Interface definition 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?

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 Woof

C speak

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. Calling s.Speak() returns "Woof!" exactly as defined.

❓ Predict Output

intermediate

Interface variable nil check

What will be printed by this Go program regarding interface nil checks?

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 Runtime panic

B r is nil

C Compilation error

D r is not nil

💡 Hint

An interface holding a nil pointer is not itself nil.

The interface r holds a *File pointer which is nil, but the interface itself is not nil, so the else branch runs.

🔧 Debug

advanced

Why does this interface assignment fail?

This code fails to compile. What is the cause of the error?

Go

package main
import "fmt"
type Writer interface {
    Write(data string)
}
type Printer struct {}
func (p Printer) Print(data string) {
    fmt.Println(data)
}
func main() {
    var w Writer = Printer{}
    w.Write("Hello")
}

A Printer does not implement Writer because method has wrong parameter type

B Printer does not implement Writer because method name is Print, not Write

C Interface Writer cannot be assigned to struct Printer

D Missing import for fmt package

💡 Hint

Check method names and signatures carefully.

The Writer interface requires a Write method, but Printer only has Print method, so it does not implement Writer.

🧠 Conceptual

advanced

Interface embedding behavior

Given these interface definitions, which statement is true about interface C?

Go

type A interface {
    Foo()
}
type B interface {
    Bar()
}
type C interface {
    A
    B
}

A C requires only Foo() method to be implemented

B C requires only Bar() method to be implemented

C C requires both Foo() and Bar() methods to be implemented

D C does not require any methods to be implemented

💡 Hint

Interface embedding combines all methods from embedded interfaces.

Interface C embeds A and B, so it requires all methods from both interfaces: Foo() and Bar().

❓ Predict Output

expert

Interface method call with pointer vs value receiver

What is the output of this Go program?

Go

package main
import "fmt"
type Speaker interface {
    Speak() string
}
type Cat struct {}
func (c *Cat) Speak() string {
    return "Meow"
}
func main() {
    var s Speaker
    c := Cat{}
    s = c
    fmt.Println(s.Speak())
}

A Compilation error: Cat does not implement Speaker

B Runtime panic

C Meow

D Empty string

💡 Hint

Check if Cat value implements Speaker when Speak has pointer receiver.

Speak has pointer receiver (*Cat), so Cat value does not implement Speaker interface. Assignment fails to compile.