What Is Interface Data Type in Golang?


Angela Bailey

What Is Interface Data Type in Golang?

In the Go programming language, an interface is a fundamental data type that allows you to define a contract for behavior. It specifies a set of methods that a concrete type must implement to satisfy the interface. Interfaces enable polymorphism, which means multiple types can be used interchangeably as long as they implement the required methods.

Defining an Interface

To define an interface in Go, you use the interface keyword followed by its name and a set of method signatures enclosed in curly braces:

type MyInterface interface {
    Method2() int
    // ..

The example above shows an interface called MyInterface with two method signatures: Method1() and Method2(). It’s important to note that interfaces only declare method signatures, not their implementations.

Implementing an Interface

A Go type can implement an interface by implementing all the methods declared in the interface. Let’s say we have a struct called MyStruct:

type MyStruct struct{}

func (s MyStruct) Method1() {
    // Implementation for Method1

func (s MyStruct) Method2() int {
    // Implementation for Method2
    return 42

In this example, MyStruct implements both Method1() and Method2(). By doing so, it satisfies the contract defined by the MyInterface.

Type Assertion and Type Switch

An interface value can hold any value as long as it implements the interface’s methods. However, when you need to access the underlying concrete type, you can use type assertion or type switch.

Type assertion allows you to extract the concrete value from an interface. Here’s an example:

var i MyInterface = MyStruct{}
s := i.(MyStruct)

In this case, i is an interface value of type MyInterface, which holds a MyStruct value. By using i.(MyStruct), we assert that the underlying concrete type of i is MyStruct, and assign it to the variable s.

Type switch provides a way to conditionally check the underlying concrete type of an interface. Here’s how you can use a type switch:

func DoSomething(i MyInterface) {
    switch v := i.(type) {
    case MyStruct:
        // Handle MyStruct
    case AnotherType:
        // Handle AnotherType
        // Handle other types

In this example, depending on the underlying type of the interface value passed to DoSomething(), you can execute different logic using a type switch.

Empty Interface

In Go, there is a special case of an interface called an empty interface ({}). It doesn’t specify any method signature and can hold values of any type.

// Accepts any value
func DoSomething(i interface{}) {
    // .

This can be useful in scenarios where you need to work with values of different types without knowing their exact type in advance.


Interfaces in Go provide a powerful way to define contracts for behavior and enable polymorphism. They allow you to write flexible and reusable code by decoupling the implementation details from the interface itself. By utilizing interfaces effectively, you can achieve code that is not only informative but also visually engaging.

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