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