Difference Between Array and ArrayList

An array and an ArrayList both store an ordered group of elements that can be accessed by index. The main difference is that an array has a fixed size, decided when it is created, while an ArrayList is dynamic: it grows and shrinks as elements are added and removed. In this lesson, you will learn all the differences between them, with an example of each.

Example: Array and ArrayList Side by Side

Let's see an example that stores marks both ways.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
 
public class ArrayVsList {
    public static void main(String[] args) {
        int[] marksArray = new int[3];
        marksArray[0] = 91;
        marksArray[1] = 88;
        marksArray[2] = 95;
        System.out.println("Array: " + Arrays.toString(marksArray)
            + ", length " + marksArray.length);
 
        List<Integer> marksList = new ArrayList<>();
        marksList.add(91);
        marksList.add(88);
        marksList.add(95);
        marksList.add(79);
        System.out.println("ArrayList: " + marksList
            + ", size " + marksList.size());
    }
}

Array: [91, 88, 95], length 3
ArrayList: [91, 88, 95, 79], size 4

Explanation: The array was created for exactly 3 marks; a fourth would need a new, bigger array. The ArrayList simply accepts a fourth mark with add. Note the different ways to get the count: length (a field) for arrays and size() (a method) for lists.

Differences Between Array and ArrayList

Converting Between Them

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
 
public class Convert {
    public static void main(String[] args) {
        String[] namesArray = {"Navika", "Srihitha", "Advitha"};
        List<String> namesList = new ArrayList<>(Arrays.asList(namesArray));
        namesList.add("Anamaneni");
        System.out.println(namesList);
 
        String[] back = namesList.toArray(new String[0]);
        System.out.println(back.length + " " + back[3]);
    }
}

[Navika, Srihitha, Advitha, Anamaneni]
4 Anamaneni

Explanation: Arrays.asList wraps the array as a fixed-size list, and new ArrayList<>(...) copies it into a real, growable list. toArray(new String[0]) converts the list back into a String[] of the right size.

Common Mistakes

Calling arr.size() or list.length. Arrays use length; lists use size().

Writing ArrayList<int>. Use ArrayList<Integer>.

Adding to the list from Arrays.asList directly. It's fixed-size; copy it into a new ArrayList<>(...) first.

Interview Corner

Practice

1. Easy: Store five temperatures in an array and in an ArrayList, then add a sixth to the list.
2. Harder: Write a method that takes an int[] and returns an ArrayList<Integer> containing only its even numbers.

Key Takeaways