Interface definition in Go - Step-by-Step Execution

Concept Flow - Interface definition

Define interface with method signatures

↓

Create struct type

↓

Implement methods on struct matching interface

↓

Assign struct instance to interface variable

↓

Call interface methods

↓

Use polymorphism

This flow shows how to define an interface, implement it with a struct, assign the struct to the interface, and call its methods.

Execution Sample

Go

package main
import "fmt"
type Speaker interface {
    Speak() string
}
type Person struct {
    Name string
}
func (p Person) Speak() string {
    return "Hello, " + p.Name
}
func main() {
    var s Speaker
    s = Person{Name: "Alice"}
    fmt.Println(s.Speak())
}

This code defines an interface Speaker with one method Speak, implements it with Person struct, assigns a Person to Speaker, and calls Speak.

Execution Table

Step	Action	Evaluation	Result
1	Define interface Speaker with method Speak()	Speaker interface created	Interface ready to use
2	Define struct Person with field Name	Person struct created	Struct ready to use
3	Implement Speak() method on Person	Method matches Speaker interface	Person implements Speaker
4	Declare variable s of type Speaker	s is nil interface variable	s ready to hold Speaker
5	Assign Person{Name: "Alice"} to s	Person instance assigned to s	s holds Person as Speaker
6	Call s.Speak()	Calls Person.Speak()	Returns "Hello, Alice"
7	Print output	Output printed to console	Hello, Alice
8	Program ends	No more code	Execution stops

💡 Program ends after printing the greeting from the interface method

Variable Tracker

Variable	Start	After Step 5	After Step 6	Final
s	nil	Person{Name: "Alice"}	Person{Name: "Alice"}	Person{Name: "Alice"}

Key Moments - 3 Insights

Why can we assign a Person struct to a Speaker interface variable?

Because Person has a method Speak() with the same signature as the Speaker interface requires, so Go treats Person as implementing Speaker (see execution_table step 5).

What happens when we call s.Speak() if s holds a Person?

The Speak() method of Person is called, returning the greeting string (see execution_table step 6).

Why is the interface variable s initially nil?

Because declaring var s Speaker creates an interface variable with no value until assigned (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of variable s after step 5?

A Person{Name: "Alice"}

B nil

C Speaker interface type

D Empty struct

💡 Hint

Check variable_tracker column 'After Step 5' for s value

At which step does the program call the Speak() method?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

See execution_table row where Action is 'Call s.Speak()'

If Person did not implement Speak(), what would happen when assigning to s?

A s would hold a nil value

B Assignment would fail with a compile error

C Program would panic at runtime

D Assignment would succeed but method call fail

💡 Hint

Interfaces require matching methods; see key_moments about assignment

Concept Snapshot

Interface definition in Go:
- Use 'type Name interface { MethodSignatures }'
- Any type with matching methods implements the interface
- Assign instances to interface variables
- Call methods via interface for polymorphism
- No explicit 'implements' keyword needed

Full Transcript

This example shows how to define an interface named Speaker with one method Speak. Then we create a struct Person with a Name field. We implement the Speak method on Person to return a greeting string. In main, we declare a variable s of type Speaker, initially nil. We assign a Person instance with Name 'Alice' to s. Because Person implements Speak, this assignment works. Calling s.Speak() calls Person's method and returns 'Hello, Alice'. Finally, we print the result. The program ends after printing. This demonstrates Go's interface definition and usage with method implementation and polymorphism.