How do you declare an interface in Golang?

Declaring an interface in go language means creating a new named type that defines a collection of method signatures. In go-language we can declare an interface using single method interface, multiple method interface as well as embedded interface. In this article we are going to understand these methods and declare an interface in go language with the help of various examples.

Method 1: Single Method Interface

The first method involves a single interface, in this approach we describe an interface that all implementing types must fulfill.


  • Create the interface with a single method called CreateSound().

  • Now, create the struct cat, that represents a cat.

  • Implement the CreateSound() function on the cat struct. It will print "meow!" to the console.

  • Now declare a variable of type Animal in the main function.

  • On the animal variable, use the CreateSound() function.

  • It outputs "meow!" to the console.


This code defines an interface Animal and a struct Cat. The interface defines a single method named “Create Sound()” which prints the output and the cat struct implement the CreateSound() function.

package main

import "fmt"

type Animal interface {

type cat struct{}

func (d cat) CreateSound() {

func main() {
   var animal Animal
   animal = cat{}
   animal.CreateSound() // Output: Woof!



Method 2: Using Multiple Method Interface

This other method involves using multiple interfaces, in this method we are going to see go language interfaces that can include many methods.


  • Define the Shape interface using two methods Area() and Perimeter.

  • Now, Create a struct named Rectangle with type float64 length and width.

  • Implement the Area() function. Return the result of multiplying the length by the width.

  • Implement the Perimeter() function. Return the perimeter that is calculated using the formula = 2 * (length + width).

  • Assign a value of type Rectangle to the shape variable, with the length set to 8 and the width set to 5.

  • Print the result of the Area() function. Print the result of the Perimeter() function.


The example given below demonstrates the use of interface in go language to calculate the area and perimeter of a rectangle.

package main

import "fmt"

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

type Rectangle struct {
   length, width float64

func (r Rectangle) Area() float64 {
   return r.length * r.width

func (r Rectangle) Perimeter() float64 {
   return 2 * (r.length + r.width)

func main() {
   var shape Shape
   shape = Rectangle{length: 8, width: 5}



Method 3: Using Embedded Interfaces

This method involves embedding an interface inside another interface, allowing the new interface to adopt all the methods from the embedded interface.


  • Create an interface named Reader which has a Read() function that returns a []byte.

  • Create the interface named Writer and define the function Write(data []byte).

  • Define a struct file.

  • Implement the File struct's Read() function.

  • Implement the File struct's Write(data []byte) function.

  • In the main function, Declare a variable rw of type ReadWrite.

  • Assign the rw variable a value of type File, indicating that a File implements the ReadWrite interface.

  • Execute the Read() method on the rw variable and save the results.

  • Pass the data variable as a parameter to the Write(data) method on the rw variable.

  • Proram use the Read() function to read data from a file and the send() method to send data to the console.


The example given below explains the notation of interface composition, which involves mixing multiple interfaces together to create a new interface.

package main

import "fmt"

type Reader interface {
   Read() []byte

type Writer interface {
   Write(data []byte)

type ReadWrite interface {

type File struct {
   // implementation details

func (f File) Read() []byte {
   return []byte("Read data from file")

func (f File) Write(data []byte) {
   fmt.Println("Write data to file:", string(data))

func main() {
   var rw ReadWrite
   rw = File{}
   data := rw.Read()


Write data to file: Read data from file


In this article we have discussed how we can declare an interface using a single interface, multiple interface and embedded interface method. Interfaces in go language are very helpful tools to achieve polymorphism in the code and make the code flexible.

Updated on: 05-Jul-2023


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