Difference between an Array and ArrayList in Java

In this post, we will discuss the difference between an array and ArrayList in Java.

 

1. Resizing

An Array is a fixed size data structure that does not permit elements to be inserted or removed after its creation. It is possible to effectively implement a dynamic version of an array by reallocating storage and copying the elements of the old array to it. This is exactly how an ArrayList works which is resizable-array implementation of the List interface.

 

2. Performance

Common operations (such as get, set for ArrayList and store, select for Arrays) takes constant time. However, an array doesn’t support insertion operation while insertions at the end of the ArrayList require only amortized constant time i.e. adding n elements requires Θ(n) time.

 

3. Primitives

Arrays in Java can hold both primitives datatypes (int, char, long, float, double, boolean, etc) and Java objects (Integer, Character, Long, Float, Double, Boolean, String, etc) while an ArrayList can hold only Java objects.

 

4. Storage

Arrays take O(n) space for n number of elements and do not reserve any additional storage while an ArrayList reserve linear O(n) additional storage. We can call trimToSize() method of ArrayList class to minimize the storage of an ArrayList instance.

 

5. Size

The size of the ArrayList can be calculated by its size() method while we can use length variable to calculate length of an array.

 

6. Dimension

The dimension of an array is the number of indices needed to select an element. Arrays in Java supports one-dimensional array, two-dimensional array, three-dimensional array and so on.. while an ArrayList has no concept of dimentions but we can easily construct an ArrayList of ArrayLists or ArrayList of ArrayList of ArrayLists.

 

7. Generics support

Generics are not supported by an array but it can be use with an ArrayList to ensure type safety.

 
References: Arrays Wiki

 
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