Mastering Interfaces in Go – Polymorphism with Real Examples

What is an Interface?

An interface defines a set of method signatures. Any type that implements those methods automatically satisfies the interface — no explicit declaration needed.

type Shape interface {
    Area() float64
}

type Circle struct {
    Radius float64
}

func (c Circle) Area() float64 {
    return 3.14 * c.Radius * c.Radius
}

Polymorphism with Interfaces

func printArea(s Shape) {
    fmt.Println("Area:", s.Area())
}

c := Circle{Radius: 5}
printArea(c) // works because Circle implements Shape

The Empty Interface

interface{} (or any in newer Go versions) can hold a value of any type, similar to Object in other languages, but should be used sparingly to keep type safety.