Interfaces and Polymorphism

Interfaces are one of Go's most powerful features. Unlike Java or C#, Go interfaces are satisfied implicitly — a type implements an interface simply by having the right methods. There is no implements keyword. This design enables flexible, decoupled code.

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Defining an Interface

An interface specifies a set of method signatures:

type Shape interface {
    Area() float64
    Perimeter() float64
}

Any type that has Area() float64 and Perimeter() float64 methods automatically satisfies the Shape interface.

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Implementing an Interface

type Circle struct {
    Radius float64
}

func (c Circle) Area() float64 {
    return math.Pi * c.Radius * c.Radius
}

func (c Circle) Perimeter() float64 {
    return 2 * math.Pi * c.Radius
}

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)
}

Both Circle and Rectangle implement Shape implicitly:

func printShape(s Shape) {
    fmt.Printf("Area: %.2f, Perimeter: %.2f\n", s.Area(), s.Perimeter())
}

printShape(Circle{Radius: 5})
printShape(Rectangle{Width: 4, Height: 6})

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Implicit vs Explicit Interface Implementation

Feature	Go (Implicit)	Java / C# (Explicit)
Keyword	None	implements / : IInterface
Coupling	Type does not know about the interface	Type must explicitly declare it
Third-party types	Can satisfy your interfaces	Cannot — must wrap or modify
Compile-time check	At point of use	At point of declaration

This implicit satisfaction means you can define an interface that existing types already satisfy — without modifying them.

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Common Standard Library Interfaces

Interface	Methods	Used For
fmt.Stringer	String() string	Custom string representation
error	Error() string	Error values
io.Reader	Read(p []byte) (n int, err error)	Reading data
io.Writer	Write(p []byte) (n int, err error)	Writing data
io.Closer	Close() error	Closing resources
io.ReadWriter	Read + Write	Read and write
sort.Interface	Len, Less, Swap	Custom sorting
http.Handler	ServeHTTP(w, r)	HTTP request handling
json.Marshaler	MarshalJSON() ([]byte, error)	Custom JSON encoding
json.Unmarshaler	UnmarshalJSON([]byte) error	Custom JSON decoding

Implementing fmt.Stringer

func (u User) String() string {
    return fmt.Sprintf("%s (%s)", u.Name, u.Email)
}

fmt.Println(u)   // Alice (alice@example.com)

Implementing io.Reader

type InfiniteZeros struct{}

func (z InfiniteZeros) Read(p []byte) (int, error) {
    for i := range p {
        p[i] = 0
    }
    return len(p), nil
}

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The Empty Interface

interface{}   // Go 1.17 and earlier
any           // Go 1.18+ (alias for interface{})

The empty interface has no methods, so every type satisfies it: