Interface use cases in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of interface method call

What is the output of this Go program that uses an interface to call a method?

Go

package main
import "fmt"
type Speaker interface {
    Speak() string
}
type Dog struct {}
func (d Dog) Speak() string {
    return "Woof!"
}
func main() {
    var s Speaker = Dog{}
    fmt.Println(s.Speak())
}

A Woof!

B Compilation error

C speak

D Woof

💡 Hint

Check the Speak method implementation and what it returns.

The Dog type implements the Speaker interface by defining Speak() string. The method returns "Woof!" exactly, so printing s.Speak() outputs "Woof!".

🧠 Conceptual

intermediate

Why use interfaces in Go?

Which of the following is the main reason to use interfaces in Go?

A To enable polymorphism and write flexible code that works with different types

B To improve program speed by avoiding function calls

C To declare variables with fixed memory size

D To enforce inheritance like in other languages

💡 Hint

Think about how interfaces allow different types to be used interchangeably.

Interfaces in Go allow different types to implement the same set of methods, enabling polymorphism and flexible code that can work with any type implementing the interface.

🔧 Debug

advanced

Identify the runtime error with interface nil check

What error will this Go program produce when run?

Go

package main
import "fmt"
type Reader interface {
    Read() string
}
type File struct {}
func (f *File) Read() string {
    return "file content"
}
func main() {
    var r Reader
    var f *File = nil
    r = f
    if r == nil {
        fmt.Println("Reader is nil")
    } else {
        fmt.Println(r.Read())
    }
}

A Compilation error

B file content

C panic: runtime error: invalid memory address or nil pointer dereference

D Reader is nil

💡 Hint

Check how interface values with nil underlying pointers behave in comparisons.

The interface r holds a *File pointer which is nil, but the interface itself is not nil. So r == nil is false, and calling r.Read() works because the method has a receiver pointer and does not dereference the pointer inside the method. It returns "file content".

📝 Syntax

advanced

Which code snippet correctly implements an interface?

Given interface Writer with method Write(string) error, which option correctly implements it?

Go

type Writer interface {
    Write(s string) error
}

A

type File struct {}
func Write(f File, s string) error {
    return nil
}

B

type File struct {}
func (f *File) Write() error {
    return nil
}

C

type File struct {}
func (f File) Write(s int) error {
    return nil
}

D

type File struct {}
func (f File) Write(s string) error {
    return nil
}

💡 Hint

Check method signature matches exactly the interface method.

Option D defines Write with receiver File and parameter string, returning error, matching the interface exactly. A misses the string parameter. C has wrong parameter type. D is a function, not a method.

🚀 Application

expert

How many types implement this interface?

Given the interface and types below, how many distinct types implement the interface Printable?

Go

package main
import "fmt"
type Printable interface {
    Print()
}
type A struct {}
func (a A) Print() { fmt.Println("A") }
type B struct {}
func (b *B) Print() { fmt.Println("B") }
type C struct {}
func (c C) Print() { fmt.Println("C") }
func main() {}

A 2

B 1

C 3

D 0

💡 Hint

Remember that in Go, types with pointer receiver methods also implement the interface via automatic address taking.

A has Print() with value receiver, so A implements Printable. B has Print() with pointer receiver, so B implements Printable (Go automatically uses &b.Print()). C has Print() with value receiver, so C implements Printable. So three types implement Printable: A, B, and C.