Implementing interfaces in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interface in Go?

An interface in Go is a type that defines a set of method signatures. Any type that implements these methods satisfies the interface automatically.

beginner

How does a type implement an interface in Go?

A type implements an interface by having all the methods that the interface requires. There is no explicit declaration needed.

beginner

Given this interface:

type Speaker interface { Speak() string }

How would a struct implement it?

The struct must have a method Speak() that returns a string. For example:

func (p Person) Speak() string { return "Hello!" }

intermediate

Can a pointer receiver implement an interface in Go?

Yes. If the interface method is implemented with a pointer receiver, only pointer types of that struct implement the interface. Value types do not.

beginner

What happens if a type does not implement all methods of an interface?

The type does not satisfy the interface and cannot be assigned to a variable of that interface type.

In Go, how do you declare that a type implements an interface?

A By defining all methods of the interface on the type

B By using the 'implements' keyword

C By embedding the interface in the type

D By importing the interface package

Go uses implicit implementation. If a type has all methods of an interface, it implements it automatically.

If an interface has method Foo(), which of these implements it?

A Type with method Foo()

B Type with method foo() (lowercase f)

C Type with method Foo(int)

D Type with no methods

Method names and signatures must match exactly, including case and parameters.

What is true about pointer receivers and interfaces?

A Interfaces require value receiver methods

B Value types always implement interfaces with pointer receiver methods

C Pointer receiver methods mean only pointer types implement the interface

D Pointer receivers are not allowed in interfaces

If methods have pointer receivers, only pointers to that type implement the interface.

Can a single type implement multiple interfaces?

A No, a type can implement only one interface

B Only if interfaces have no methods

C Only if interfaces are embedded

D Yes, if it has all methods required by each interface

A type can implement any number of interfaces by having the required methods.

What happens if you assign a type that does not implement an interface to a variable of that interface type?

A It works but methods are empty

B Compilation error

C Runtime panic

D It silently ignores missing methods

Go compiler checks interface implementation at compile time and will error if not satisfied.

Explain how a Go type implements an interface and why no explicit declaration is needed.

Think about how Go checks methods rather than declarations.

Describe the difference between pointer receiver and value receiver methods in interface implementation.

Consider which types can call the methods.

Practice

1. What does it mean for a type in Go to implement an interface?

easy

A. The type can only have one method.

B. The type inherits from the interface.

C. The type must be explicitly declared as implementing the interface.

D. The type has all the methods declared in the interface.

Solution

1. Step 1: Understand interface implementation in Go

   In Go, a type implements an interface by having all the methods the interface declares.

2. Step 2: Compare options with Go's rules

   Go does not use inheritance or explicit declarations for interfaces; it uses method sets.

3. Final Answer:

   The type has all the methods declared in the interface. -> Option D

4. Quick Check:

   Interface implementation = method set match [OK]

Hint: Check if all interface methods exist in the type [OK]

Common Mistakes:

• Thinking Go uses inheritance for interfaces
• Believing explicit declaration is needed
• Confusing interfaces with structs

2. Which of the following is the correct way to declare that a type implements an interface in Go?

easy

A. type MyType struct implements MyInterface {}

B. type MyType struct { MyInterface }

C. No explicit declaration needed; implement all interface methods

D. func (m MyType) implements() MyInterface {}

Solution

1. Step 1: Recall Go interface implementation syntax

   Go does not require explicit declarations; a type implements an interface by having all its methods.

2. Step 2: Analyze each option

   Options A, B, and D show invalid or non-Go syntax; only C correctly states Go's implicit implementation.

3. Final Answer:

   No explicit declaration needed; implement all interface methods -> Option C

4. Quick Check:

   Implicit implementation = No explicit declaration needed; implement all interface methods [OK]

Hint: Remember: Go uses implicit interface implementation [OK]

Common Mistakes:

• Trying to declare implementation explicitly
• Using inheritance syntax from other languages
• Adding interface as a struct field

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

package main
import "fmt"
type Speaker interface {
    Speak() string
}
type Dog struct {}
func (d Dog) Speak() string {
    return "Woof!"
}
func saySomething(s Speaker) {
    fmt.Println(s.Speak())
}
func main() {
    var d Dog
    saySomething(d)
}

medium

A. Woof!

B. Compilation error: Dog does not implement Speaker

C. Runtime error

D. No output

Solution

1. Step 1: Check if Dog implements Speaker

   Dog has a method Speak() string, so it implements Speaker interface.

2. Step 2: Trace the function call saySomething(d)

   saySomething calls s.Speak(), which returns "Woof!" and prints it.

3. Final Answer:

   Woof! -> Option A

4. Quick Check:

   Interface method call prints "Woof!" [OK]

Hint: Check if method signatures match interface exactly [OK]

Common Mistakes:

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

4. Identify the error in this Go code that tries to implement an interface:

package main
import "fmt"
type Runner interface {
    Run()
}
type Person struct {}
func (p *Person) Run() {
    fmt.Println("Running")
}
func main() {
    var r Runner
    p := Person{}
    r = p
    r.Run()
}

medium

A. Missing Run() method in Person

B. Cannot assign Person value to Runner interface because Run() has pointer receiver

C. Interface Runner must have a return type

D. No error, code runs fine

Solution

1. Step 1: Check method receiver type

   Run() is defined with pointer receiver (*Person), so only *Person implements Runner.

2. Step 2: Analyze assignment r = p

   Variable p is a Person value, not *Person pointer, so it does not implement Runner.

3. Final Answer:

   Cannot assign Person value to Runner interface because Run() has pointer receiver -> Option B

4. Quick Check:

   Pointer receiver method requires pointer type [OK]

Hint: Match method receiver type with assigned variable type [OK]

Common Mistakes:

• Ignoring pointer vs value receiver difference
• Assuming interface methods must return values
• Thinking interface assignment always works

5. Given these interfaces and types, which assignment is valid in Go?

type Reader interface {
    Read() string
}
type Writer interface {
    Write(string)
}
type ReadWriter interface {
    Reader
    Writer
}
type File struct {}
func (f *File) Read() string {
    return "data"
}
func (f *File) Write(s string) {
    // write data
}
func main() {
    var rw ReadWriter
    var f File
    // Which assignment is valid?
}

hard

A. rw = &f

B. rw = f

C. rw = File{} (without methods)

D. rw = nil

Solution

1. Step 1: Check if File implements ReadWriter

   File has Read() and Write(string) with pointer receivers (*File), so only *File implements ReadWriter; File value does not.

2. Step 2: Analyze assignment options

   rw = &f is valid (&f is *File). rw = f fails (File doesn't implement). C invalid. rw = nil compiles but doesn't hold usable ReadWriter.

3. Final Answer:

   rw = &f -> Option A

4. Quick Check:

   Pointer receiver requires pointer type [OK]

Hint: Pointer receiver methods mean only pointer type implements interface [OK]

Common Mistakes:

• Thinking value type implements pointer receiver interface
• Assigning nil to interface without initialization
• Confusing interface embedding with inheritance