Array Length vs. Capacity

How long is an array?

If someone asks you how long an array is, there could be two possible answers when discussing how long an array is.

1. How many items can an array hold, and
2. How many items currently an array has?

The first point is about capacity, and the second is about length.

Let us create an array A[10] whose capacity is10, but no items are added. Technically we can say the length is 0.

int[] A = new int[10];

Let's insert integers 1, 2, 3, and 4 into the above array.

A[0] = 1;
A[1] = 2;
A[2] = 3;
A[3] = 4;

At this point, the length/size of the array is 4 and the capacity of the array that has room to store elements is 10.

The following code snippets explain the difference between array length vs. capacity.

Capacity

The capacity of an array in Java can be checked by looking at the value of its length attribute. This is done using the code A.lengthwhere A the Array's name is.

public class ArrayCapacityLength {
    public static void main(String[] args) {
        int[] A = new int[10];

        System.out.println("Array Capacity " + A.length); // 10
    }
}

Running the above snippet gives

Array Capacity is 10

Length

This is the number of items currently in the A[] array.

import java.util.Arrays;

public class ArrayCapacityLength {
    public static void main(String[] args) {
        int[] A = new int[10];

        int currentItemsLength = 0;
        for (int i = 0; i < 4; i++) {
            currentItemsLength += 1;
            A[i] = i + 10;
        }

        System.out.println(Arrays.toString(A)); // [10, 11, 12, 13, 0, 0, 0, 0, 0, 0]
        System.out.println("Array length is " + currentItemsLength); // 4
        System.out.println("Array Capacity is " + A.length); // 10
    }
}

Running the above snippet gives

Array length is 4
Array Capacity is 10