Array vs ArrayList in Java

Lakshmi Ramasamy 24th July 2020 Leave a Comment

In Java, we have two ways to create an array.

Array – Simple fixed sized arrays. We cannot change its size.
ArrayList – Is a class of Java Collections framework. It belongs to java.util package.

//Array
int arr[] = new int[10];
//ArrayList - Here Type is the type of elements
ArrayList<Type> arrL = new ArrayList<Type>();

Similarities

Both are used for storing elements.
Array and ArrayList both can store null values.
They both can allow duplicate values.
Don’t maintain the order of elements.

Differences

Array	ArrayList
static in size.	Dynamic in size.
fixed-length data structure	variable-length data structure. It can be resized itself when needed.
It is mandatory to initializing the size of an array directly or indirectly.	Can create an instance of `ArrayList` without specifying its size.
It performs fast in comparison to `ArrayList` because of fixed size.	`ArrayList` is internally backed by the array in Java. The resize operation in `ArrayList` slows down the performance.
Stores both objects and primitives type.	It automatically converts primitive type to object.
Use `for` loop or `for each` loop to iterate over an array.	Use `for` loop or `for each` loop or iterator to iterate over `ArrayList`.
`.length` to get the length of an Array	`size()` method to determine the size.
We can add elements in an array by using the assignment operator. eg: arr[0] = 0;	`add()` method to add elements in the ArrayList. eg: arrL.add(1);
multi-dimensional.	Always single-dimensional.

Example for Array in Java

public class ArrayFunction {
	public static void main(String[] args) {
		// creating an array of integer type
		int arr[] = new int[5];
		// adding elements into array
		arr[0] = 12;
		arr[1] = 2;
		arr[2] = 15;
		arr[3] = 67;
		arr[4] = 12;
		System.out.println("Array Length: " + arr.length);
		for (int i = 0; i < arr.length; i++) {
			System.out.println("Index::" + i + "   Value::" + arr[i]);
		}
	}
}

Output

Array Length: 5
Index::0   Value::12
Index::1   Value::2
Index::2   Value::15
Index::3   Value::67
Index::4   Value::12

If you add a value for the index 5 it will throw java.lang.ArrayIndexOutOfBoundsException: 5. This is fixed-length data structure.

It is mandatory to initializing the size of an array otherwise it will shows the error as

Variable must provide either dimension expressions or an array initializer.

Example for ArrayList in Java

public class ArrayListFunction {
	public static void main(String[] args) {
        //creating an instance of ArrayList  
		ArrayList<Integer> list = new ArrayList<Integer>();   
		//adding element to arraylist  
		list.add(12);   
		list.add(34);   
		list.add(56);   
		list.add(78);   
		//iteration over ArrayList using for-each loop  
		for(Integer i:list)  
		{  
			System.out.println("Value: " + i);   
		}  
		}   
}