Dynamic Array in Golang

array.go

package array

const DefaultCapacity int = 10

type Array struct {
	capacity int
	size     int
	items    []interface{}
}

func DynamicArray() *Array {
	items := make([]interface{}, DefaultCapacity)

	return &Array{capacity: DefaultCapacity, items: items}
}

func (array *Array) Add(element interface{}) {
	if array.size == array.capacity {
		newCapacity := array.capacity * 2
		newArray := make([]interface{}, newCapacity)

		for index, item := range array.items {
			newArray[index] = item
		}

		array.capacity = newCapacity
		array.items = newArray
	}

	array.items[array.size] = element
	array.size++
}

func (array *Array) Get(index int) interface{} {
	return array.items[index]
}

func (array *Array) Size() int {
	return array.size
}

array_test.go

package array

import (
	"fmt"
	"testing"
)

func TestDynamicArray(t *testing.T) {
	array := DynamicArray()

	t.Run("when a new array it returns a new Dynamic array", func(t *testing.T) {
		if array == nil {
			t.Errorf("should return an dynamic array")
		}
	})

	t.Run("when adding a new element to the Array", func(t *testing.T) {
		array.Add(10)
		if got := array.Size(); got != 1 {
			t.Errorf("Size should be 1, but got %d", got)
		}

		array.Add(20)
		if got := array.Size(); got != 2 {
			t.Errorf("Size should be 2, but got %d", got)
		}

		if got := array.Get(0); got != 10 {
			t.Errorf("should return the first added element %d", got)
		}

		if got := array.Get(1); got != 20 {
			t.Errorf("should return the second added element %d", got)
		}
	})

	t.Run("when adding new element up to the capacity", func(t *testing.T) {
		for index := 0; index < DefaultCapacity; index++ {
			fmt.Printf("index %d, size %d\n", index, array.Size())

			array.Add(index)
		}

		if array.capacity != 20 {
			t.Errorf("it should double the array capacity")
		}

		array.Add(99)

		if got := array.Get(12); got != 99 {
			t.Errorf("should be able to add items beyound previous capacity %d", got)
		}
	})
}