Interface declaration

Learn to declare an interface with type ... interface.

Defining interfaces

An interface is a list of methods describing what a type can do. What matters is not which type it is but which behavior it offers:

type Sekil interface {
    Alan() float64
}

func toplamAlan(sekiller []Sekil) float64 {
    t := 0.0
    for _, s := range sekiller {
        t += s.Alan()
    }
    return t
}

toplamAlan works with circles, squares or any shape added later, as long as it has an Alan() float64 method.

Small interfaces

In Go interfaces are usually very small; most have a single method: io.Reader (Read), io.Writer (Write), fmt.Stringer (String), error (Error). Small interfaces are easy to satisfy and can be combined: type ReadWriter interface { Reader; Writer }. The proverb: "The bigger the interface, the weaker the abstraction."

Exercise Shapes

Write the Sekil interface (Alan() float64) and the types Kare (Kenar float64) and Dikdortgen (En, Boy float64). Print the total area for []Sekil{Kare{2}, Dikdortgen{3, 4}}.

Show example solution

package main

import "fmt"

type Sekil interface {
    Alan() float64
}

type Kare struct{ Kenar float64 }
type Dikdortgen struct{ En, Boy float64 }

func (k Kare) Alan() float64       { return k.Kenar * k.Kenar }
func (d Dikdortgen) Alan() float64 { return d.En * d.Boy }

func main() {
    t := 0.0
    for _, s := range []Sekil{Kare{2}, Dikdortgen{3, 4}} {
        t += s.Alan()
    }
    fmt.Println(t)
}

Test yourself

What is the zero value of an interface variable?
- nil
- An empty struct
- 0
- Undefined
Show answer

nil — An interface with no value assigned is nil; calling a method on a nil interface panics.

Interface type

type Sesli ____ {
    Ses() string
}

Show answer

type Sesli interface {
    Ses() string
}

Interface and its method

io.Reader

Read
io.Writer

Write
fmt.Stringer

String
error

Error

Next lesson: Implicit satisfaction

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