Optimized Implementation

So with that context we can update the simple implementation as follows.

Note the use of IndexOutOfBoundsException. It is an unchecked exception that comes with Java specifically for when an index is out of bounds.¹

class GrowableIntArray {
    private int[] data;
    // We need a different variable to store the size
    // since the array's size will different than we want.
    private int size;

    GrowableIntArray() {
        this.data = new int[0];
        this.size = 0;
    }

    int get(int index) {
        // Now that we manage the size, we need to do
        // "bounds checks" ourselves
        if (index >= size) {
            throw new IndexOutOfBoundsException(index);
        }
        return this.data[index];
    }

    void set(int index, int value) {
        // For setting as well
        if (index >= size) {
            throw new IndexOutOfBoundsException(index);
        }
        this.data[index] = value;
    }

    int size() {
        // We use the size we store rather than the length of the array
        return this.size;
    }

    void add(int value) {
        // if we won't have enough room in the array, make a new one.
        if (size >= this.data.length - 1) {
            // Overallocate by 2x
            int newLength = this.data.length * 2;
            if (newLength == 0) {
                newLength = 2; // Unfortunately 0 * 2 is 0, so account for that
            }

            int[] newArray = new int[newLength];
            for (int i = 0; i < this.data.length; i++) {
                newArray[i] = this.data[i];
            }
            this.data = newArray; 
        }

        // And at this point we know the array is big enough
        this.data[size] = value;
        this.size++;
    }
}