Empty interface in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of assigning different types to empty interface

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var x interface{}
    x = 42
    fmt.Printf("%T %v\n", x, x)
    x = "hello"
    fmt.Printf("%T %v\n", x, x)
}

A

interface {} 42
interface {} hello

B Compilation error

C

42
hello

D

int 42
string hello

💡 Hint

Empty interface can hold any type, and %T prints the dynamic type.

The empty interface can hold any value. When printing with %T, it shows the actual type stored, so first int then string.

🧠 Conceptual

intermediate

Understanding empty interface usage

Which statement about the empty interface in Go is TRUE?

A It can hold values of any type.

B It can only hold values of type interface{}.

C It requires explicit type conversion to assign values.

D It cannot be used as a function parameter.

💡 Hint

Think about what 'empty interface' means in Go.

The empty interface has zero methods, so all types implement it and can be assigned to it.

❓ Predict Output

advanced

Output of type assertion with empty interface

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var x interface{} = 3.14
    v, ok := x.(int)
    fmt.Println(v, ok)
}

A 3 true

B 0 false

C 3.14 true

D Compilation error

💡 Hint

Type assertion fails if the dynamic type does not match.

x holds a float64, but we assert int, so v is zero value 0 and ok is false.

🔧 Debug

advanced

Identify the runtime error with empty interface

What runtime error will this Go program produce?

Go

package main
import "fmt"
func main() {
    var x interface{} = "golang"
    v := x.(int)
    fmt.Println(v)
}

A panic: interface conversion: interface {} is string, not int

B Compilation error: cannot convert string to int

C 0

D golang

💡 Hint

Direct type assertion without checking can cause panic if types mismatch.

The program panics because x holds a string but is asserted as int without checking.

🚀 Application

expert

Number of elements in slice of empty interfaces

What is the length of the slice 'data' after this code runs?

Go

package main
func main() {
    data := []interface{}{1, "two", 3.0, true}
    // What is len(data)?
}

A 3

B 1

C 4

D 0

💡 Hint

Count the number of elements inside the slice literal.

The slice contains four elements of different types, all stored as interface{} values.