Interface definition 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 do you define an interface in Go?

You define an interface using the type keyword followed by the interface name and the interface keyword with method signatures inside curly braces.

Example:

type Speaker interface {
    Speak() string
}

beginner

True or False: In Go, a type must explicitly declare it implements an interface.

False. In Go, a type implements an interface implicitly by having the required methods. No explicit declaration is needed.

intermediate

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

The Go compiler will raise an error because the type does not satisfy the interface. All methods must be implemented to satisfy the interface.

intermediate

Explain how interfaces help in Go programming.

Interfaces allow writing flexible and reusable code by defining behavior without specifying implementation. Different types can implement the same interface, enabling polymorphism.

How do you declare an interface named 'Mover' with a method 'Move()' in Go?

A func Mover() interface { Move() }

B interface Mover { Move() }

C type Mover struct { Move() }

D type Mover interface { Move() }

In Go, interfaces are declared with 'type' keyword followed by the interface name and 'interface' keyword with method signatures inside braces.

If a type has all methods required by an interface, what must you do to implement that interface?

A Nothing, it implements automatically

B Explicitly declare it implements the interface

C Embed the interface in the type

D Use a special keyword 'implements'

Go uses implicit implementation. If a type has all methods, it automatically satisfies the interface.

What error occurs if a type does not implement all interface methods?

A No error, it works fine

B Runtime panic

C Compile-time error

D Linker error

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

Which of these is NOT true about Go interfaces?

A Interfaces specify method implementations

B Interfaces are satisfied implicitly

C Interfaces enable polymorphism

D Interfaces can hold any type that implements their methods

Interfaces specify method signatures, not implementations.

What keyword is used to define an interface in Go?

A interface

B type

C struct

D func

The 'type' keyword is used to define interfaces, combined with the 'interface' keyword inside the definition.

Describe how you define and use an interface in Go with a simple example.

Think about how you declare an interface and how a struct can implement it without extra keywords.

Explain why interfaces are useful in Go programming and how they support flexible code design.

Consider how interfaces let different types behave similarly without sharing code.

Practice

1. What does an interface in Go define?

easy

A. A concrete type with fields and methods

B. A variable that holds data

C. A set of method signatures without implementation

D. A function that returns multiple values

Solution

1. Step 1: Understand what an interface is

   An interface in Go specifies method signatures but does not provide their implementation.

2. Step 2: Differentiate from other types

   Concrete types have fields and method implementations, unlike interfaces.

3. Final Answer:

   A set of method signatures without implementation -> Option C

4. Quick Check:

   Interface = method signatures only [OK]

Hint: Interfaces list methods only, no code inside [OK]

Common Mistakes:

• Thinking interfaces hold data
• Confusing interfaces with structs
• Assuming interfaces implement methods

2. Which of the following is the correct syntax to define an interface named Shape with a method Area() returning a float64?

easy

A. func Shape interface { Area() float64 }

B. interface Shape { func Area() float64 }

C. type Shape struct { Area() float64 }

D. type Shape interface { Area() float64 }

Solution

1. Step 1: Recall Go interface syntax

   Interfaces are defined with the keyword type, name, then interface and method signatures inside braces.

2. Step 2: Check each option

   type Shape interface { Area() float64 } matches correct syntax. Options B, C, and D misuse keywords or structure.

3. Final Answer:

   type Shape interface { Area() float64 } -> Option D

4. Quick Check:

   Correct interface syntax = type Shape interface { Area() float64 } [OK]

Hint: Use 'type Name interface { methods }' for interfaces [OK]

Common Mistakes:

• Using 'interface' keyword alone without 'type'
• Defining methods inside struct instead of interface
• Placing 'func' keyword inside interface method

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

package main
import "fmt"
type Printer interface {
    Print()
}
type Document struct {
    content string
}
func (d Document) Print() {
    fmt.Println(d.content)
}
func main() {
    var p Printer
    d := Document{"Hello, Go!"}
    p = d
    p.Print()
}

medium

A. Runtime panic: nil pointer dereference

B. Hello, Go!

C. Compilation error: cannot assign Document to Printer

D. No output

Solution

1. Step 1: Check interface implementation

   Document has a method Print() matching Printer interface, so Document implements Printer.

2. Step 2: Assign and call method

   Variable p of type Printer is assigned d (Document), then p.Print() calls Document.Print(), printing content.

3. Final Answer:

   Hello, Go! -> Option B

4. Quick Check:

   Interface method calls concrete method [OK]

Hint: Assign concrete type with matching methods to interface [OK]

Common Mistakes:

• Thinking interface needs explicit implementation declaration
• Expecting compile error on assignment
• Assuming interface variable is nil

4. Identify the error in this Go code snippet:

type Reader interface {
    Read(p []byte) (n int, err error)
}
type File struct {}
func (f File) Read() (int, error) {
    return 0, nil
}

medium

A. Method Read has wrong signature, missing parameter

B. File does not implement Reader because it lacks Read method

C. Interface Reader cannot have methods with parameters

D. No error, code is correct

Solution

1. Step 1: Compare method signatures

   Reader interface expects Read with parameter []byte and two return values.

2. Step 2: Check File's Read method

   File's Read method has no parameters, so signature does not match interface.

3. Final Answer:

   Method Read has wrong signature, missing parameter -> Option A

4. Quick Check:

   Method signature must match interface [OK]

Hint: Method signatures must exactly match interface methods [OK]

Common Mistakes:

• Ignoring method parameters in implementation
• Assuming method name alone is enough
• Thinking interface methods can't have parameters

5. Given these interfaces and types:

type Reader interface {
    Read(p []byte) (n int, err error)
}
type Writer interface {
    Write(p []byte) (n int, err error)
}
type ReadWriter interface {
    Reader
    Writer
}
type MyFile struct {}
func (f MyFile) Read(p []byte) (int, error) {
    return len(p), nil
}
func (f MyFile) Write(p []byte) (int, error) {
    return len(p), nil
}

Which statement is true about MyFile and ReadWriter?

hard

A. MyFile implements ReadWriter because it has both Read and Write methods

B. MyFile only implements Reader, not ReadWriter

C. MyFile does not implement any interface

D. ReadWriter cannot embed other interfaces

Solution

1. Step 1: Understand interface embedding

   ReadWriter embeds Reader and Writer, so it requires both methods.

2. Step 2: Check MyFile methods

   MyFile has both Read and Write methods matching signatures.

3. Step 3: Confirm implementation

   Since MyFile has all required methods, it implements ReadWriter.

4. Final Answer:

   MyFile implements ReadWriter because it has both Read and Write methods -> Option A

5. Quick Check:

   Embedded interfaces require all methods [OK]

Hint: Embedded interfaces combine methods; implement all to satisfy [OK]

Common Mistakes:

• Thinking embedding interfaces is invalid
• Assuming partial method implementation suffices
• Confusing interface embedding with inheritance