go interfaces in golang

From a first view, interfaces in Go might seem confusing at first. But fear not! In this comprehensive guide, we’ll break down the concepts, show you why they’re important, and help you master Golang interfaces with ease. By the end, you’ll be well-equipped to write more flexible and efficient Go code.

This post is part of a series on Golang programming. Check out our other articles for more valuable insights:

Introduction to interfaces in Go

Before we dive into the world of interfaces, let’s briefly explain what an interface is. In simple terms, an interface is a collection of method signatures that a type can implement. It’s a way to define the behavior of an object without specifying its actual implementation.

In Go, interfaces are incredibly powerful, allowing for more flexible and modular code design. They enable you to write functions that accept a variety of types, making it easier to reuse and extend your code.

What is the difference between an interface and a struct in Go?

golang interfaces vs structs

In Go, a struct is a composite data type that groups together properties (fields) under a single name. Structs are used to define custom types representing real-world entities or data structures.

An interface, on the other hand, is a collection of method signatures that a type can implement. It defines a contract for behavior without specifying the actual implementation. Interfaces enable you to write more flexible and modular code by allowing functions to accept various types that implement the same methods.

Why interfaces are important in Go

golang importance of interfaces

Interfaces are essential in Go for several reasons:

  1. Polymorphism: Interfaces enable you to write functions that work with multiple types, making your code more flexible and reusable.
  2. Abstraction: By using interfaces, you can hide the implementation details of a type, exposing only the relevant methods and properties.
  3. Decoupling: Interfaces help to reduce the tight coupling between components, making it easier to change or replace parts of your system without affecting others.

Defining and implementing interfaces

To define an interface in Go, use the type keyword, followed by the interface name and the interface keyword. Inside the curly braces, you can list the method signatures that the interface requires.

Here’s a simple example:

type Animal interface {
    Speak() string

To implement an interface, a type must provide implementations for all the methods listed in the interface. In Go, this is done implicitly; you don’t need to explicitly specify that a type implements an interface.

type Dog struct{}

func (d Dog) Speak() string {
    return "Woof!"

type Cat struct{}

func (c Cat) Speak() string {
    return "Meow!"

In this example, both Dog and Cat implicitly implement the Animal interface because they both provide a Speak() method.

Common use cases for interfaces

Some common use cases for interfaces in Go include:

  • Writing functions that work with multiple types
  • Encapsulating functionality for reuse and modularity
  • Defining contracts for external packages or libraries
  • Creating mock objects for testing

Interface composition and type embedding

golang interface athlete

Go allows you to compose interfaces by embedding other interfaces within them. This feature is useful when you want to create a new interface that extends the functionality of existing ones.

type Walker interface {
    Walk() string

type Runner interface {
    Run() string

type Athlete interface {

In this example, the Athlete interface composes the Walker and Runner interfaces. Any type that implements both Walk() and Run() methods will implicitly satisfy the Athlete interface.

Type embedding is another powerful feature in Go that allows you to embed one struct type within another, effectively inheriting its methods and properties. This can be particularly useful when working with interfaces.

Consider the following example:

type Vehicle struct {
    Speed int

func (v Vehicle) Move() string {
    return fmt.Sprintf("Moving at %d km/h", v.Speed)

type Car struct {

type Bicycle struct {

func main() {
    car := Car{Vehicle{Speed: 120}}
    bicycle := Bicycle{Vehicle{Speed: 25}}

    fmt.Println("Car:", car.Move())
    fmt.Println("Bicycle:", bicycle.Move())

Here, both Car and Bicycle embed the Vehicle type, inheriting its Speed property and Move() method. This approach helps reduce code duplication and promotes reusability.

In the main() function, we first create instances of Car and Bicycle, initializing their Speed properties with appropriate values. We then use the Move() method on each instance to display their respective speeds.

When you run this program, you should see output like:

Car: Moving at 120 km/h
Bicycle: Moving at 25 km/h

Best practices for working with interfaces in Go

  1. Keep interfaces small and focused: Design your interfaces to be small and focused on a specific set of functionality. This makes them easier to understand, implement, and maintain.
  2. Use the interface segregation principle: Break down large interfaces into smaller, more focused ones. This reduces the burden on types that need to implement them and promotes better code organization.
  3. Don’t export interfaces unnecessarily: Only export interfaces when it’s necessary for other packages to use them. Keep interfaces unexported whenever possible to minimize coupling between packages.
  4. Rely on interface composition: Compose interfaces to extend their functionality rather than creating large monolithic interfaces. This promotes code reuse and modularity.

Frequently asked questions

In Go, types implicitly implement interfaces by providing methods that match the method signatures defined in the interface. You don’t need to explicitly specify that a type implements an interface.

To check if a type implements an interface at runtime, you can use the type assertion or type switch constructs. Here’s an example using type assertion:

var myAnimal Animal = Dog{}

if _, ok := myAnimal.(Dog); ok {
    fmt.Println("The myAnimal variable is of type Dog")
} else {
    fmt.Println("The myAnimal variable is not of type Dog")

Yes, a type can implement multiple interfaces in Go. As long as the type provides implementations for all the methods required by the interfaces, it will implicitly implement them. For example, if you have two interfaces Walker and Runner, a type can implement both by providing Walk() and Run() methods.

No, interfaces in Go cannot have fields or properties. They can only define method signatures. If you need to associate data with an interface, consider defining methods that retrieve or set the data in the implementing types.

The empty interface (interface{}) is a special case in Go that represents any type since it has no method requirements. It can be used as a function parameter or return type when you want to accept or return values of any type.

However, using the empty interface should be done with caution, as it can lead to less type-safe code and make it harder to understand the expected behavior. Prefer using more specific interfaces or concrete types whenever possible to maintain type safety and readability.


By now, you should have a solid understanding of interfaces in Go and their importance in writing flexible, reusable, and efficient code. With this newfound knowledge, you can effectively leverage the power of interfaces to improve your Go programs.

Remember to keep your interfaces small and focused, practice interface composition, and minimize unnecessary exports. As you continue exploring Golang, be sure to check out our other articles in the series for even more valuable insights.

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