently calling its get(index) operations.

Read mostly: If the application only displays or iterates through elements with little modification (like a product list or dataset), an ArrayList is more efficient.

Insertions and deletions usually happen at the end: Appending at the end is a fast operation (O(1) amortized). In contrast, inserting or deleting from the middle or start requires a shifting of elements, which can result in an expensive operation (O(n)).

If memory matters: In memory per element, ArrayList is more efficient than LinkedList, which uses additional references for previous and next nodes.

When Should We Use LinkedList?

LinkedList is backed by a doubly-linked list, and shines when:

Your application involves frequent insertions and deletions, especially in the middle or at the beginning of the list.

Write-heavy operations dominate: LinkedList allows efficient additions/removals (O(1) if you have a reference to the node), without the need to shift elements like in ArrayList.

You don’t need fast index-based access: Accessing elements by index is a linear operation (O(n)) since the list has to be traversed from the start (or end).

You’re implementing data structures like stacks, queues, or deques: LinkedList is a natural fit due to its ability to insert and remove from both ends efficiently.

Conclusion

Both ArrayList and LinkedList are versatile and widely used implementations of the List interface in Java, each offering unique advantages and trade-offs. ArrayList excels in scenarios that require fast access and minimal modifications, making it ideal for applications where data is read frequently and updated infrequently. On the other hand, LinkedList shines in use cases that involve frequent insertions and deletions, especially in the middle or at the beginning of the list, thanks to its node-based structure.

Ultimately, the decision between ArrayList and LinkedList depends on the nature of the operations your application performs most often. By understanding the internal mechanics and performance implications of each, developers can choose the most efficient list implementation for their specific use case, leading to more optimized and maintainable

Frequently Asked Questions (FAQs)

Q1. What is the difference between ArrayList and LinkedList memory allocation?

ArrayList uses a dynamic array internally, meaning it allocates a contiguous block of memory. When the array reaches its maximum capacity, a new, larger array is created and all existing elements are copied into it, which can be a costly operation. This approach makes ArrayList memory efficient for random access but expensive for frequent insertions and deletions.
In contrast, LinkedList uses a doubly-linked list data structure where each element (node) contains references to both its previous and next elements.

Memory is allocated individually for each node, and elements do not need to be contiguous in memory. This makes LinkedList efficient for frequent additions and deletions, especially at the beginning or middle of the list, but it also introduces extra overhead due to storing references with each node and can lead to fragmented memory.

Q2. Which is faster: ArrayList or LinkedList?

The performance of ArrayList and LinkedList depends on the type of operations being performed. ArrayList is faster for accessing elements using an index (O(1) time complexity), making it ideal for scenarios that involve a lot of random access. However, it becomes slower during insertions and deletions, particularly in the middle or beginning of the list, because it requires shifting elements.

LinkedList, on the other hand, offers better performance for frequent insertions and deletions (O(1) if using an iterator or modifying from ends), but it is slower for index-based access because it must traverse the list node by node (O(n) time complexity).

Q3. Can we store null values in ArrayList and LinkedList?

Yes, both ArrayList and LinkedList in Java allow null values. You can add one or more null entries to either of these data structures. The key difference lies in how null values may impact performance. Since LinkedList relies on node references, excessive null entries could lead to harder debugging or misuse if not handled properly, while ArrayList may waste space if nulls are inserted unintentionally in its indexed positions.

Q4. How do ArrayList and LinkedList differ in terms of iteration performance?

ArrayList offers better performance for iteration using the standard for-loop or enhanced for-loop because its elements are stored in a contiguous block of memory, making them easily accessible by index. This enables faster traversal with less overhead.

LinkedList iteration is typically slower since it involves traversing each node sequentially through pointers. However, LinkedList performs well when using a ListIterator for additions and deletions during iteration, where it avoids the need to shift elements as in an ArrayList.

Q5. When should we prefer LinkedList over ArrayList?

LinkedList is preferred over ArrayList when your application involves frequent insertions and deletions of elements, especially in the middle or at the beginning of the list. It is also suitable when memory fragmentation isn't a concern and object references are more manageable.

On the contrary, for applications requiring a lot of read operations or random access with minimal modifications, ArrayList is the better choice due to its faster index-based retrieval. Understanding the specific use case is crucial to choosing the right collection.

This article was contributed by Johns Joseph, Unstop Intern and Campus Ambassador.

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