go_polymorphism.go

package main

import (
	"errors"
	"fmt"
	"testing"

	"github.com/stretchr/testify/require"
)

// TestPolymorphism tests treating a savings account like a basic account
func TestPolymorphism(t *testing.T) {
	savingsAccount, err := NewSavingsAccount("mysavings")
	require.NoError(t, err)
	require.NotNil(t, savingsAccount)

	savingsAccount.SetWithdrawalLimit(4.0)

	// Treat the savings account as a basic bank account
	basicAccount := BankAccount(savingsAccount)

	b1, err := basicAccount.Balance()
	require.NoError(t, err)
	require.Equal(t, 0.0, b1)

	require.NoError(t, basicAccount.Deposit(12.0))

	b2, err := basicAccount.Balance()
	require.NoError(t, err)
	require.Equal(t, 12.0, b2)

	require.Error(t, basicAccount.Withdraw(5.0))

	require.NoError(t, basicAccount.Withdraw(4.0))

	b3, err := basicAccount.Balance()
	require.NoError(t, err)
	require.Equal(t, 8.0, b3)
}

// BankAccount represents a bank account
type BankAccount interface {
	// Balance returns the current balance of the account
	Balance() (float64, error)

	// Deposit deposits money onto the account
	Deposit(value float64) error

	// Withdraw widthdraws money from the account
	Withdraw(value float64) error
}

// SavingsAccount represents a savings bank account
type SavingsAccount interface {
	// Embedded public interface
	BankAccount

	// SetWithdrawalLimit sets a new withdrawal limit
	SetWithdrawalLimit(newLimit float64) error
}

/*************************************************\
  BASIC BANK ACCOUNT
\*************************************************/

// NewBankAccount creates a new bank account instance
func NewBankAccount(id string) (BankAccount, error) {
	return newBankAccount(id)
}

func newBankAccount(id string) (*bankAccount, error) {
	if len(id) < 1 {
		return nil, errors.New("invalid bank account id")
	}
	return &bankAccount{
		id:      id,
		blocked: false,
		balance: 0,
	}, nil
}

type bankAccount struct {
	// Private fields
	id      string
	blocked bool
	balance float64
}

func (ba *bankAccount) checkBlocked() error {
	if ba.blocked {
		return fmt.Errorf("account %s is blocked", ba.id)
	}
	return nil
}

func (ba *bankAccount) Balance() (float64, error) {
	if err := ba.checkBlocked(); err != nil {
		return 0, err
	}
	return ba.balance, nil
}

func (ba *bankAccount) Deposit(value float64) error {
	if err := ba.checkBlocked(); err != nil {
		return err
	}
	if value > 0 {
		ba.balance += value
		return nil
	}
	return fmt.Errorf("invalid deposit value: %f", value)
}

func (ba *bankAccount) Withdraw(value float64) error {
	if err := ba.checkBlocked(); err != nil {
		return err
	}
	if ba.balance-value < 0 {
		return errors.New("insufficient funds")
	}
	ba.balance -= value
	return nil
}

// TestBankAccount tests BankAccount
func TestBankAccount(t *testing.T) {
	account, err := NewBankAccount("ABC")
	require.NoError(t, err)
	require.NotNil(t, account)

	b1, err := account.Balance()
	require.NoError(t, err)
	require.Equal(t, 0.0, b1)

	require.NoError(t, account.Deposit(4.5))

	b2, err := account.Balance()
	require.NoError(t, err)
	require.Equal(t, 4.5, b2)

	require.NoError(t, account.Withdraw(2.5))

	b3, err := account.Balance()
	require.NoError(t, err)
	require.Equal(t, 2.0, b3)
}

/*************************************************\
  SAVINGS BANK ACCOUNT
    - composes a basic account
    - overwrites method Withdraw
\*************************************************/

// NewSavingsAccount creates a new savings bank account instance
func NewSavingsAccount(id string) (SavingsAccount, error) {
	return newSavingsAccount(id)
}

func newSavingsAccount(id string) (*savingsAccount, error) {
	newBankAccount, err := newBankAccount(id)
	if err != nil {
		return nil, err
	}

	return &savingsAccount{
		bankAccount:     newBankAccount,
		withdrawalLimit: 0.0,
	}, nil
}

type savingsAccount struct {
	// Embedded private struct
	*bankAccount

	// Private fields
	withdrawalLimit float64
}

func (ba *savingsAccount) Withdraw(value float64) error {
	if value > ba.withdrawalLimit {
		return fmt.Errorf("withdrawal limit (%f) exceeded", ba.withdrawalLimit)
	}
	return ba.bankAccount.Withdraw(value)
}

func (ba *savingsAccount) SetWithdrawalLimit(newLimit float64) error {
	if newLimit < 0 {
		return fmt.Errorf("invalid withdrawal limit: %f", newLimit)
	}
	ba.withdrawalLimit = newLimit
	return nil
}

// TestSavingsAccount tests SavingsAccount
func TestSavingsAccount(t *testing.T) {
	account, err := NewSavingsAccount("mysavings")
	require.NoError(t, err)
	require.NotNil(t, account)

	b1, err := account.Balance()
	require.NoError(t, err)
	require.Equal(t, 0.0, b1)

	require.NoError(t, account.Deposit(12.0))

	b2, err := account.Balance()
	require.NoError(t, err)
	require.Equal(t, 12.0, b2)

	account.SetWithdrawalLimit(4.0)

	require.Error(t, account.Withdraw(5.0))

	require.NoError(t, account.Withdraw(4.0))

	b3, err := account.Balance()
	require.NoError(t, err)
	require.Equal(t, 8.0, b3)
}