Go Interfaces
Master Go's interface system for creating flexible, decoupled code through implicit implementation and composition patterns.
Basic Interfaces
Defining and implementing interfaces:
package main
import "fmt"
// Define interface
type Writer interface {
Write(p []byte) (n int, err error)
}
// Implement interface (implicit)
type ConsoleWriter struct{}
func (cw ConsoleWriter) Write(p []byte) (n int, err error) {
fmt.Print(string(p))
return len(p), nil
}
func main() {
var w Writer = ConsoleWriter{}
w.Write([]byte("Hello, World!\n"))
}Multiple methods in interface:
type Reader interface {
Read(p []byte) (n int, err error)
}
type ReadWriter interface {
Read(p []byte) (n int, err error)
Write(p []byte) (n int, err error)
}
// Implement ReadWriter
type File struct {
name string
}
func (f *File) Read(p []byte) (n int, err error) {
// Implementation
return 0, nil
}
func (f *File) Write(p []byte) (n int, err error) {
// Implementation
return len(p), nil
}Empty Interface
Using interface{} (any in Go 1.18+):
// Accepts any type
func printValue(v interface{}) {
fmt.Println(v)
}
// Modern syntax (Go 1.18+)
func printAny(v any) {
fmt.Println(v)
}
func main() {
printValue(42)
printValue("hello")
printValue(true)
printAny(3.14)
}Type assertions:
func processValue(v interface{}) {
// Type assertion
if str, ok := v.(string); ok {
fmt.Println("String:", str)
}
// Type switch
switch val := v.(type) {
case int:
fmt.Println("Integer:", val)
case string:
fmt.Println("String:", val)
case bool:
fmt.Println("Boolean:", val)
default:
fmt.Println("Unknown type")
}
}Interface Composition
Embedding interfaces:
type Reader interface {
Read(p []byte) (n int, err error)
}
type Writer interface {
Write(p []byte) (n int, err error)
}
type Closer interface {
Close() error
}
// Compose interfaces
type ReadWriter interface {
Reader
Writer
}
type ReadWriteCloser interface {
Reader
Writer
Closer
}
// Standard library example
import "io"
func useReadWriteCloser(rwc io.ReadWriteCloser) {
// Can call Read, Write, and Close
rwc.Write([]byte("data"))
rwc.Close()
}Common Interfaces
Standard library interfaces:
// Stringer interface
type Stringer interface {
String() string
}
type Person struct {
Name string
Age int
}
func (p Person) String() string {
return fmt.Sprintf("%s (%d years old)", p.Name, p.Age)
}
// error interface
type error interface {
Error() string
}
type MyError struct {
Message string
}
func (e MyError) Error() string {
return e.Message
}
// sort.Interface
type Interface interface {
Len() int
Less(i, j int) bool
Swap(i, j int)
}
type ByAge []Person
func (a ByAge) Len() int { return len(a) }
func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }
func (a ByAge) Swap(i, j int) { a[i], a[j] = a[j], a[i] }Interface Design Patterns
Small interfaces:
// Good: small, focused interfaces
type Getter interface {
Get(key string) (value string, exists bool)
}
type Setter interface {
Set(key, value string)
}
type Deleter interface {
Delete(key string)
}
// Compose as needed
type Cache interface {
Getter
Setter
Deleter
}Accept interfaces, return structs:
// Accept interface parameter
func processReader(r io.Reader) error {
data, err := io.ReadAll(r)
if err != nil {
return err
}
fmt.Println(string(data))
return nil
}
// Return concrete type
func newConfig() *Config {
return &Config{
Host: "localhost",
Port: 8080,
}
}
type Config struct {
Host string
Port int
}Nil Interfaces
Understanding nil interfaces:
func checkNil() {
var i interface{}
fmt.Println(i == nil) // true
var p *Person
i = p
fmt.Println(i == nil) // false! (type is set, value is nil)
// Proper nil check
v, ok := i.(*Person)
fmt.Println(v == nil, ok) // true, true
}Interface Satisfaction
Checking interface implementation:
// Compile-time check
var _ io.Writer = (*MyWriter)(nil)
var _ io.Reader = (*MyReader)(nil)
type MyWriter struct{}
func (w *MyWriter) Write(p []byte) (n int, err error) {
return len(p), nil
}
// If MyWriter doesn't implement Writer, compilation failsDuck Typing
Implicit interface satisfaction:
// No explicit "implements" keyword needed
type Duck interface {
Quack()
Walk()
}
type RealDuck struct{}
func (d RealDuck) Quack() {
fmt.Println("Quack!")
}
func (d RealDuck) Walk() {
fmt.Println("Waddle waddle")
}
type Robot struct{}
func (r Robot) Quack() {
fmt.Println("Beep boop quack")
}
func (r Robot) Walk() {
fmt.Println("*mechanical walking sounds*")
}
func makeDuckDoThings(d Duck) {
d.Quack()
d.Walk()
}
func main() {
makeDuckDoThings(RealDuck{})
makeDuckDoThings(Robot{})
}Polymorphism
Using interfaces for polymorphism:
type Shape interface {
Area() float64
Perimeter() float64
}
type Rectangle struct {
Width, Height float64
}
func (r Rectangle) Area() float64 {
return r.Width * r.Height
}
func (r Rectangle) Perimeter() float64 {
return 2 * (r.Width + r.Height)
}
type Circle struct {
Radius float64
}
func (c Circle) Area() float64 {
return 3.14159 * c.Radius * c.Radius
}
func (c Circle) Perimeter() float64 {
return 2 * 3.14159 * c.Radius
}
func printShapeInfo(s Shape) {
fmt.Printf("Area: %.2f, Perimeter: %.2f\n",
s.Area(), s.Perimeter())
}
func main() {
shapes := []Shape{
Rectangle{Width: 10, Height: 5},
Circle{Radius: 7},
}
for _, shape := range shapes {
printShapeInfo(shape)
}
}Dependency Injection
Using interfaces for testability:
// Define interface for dependency
type UserRepository interface {
GetUser(id int) (*User, error)
SaveUser(user *User) error
}
// Production implementation
type PostgresUserRepo struct {
db *sql.DB
}
func (r *PostgresUserRepo) GetUser(id int) (*User, error) {
// Database query
return &User{}, nil
}
func (r *PostgresUserRepo) SaveUser(user *User) error {
// Database insert/update
return nil
}
// Test implementation
type MockUserRepo struct {
users map[int]*User
}
func (m *MockUserRepo) GetUser(id int) (*User, error) {
user, exists := m.users[id]
if !exists {
return nil, errors.New("user not found")
}
return user, nil
}
func (m *MockUserRepo) SaveUser(user *User) error {
m.users[user.ID] = user
return nil
}
// Service depends on interface, not concrete type
type UserService struct {
repo UserRepository
}
func (s *UserService) GetUserName(id int) (string, error) {
user, err := s.repo.GetUser(id)
if err != nil {
return "", err
}
return user.Name, nil
}
type User struct {
ID int
Name string
}Builder Pattern with Interfaces
Fluent interface pattern:
type QueryBuilder interface {
Select(fields ...string) QueryBuilder
From(table string) QueryBuilder
Where(condition string) QueryBuilder
Build() string
}
type sqlQueryBuilder struct {
selectFields []string
fromTable string
whereClause string
}
func NewQueryBuilder() QueryBuilder {
return &sqlQueryBuilder{}
}
func (b *sqlQueryBuilder) Select(fields ...string) QueryBuilder {
b.selectFields = fields
return b
}
func (b *sqlQueryBuilder) From(table string) QueryBuilder {
b.fromTable = table
return b
}
func (b *sqlQueryBuilder) Where(condition string) QueryBuilder {
b.whereClause = condition
return b
}
func (b *sqlQueryBuilder) Build() string {
query := "SELECT " + strings.Join(b.selectFields, ", ")
query += " FROM " + b.fromTable
if b.whereClause != "" {
query += " WHERE " + b.whereClause
}
return query
}
func main() {
query := NewQueryBuilder().
Select("id", "name", "email").
From("users").
Where("age > 18").
Build()
fmt.Println(query)
}Strategy Pattern
Implementing strategy pattern:
type PaymentStrategy interface {
Pay(amount float64) error
}
type CreditCardPayment struct {
CardNumber string
}
func (c *CreditCardPayment) Pay(amount float64) error {
fmt.Printf("Paying %.2f with credit card %s\n",
amount, c.CardNumber)
return nil
}
type PayPalPayment struct {
Email string
}
func (p *PayPalPayment) Pay(amount float64) error {
fmt.Printf("Paying %.2f via PayPal to %s\n",
amount, p.Email)
return nil
}
type ShoppingCart struct {
paymentMethod PaymentStrategy
}
func (cart *ShoppingCart) SetPaymentMethod(pm PaymentStrategy) {
cart.paymentMethod = pm
}
func (cart *ShoppingCart) Checkout(amount float64) error {
return cart.paymentMethod.Pay(amount)
}
func main() {
cart := &ShoppingCart{}
cart.SetPaymentMethod(&CreditCardPayment{CardNumber: "1234-5678"})
cart.Checkout(100.00)
cart.SetPaymentMethod(&PayPalPayment{Email: "user@example.com"})
cart.Checkout(50.00)
}Adapter Pattern
Adapting interfaces:
// Third-party logger
type ThirdPartyLogger struct{}
func (t *ThirdPartyLogger) LogMessage(msg string, level int) {
fmt.Printf("[Level %d] %s\n", level, msg)
}
// Our application interface
type Logger interface {
Info(msg string)
Error(msg string)
}
// Adapter
type LoggerAdapter struct {
thirdParty *ThirdPartyLogger
}
func (a *LoggerAdapter) Info(msg string) {
a.thirdParty.LogMessage(msg, 0)
}
func (a *LoggerAdapter) Error(msg string) {
a.thirdParty.LogMessage(msg, 2)
}
func useLogger(logger Logger) {
logger.Info("Application started")
logger.Error("An error occurred")
}
func main() {
adapter := &LoggerAdapter{
thirdParty: &ThirdPartyLogger{},
}
useLogger(adapter)
}When to Use This Skill
Use go-interfaces when you need to:
- Define contracts for behavior without implementation
- Enable polymorphism and code reuse
- Create testable code with dependency injection
- Implement design patterns (strategy, adapter, etc.)
- Build plugin systems or extensible architectures
- Decouple components in large applications
- Mock dependencies in tests
- Follow SOLID principles in Go
- Create flexible, maintainable APIs
- Support multiple implementations of same behavior
Best Practices
- Keep interfaces small and focused (1-3 methods)
- Accept interfaces, return concrete types
- Define interfaces where they're used, not implemented
- Use interface composition for complex interfaces
- Don't use empty interface unless absolutely necessary
- Verify interface implementation at compile time
- Document expected behavior in interface comments
- Prefer many small interfaces over large ones
- Use standard library interfaces when applicable
- Name interfaces with -er suffix (Reader, Writer, etc.)
Common Pitfalls
- Making interfaces too large or generic
- Defining unused interfaces "just in case"
- Returning interfaces instead of concrete types
- Not checking for nil interface values properly
- Over-abstracting simple code
- Forgetting that interfaces are satisfied implicitly
- Using empty interface excessively
- Not documenting interface contracts
- Creating interfaces for single implementation
- Confusing nil value vs nil interface