Implementing interfaces 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 and a struct implementing it?

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 speak

B Woof

C Woof!

D Compilation error

💡 Hint

Check the Speak method's return value and how it's printed.

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

❓ Predict Output

intermediate

Interface value nil check

What will be printed by this Go program that checks if an interface value is nil?

Go

package main
import "fmt"

type Reader interface {
    Read() string
}

type File struct {}

func (f *File) Read() string {
    return "data"
}

func main() {
    var r Reader
    var f *File = nil
    r = f
    if r == nil {
        fmt.Println("r is nil")
    } else {
        fmt.Println("r is not nil")
    }
}

A r is nil

B r is not nil

C Compilation error

D Runtime panic

💡 Hint

An interface value is nil only if both type and value are nil.

The interface r holds a type *File and a nil value. Since the type is not nil, r != nil, so it prints "r is not nil".

🔧 Debug

advanced

Why does this interface implementation fail?

This Go code tries to assign a struct to an interface variable but fails to compile. Why?

Go

package main
import "fmt"

type Walker interface {
    Walk()
}

type Person struct {}

func (p Person) walk() {
    fmt.Println("Walking")
}

func main() {
    var w Walker = Person{}
    w.Walk()
}

A Compilation error: Person does not implement Walker (missing Walk method)

B Compilation error: cannot assign Person to Walker because of pointer receiver

C Runtime panic: method walk not found

D Program runs and prints 'Walking'

💡 Hint

Check method names and case sensitivity in Go.

The method is named 'walk' with lowercase 'w', but interface requires 'Walk' with uppercase 'W'. Go is case-sensitive, so Person does not implement Walker.

📝 Syntax

advanced

Which option correctly implements this interface?

Given this interface, which option correctly implements it without syntax errors?

Go

type Printer interface {
    Print(msg string) string
}

A

type MyPrinter struct {}
func (p MyPrinter) Print() string {
    return "done"
}

B

type MyPrinter struct {}
func (p *MyPrinter) Print(msg string) {
    fmt.Println(msg)
}

C

type MyPrinter struct {}
func Print(msg string) string {
    return msg
}

D

type MyPrinter struct {}
func (p MyPrinter) Print(msg string) string {
    return msg
}

💡 Hint

Check method signature matches interface exactly.

Option D matches the interface method signature exactly: method name, parameter, and return type. Others differ in signature or receiver.

🚀 Application

expert

How many items in the slice after interface assignment?

Consider this Go code. How many elements does the slice 'items' contain after execution?

Go

package main
import "fmt"

type Item interface {
    ID() int
}

type Product struct {
    id int
}

func (p Product) ID() int {
    return p.id
}

func main() {
    var items []Item
    p1 := Product{id: 1}
    p2 := Product{id: 2}
    items = append(items, p1)
    items = append(items, &p2)
    fmt.Println(len(items))
}

A 2

B 1

C 0

D Compilation error

💡 Hint

Both Product value and pointer implement Item interface.

Both p1 (value) and &p2 (pointer) implement the Item interface, so both can be appended. The slice length is 2.