Interface satisfaction in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean for a type to satisfy an interface in Go?

A type satisfies an interface in Go if it implements all the methods declared in that interface. This means the type has the same method signatures as the interface requires.

beginner

How does Go check if a type satisfies an interface?

Go uses implicit satisfaction, meaning a type automatically satisfies an interface if it has all the required methods. No explicit declaration is needed.

intermediate

Can a pointer receiver method satisfy an interface for a value of the type in Go?

No. A type T with pointer receiver methods (*T) does not satisfy the interface; only *T does. A type with value receiver methods satisfies the interface for both T and *T.

beginner

Why is interface satisfaction useful in Go?

It allows flexible and reusable code by letting different types be used interchangeably if they satisfy the same interface, enabling polymorphism without inheritance.

beginner

What happens if a type does not implement all methods of an interface it is assigned to?

The Go compiler will produce an error because the type does not satisfy the interface, so it cannot be assigned to a variable of that interface type.

In Go, how do you make a type satisfy an interface?

A Implement all methods declared in the interface

B Explicitly declare it with a keyword

C Embed the interface in the type

D Use a special annotation

Go uses implicit satisfaction. A type satisfies an interface by implementing all its methods.

For a value of the type to satisfy an interface, which method receiver satisfies it?

A Only value receiver methods

B Only pointer receiver methods

C Both pointer and value receiver methods

D Neither pointer nor value receiver methods

Only value receiver methods allow the value type T to satisfy the interface. Pointer receiver methods allow only *T to satisfy it.

What is the benefit of interface satisfaction in Go?

A Prevents type conversions

B Requires less code to declare types

C Enforces strict type hierarchies

D Allows polymorphism without explicit inheritance

Interface satisfaction enables polymorphism by allowing different types to be used interchangeably if they implement the same interface.

What happens if a type does not implement all interface methods it is assigned to?

A Silent failure

B Runtime panic

C Compilation error

D Automatic method generation

The Go compiler checks interface satisfaction at compile time and will produce an error if methods are missing.

Does Go require explicit declaration that a type implements an interface?

A Yes, by embedding the interface

B No, it is implicit

C Yes, with the 'implements' keyword

D Yes, by using a special comment

Go uses implicit interface satisfaction, so no explicit declaration is needed.

Explain in your own words what it means for a type to satisfy an interface in Go.

Think about how Go checks if a type can be used where an interface is expected.

Describe why interface satisfaction is important for writing flexible Go programs.

Consider how interfaces help different types work together.

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