Interface satisfaction in Go - Mini Project: Build & Apply

Interface satisfaction

📖 Scenario: You are building a simple program to check if different types satisfy a common interface. This is useful when you want to ensure that different objects can be used interchangeably based on shared behavior.

🎯 Goal: Create a Go program that defines an interface and two structs. Then check if these structs satisfy the interface by assigning them to interface variables.

📋 What You'll Learn

Define an interface called Speaker with a method Speak() string

Create a struct called Dog with a method Speak() string that returns "Woof!"

Create a struct called Cat with a method Speak() string that returns "Meow!"

Assign instances of Dog and Cat to variables of type Speaker

Print the result of calling Speak() on both interface variables

💡 Why This Matters

🌍 Real World

Interfaces let you write flexible programs where different types can be used interchangeably if they share behavior. For example, different animals can be treated as speakers without knowing their exact type.

💼 Career

Understanding interface satisfaction is key for Go developers to design clean, modular, and testable code. It is widely used in real-world Go projects and libraries.

1

Define the Speaker interface and Dog struct

Write Go code to define an interface called Speaker with a method Speak() string. Then define a struct called Dog.

Go

// Define the Speaker interface
// Define the Dog struct
// Your code here

Hint

Interfaces in Go are defined with the type keyword followed by the interface name and the interface keyword. Methods inside interfaces have no body.

2

Add the Speak() method to Dog and define Cat struct

Add a method Speak() string to the Dog struct that returns "Woof!". Then define a struct called Cat.

Go

// Add Speak() method to Dog that returns "Woof!"
// Define the Cat struct
// Your code here

Hint

Methods are defined with func (receiver Type) MethodName() ReturnType. The receiver d Dog means the method belongs to Dog.

3

Add the Speak() method to Cat

Add a method Speak() string to the Cat struct that returns "Meow!".

Go

// Add Speak() method to Cat that returns "Meow!"
// Your code here

Hint

Use the same method pattern as for Dog, but change the receiver to c Cat and the return string to "Meow!".

4

Assign Dog and Cat to Speaker and print results

In the main function, create variables dogSpeaker and catSpeaker of type Speaker. Assign instances of Dog and Cat to them respectively. Then print the result of calling Speak() on both variables.

Go

func main() {
    // Create dogSpeaker and catSpeaker variables of type Speaker
    // Assign Dog{} to dogSpeaker and Cat{} to catSpeaker
    // Print dogSpeaker.Speak() and catSpeaker.Speak()
    // Your code here
}

Hint

Use var variableName Speaker = Type{} to assign structs to interface variables. Use println() to print the results.

Practice

1. Which statement best describes how a type satisfies an interface in Go?

easy

A. A type satisfies an interface by implementing all the methods declared in the interface.

B. A type satisfies an interface only if it explicitly declares it implements the interface.

C. A type satisfies an interface by embedding the interface in its struct definition.

D. A type satisfies an interface by having the same name as the interface.

Solution

1. Step 1: Understand interface satisfaction in Go

   In Go, a type satisfies an interface if it implements all the methods declared in that interface.

2. Step 2: Check explicit declaration requirement

   Go does not require explicit declaration; satisfaction is implicit by method implementation.

3. Final Answer:

   A type satisfies an interface by implementing all the methods declared in the interface. -> Option A

4. Quick Check:

   Interface satisfaction = implicit method implementation [OK]

Hint: Remember: Implement all methods, no explicit declaration needed [OK]

Common Mistakes:

• Thinking explicit declaration is required
• Confusing embedding with satisfaction
• Matching names instead of methods

2. Which of the following Go code snippets correctly shows a type satisfying an interface Speaker with method Speak()?

easy

A. type Speaker interface { Speak() string }
type Dog struct {}
func (d Dog) speak() string { return "Woof" }

B. type Speaker interface { Speak() string }
type Dog struct {}
func (d Dog) Speak() string { return "Woof" }

C. type Speaker interface { Speak() string }
type Dog struct {}
func Speak(d Dog) string { return "Woof" }

D. type Speaker interface { Speak() string }
type Dog struct {}
func (d *Dog) Speak() int { return 1 }

Solution

1. Step 1: Check method signature and receiver

   type Speaker interface { Speak() string } type Dog struct {} func (d Dog) Speak() string { return "Woof" } defines Speak() with correct receiver (d Dog) and return type string.

2. Step 2: Verify method name and return type match interface

   type Speaker interface { Speak() string } type Dog struct {} func (d Dog) speak() string { return "Woof" } has lowercase speak(), so it does not satisfy interface. type Speaker interface { Speak() string } type Dog struct {} func Speak(d Dog) string { return "Woof" } defines a function, not a method. type Speaker interface { Speak() string } type Dog struct {} func (d *Dog) Speak() int { return 1 } has wrong return type int.

3. Final Answer:

   type Speaker interface { Speak() string } type Dog struct {} func (d Dog) Speak() string { return "Woof" } -> Option B

4. Quick Check:

   Method name and signature must match exactly [OK]

Hint: Method name and signature must match interface exactly [OK]

Common Mistakes:

• Using lowercase method names
• Defining functions instead of methods
• Mismatching return types

3. What will be the output of the following Go program?

package main
import "fmt"

type Reader interface {
  Read() string
}

type File struct {}
func (f File) Read() string {
  return "file content"
}

func printContent(r Reader) {
  fmt.Println(r.Read())
}

func main() {
  f := File{}
  printContent(f)
}

medium

A. No output

B. Compilation error: File does not satisfy Reader

C. file content

D. Runtime panic

Solution

1. Step 1: Check if File satisfies Reader interface

   File has method Read() string, so it satisfies Reader.

2. Step 2: Trace function call and output

   In main, f is passed to printContent, which calls Read() and prints "file content".

3. Final Answer:

   file content -> Option C

4. Quick Check:

   Interface satisfied, method called, prints string [OK]

Hint: Check method presence to confirm interface satisfaction [OK]

Common Mistakes:

• Assuming explicit declaration needed
• Confusing method receiver types
• Expecting runtime errors without cause

4. Identify the error in the following Go code that prevents Car from satisfying the Vehicle interface:

package main
import "fmt"

type Vehicle interface {
  Drive()
}

type Car struct {}
func (c *Car) Drive() {
  fmt.Println("Driving")
}

func main() {
  var v Vehicle
  c := Car{}
  v = c
  v.Drive()
}

medium

A. Car does not satisfy Vehicle because Drive has pointer receiver but v is assigned a value.

B. Drive method has wrong return type.

C. Vehicle interface requires Drive with parameters.

D. No error, code runs fine.

Solution

1. Step 1: Check method receiver type

   Drive is defined with pointer receiver (*Car), so only *Car satisfies Vehicle, not Car value.

2. Step 2: Check assignment in main

   Variable v is assigned c which is a value, not pointer, causing mismatch.

3. Final Answer:

   Car does not satisfy Vehicle because Drive has pointer receiver but v is assigned a value. -> Option A

4. Quick Check:

   Pointer receiver methods require pointer type for interface satisfaction [OK]

Hint: Pointer receiver methods need pointer type to satisfy interface [OK]

Common Mistakes:

• Assigning value instead of pointer to interface
• Ignoring receiver type in method
• Assuming method signature mismatch

5. Given the interface and types below, which assignment will cause a compile-time error due to interface satisfaction rules?

type Writer interface {
  Write(data string) error
}

type Logger struct {}
func (l Logger) Write(data string) error {
  return nil
}

type FileWriter struct {}
func (f *FileWriter) Write(data string) error {
  return nil
}

var w Writer

// Which assignment is invalid?
// A) w = Logger{}
// B) w = &Logger{}
// C) w = FileWriter{}
// D) w = &FileWriter{}

hard

A. w = &FileWriter{}

B. w = &Logger{}

C. w = Logger{}

D. w = FileWriter{}

Solution

1. Step 1: Analyze Logger methods

   Logger has Write with value receiver, so both Logger and *Logger satisfy Writer.

2. Step 2: Analyze FileWriter methods

   FileWriter has Write with pointer receiver, so only *FileWriter satisfies Writer, not FileWriter value.

3. Step 3: Check assignments

   Assignment C tries to assign FileWriter{} value to Writer, which is invalid.

4. Final Answer:

   w = FileWriter{} causes compile-time error due to missing method on value receiver. -> Option D

5. Quick Check:

   Pointer receiver methods require pointer type for interface [OK]

Hint: Pointer receiver methods need pointer type for interface satisfaction [OK]

Common Mistakes:

• Assigning value type when method has pointer receiver
• Assuming pointer and value types are interchangeable
• Ignoring method receiver type