golang how to use interface

interface.go

package main

import (
	"fmt"
)

type StorageLogic interface {
	Create(id int32) string
	Delete(id int32)
}

type defaultStorageLogic struct{}

func NewStorageLogic() StorageLogic {
	return (*defaultStorageLogic)(nil)
}

var storageHandler *Storage

type Storage struct {
	StorageDriver StorageLogic
}

func NewStorageHandler() *Storage {
	if storageHandler == nil {
		storageHandler = &Storage{
			StorageDriver: NewStorageLogic(),
		}
	}
	return storageHandler
}

var volume map[int32]string

func (p *defaultStorageLogic) Create(id int32) string {
	volume = make(map[int32]string)
	volume[id] = fmt.Sprintf("VolumeDriver%d", id)
	return volume[id]
}

func (p *defaultStorageLogic) Delete(id int32) {
	delete(volume, id)
}

func (p *Storage) CreateStorage(id int32) string {
	return p.StorageDriver.Create(id)
}

func (p *Storage) DeleteStorage(id int32) {
	p.StorageDriver.Delete(id)
}

func main() {
	//第一种场景
	storageLogic := NewStorageLogic()
	v := storageLogic.Create(1)
	fmt.Println(v)
	fmt.Println(volume)
	storageLogic.Delete(1)
	fmt.Println(volume)

	//第二种场景
	storageHandler = NewStorageHandler()
	v2 := storageHandler.CreateStorage(2)
	fmt.Println(v2)
	fmt.Println(volume)
	storageHandler.DeleteStorage(2)
	fmt.Println(volume)
}