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Difference Between Array and ArrayList in Java – The Ultimate Guide

When learning Java, understanding arrays and ArrayList is essential. At first glance they both hold sequences of data, but they have important differences in size, flexibility, type support, and performance. This article explains the difference between array and ArrayList in Java, with examples, pros and cons, and guidance on when to use each.


What Is an Array in Java?

A Java array is a built-in data structure. It’s fixed‑size: once created, its length cannot change. Arrays support both primitive types (e.g. int, char) and object references (e.g. String, custom classes). Elements are stored in contiguous memory; primitive arrays store actual values directly, object arrays store references.

Access is via simple indexing using [] syntax. Syntax examples:

javaCopyEditint[] arr = new int[5];        
String[] names = {"Alice", "Bob", "Carol"};

What Is an ArrayList in Java?

An ArrayList is part of the Java Collections Framework (java.util) and implements the List interface.

  • It is dynamic in size: grows automatically as elements are added.
  • Stores only object references, not primitives; uses autoboxing for types like int to Integer.
  • Provides built‑in methods like add(), remove(), get(), size(), and more.
  • Internally backed by an array and resizes when capacity is exceeded.

Example usage:

javaCopyEditimport java.util.ArrayList;

ArrayList<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
System.out.println(list.get(0));    
System.out.println(list.size());    

Key Differences at a Glance

FeatureJava ArrayArrayList
SizeFixed at creationDynamic (can grow or shrink)
Type of elementsPrimitives & objectsObjects only (via generics)
Syntax accessarr[index]list.get(index)
Adding/removingNot built-inadd() and remove() methods
Resizing performanceN/A (cannot)May be slower due to internal array copy
GenericsNot supportedSupported (type-safe)
Memory layoutContiguous blockReferences contiguous; actual objects separate
Multi-dimensionalSupported (e.g. int[][] arr)Only single-dimensional
Useful methodsNone built-inindexOf(), contains(), clear()
PerformanceVery fast access, low overheadSlight overhead, resizing cost
Null handlingObjects allow null, primitives do notNulls allowed in object types
Use casesKnown fixed-size, performance-criticalFlexible, frequently modified lists

Detailed Comparison

Difference Between Array and ArrayList in Java

Size and Flexibility

Array size is fixed: int[] arr = new int[10];. You cannot add more than 10 elements.

ArrayList can grow and shrink: new ArrayList<>(). It resizes as elements are added or removed.

Type Support and Generics

Arrays support both primitives and objects.

ArrayList supports only object types. For primitives, you must use wrapper classes like Integer, Double, etc.

Generics are not supported by arrays, but ArrayList supports them fully, providing type safety at compile time (source)

Performance

Arrays provide fast, direct access (O(1)) and low overhead.

ArrayList provides similar access speed but may occasionally slow down due to resizing. Internally, when capacity is exceeded, a new array is created and elements are copied.

Built-in Functionality

Arrays are minimal. No built-in methods beyond length.

ArrayList offers a wide range of methods: add(), remove(), indexOf(), contains(), clear(), and supports iteration and Java Streams.

Memory Layout

Primitive arrays store values in a contiguous block.

Object arrays and ArrayLists store references contiguously, but the actual objects can reside anywhere on the heap.

Multi-dimensional Data

Arrays can be multi-dimensional (e.g. int[][] matrix).

ArrayLists support only single-dimension natively, although nested ArrayLists can simulate multi-dimensionality.


Example Code Snippets

Fixed-size Array Example

javaCopyEditpublic class ArrayExample {
    public static void main(String[] args) {
        int[] arr = new int[3];
        arr[0] = 10;
        arr[1] = 20;
        arr[2] = 30;
        System.out.println("Length: " + arr.length);
        for (int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }
    }
}

Dynamic ArrayList Example

javaCopyEditimport java.util.ArrayList;

public class ArrayListExample {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");
        System.out.println("Size: " + fruits.size());
        for (String f : fruits) {
            System.out.println(f);
        }
        fruits.remove(1);
        fruits.add("Date");
        System.out.println(fruits);
        System.out.println("Contains Apple? " + fruits.contains("Apple"));
    }
}

Primitive vs Object Types

javaCopyEditint[] primitiveArr = {1, 2, 3};               
Integer[] objArr = {1, 2, 3};                 
ArrayList<Integer> list = new ArrayList<>();
list.add(1);  // autoboxing

ArrayList Resizing Behavior

javaCopyEditArrayList<Integer> numbers = new ArrayList<>(2);
numbers.add(10);
numbers.add(20);
numbers.add(30);  // triggers internal resizing
System.out.println(numbers);

When to Use Each

Use Array When:

  • The size is known and fixed.
  • You want to store primitive types.
  • You need performance and memory efficiency.
  • You require multi-dimensional data storage.

Use ArrayList When:

  • The size can change dynamically.
  • You are working with objects and prefer generics.
  • You need built-in methods for adding, removing, and searching.
  • You’re dealing with data that’s frequently modified.

Performance Considerations

  • Access time for both is constant (O(1)).
  • Adding in arrays requires manual resizing; ArrayLists handle this internally.
  • Removing in arrays involves shifting manually; ArrayLists do it automatically.
  • Memory use is typically lower for arrays, unless you’re storing a lot of nulls in a larger capacity ArrayList.
  • For performance-critical applications, arrays are often preferred. For convenience, ArrayList is superior.

Advanced Examples

Wrapping an Array as a List

javaCopyEditString[] planets = { "Mercury", "Venus", "Earth", "Mars" };
List<String> fixed = Arrays.asList(planets);
List<String> dynamic = new ArrayList<>(Arrays.asList(planets));

planets[0] = "Pluto";
System.out.println(fixed);   // reflects change
System.out.println(dynamic); // independent copy

Converting ArrayList to Array

javaCopyEditArrayList<String> list = new ArrayList<>();
list.add("A");
list.add("B");

String[] arr = list.toArray(new String[list.size()]);

Summary

  • Array: Fast, lightweight, fixed-size, supports primitives and objects.
  • ArrayList: Dynamic, flexible, object-only, built-in methods, type-safe via generics (read more).
  • Choose arrays for performance-sensitive and size-stable needs.
  • Choose ArrayList for flexible, evolving collections where convenience and readability matter.

Code Comparison Side-by-Side

javaCopyEdit// Array
int[] arr = new int[3];
arr[0] = 1; arr[1] = 2; arr[2] = 3;

// ArrayList
ArrayList<Integer> al = new ArrayList<>();
al.add(1); al.add(2); al.add(3);
al.add(4);
System.out.println(al.get(2));
al.remove(Integer.valueOf(2));
System.out.println(al.contains(3));

Real‑World Use Cases

  • Configuration constants → Use arrays
  • Dynamic user input → Use ArrayList
  • Data matrices or grids → Use multi-dimensional arrays
  • Object collections with sorting/filtering → Use ArrayList with Streams

Best Practices & Tips

  • Always use generics with ArrayList.
  • Be cautious of autoboxing when storing primitives.
  • Pre-allocate ArrayList capacity if you expect many elements.
  • Use List as the interface type, e.g., List<String> list = new ArrayList<>(); for better flexibility.

The Take Home – Difference Between Array and ArrayList in Java

The difference between array and ArrayList in Java lies in size management, type compatibility, flexibility, and functionality. Arrays offer simplicity and performance, ideal for fixed-size collections. ArrayLists, on the other hand, provide versatility and ease of use, ideal for dynamic and complex data operations.

By understanding their distinctions, Java developers can make smarter choices depending on the use case—whether optimizing for speed and memory or leveraging modern programming conveniences.

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