Prev Next

Golang / GoLang Interfaces and Object Oriented Interview Questions

What is the internal two-word structure of a Go interface value?

Internally, every non-empty interface value is a two-word pair stored on the stack (or heap if it escapes):

Interface Value Layout
WordNameContent
Word 1itab (type word)Pointer to an itab struct containing: the interface type descriptor, the concrete type descriptor, a hash of the concrete type, and the method dispatch table (function pointers)
Word 2data (value word)Pointer to the concrete value on the heap, or the value itself if it fits in one pointer and is pointer-shaped
type Animal interface { Sound() string }

type Dog struct{ Name string }
func (d Dog) Sound() string { return "Woof" }

var a Animal = Dog{Name: "Rex"}
// Memory layout of 'a':
// word1 → itab{itype=Animal, ctype=Dog, hash=..., fun=[Dog.Sound]}
// word2 → *Dog{Name: "Rex"}  (heap-allocated copy)

// Nil interface — BOTH words are nil
var a2 Animal // nil interface: word1=nil, word2=nil
fmt.Println(a2 == nil) // true

// Non-nil interface holding nil pointer — word1 is non-nil!
var d *Dog
a = d  // word1 → itab{ctype=*Dog,...}, word2 → nil
fmt.Println(a == nil) // FALSE — classic trap

// Inspect with reflect
import "reflect"
v := reflect.ValueOf(a)
fmt.Println(v.IsNil())  // true  — the *Dog data pointer is nil
fmt.Println(a == nil)   // false — the interface itself is non-nil

The empty interface any (alias for interface{}) uses a simpler two-word layout called eface: word1 is a plain type pointer (no method table), word2 is the data pointer. Non-empty interfaces use iface with the full itab. The itab is cached globally per (interface type, concrete type) pair, so interface assignment is effectively free after the first occurrence.

In Go's internal interface representation, what does the first word (itab) contain?
What is the difference between eface (empty interface) and iface (non-empty interface) internally?

More Related questions...

What is an interface in Go and how does it differ from interfaces in Java or C#? What is the internal two-word structure of a Go interface value? Explain the nil interface trap in Go. Why does a typed nil fail the '!= nil' check? How does Go's implicit interface satisfaction enable duck typing and loose coupling? What is the empty interface (any) in Go and what are its trade-offs? What is the difference between value receivers and pointer receivers in Go methods? How does interface embedding (composition) work in Go? How does Go achieve runtime polymorphism using interfaces? How does Go implement code reuse without inheritance? Explain composition via struct embedding. How does embedding help a struct satisfy an interface? How is the built-in error type defined and how do you implement custom errors? What is the fmt.Stringer interface and how do you implement it? How do type assertions work on interface values and when do you use them? Explain the Go design principle: 'Accept interfaces, return concrete types'. How do Go interfaces enable dependency injection and improve testability? How do you sort custom types using sort.Interface in Go? How do io.Reader and io.Writer demonstrate Go's interface design philosophy? Why can't you always use a value of type T where *T is needed for interface satisfaction? How can a single Go type implement multiple interfaces simultaneously? How does Go implement encapsulation without private/public class modifiers? How do the fmt.Stringer and error interfaces work together and how do you avoid infinite recursion? How do generics relate to interfaces in Go 1.18+ and what are type constraints? How do you implement an abstract type pattern in Go using interfaces and constructors? What is the compile-time interface check idiom and why is it important? How does equality work for interface values in Go? How does the io.Closer interface work with defer for resource management? How does Go differ from classical OOP in terms of inheritance and method overriding? How does http.Handler demonstrate real-world interface design in Go? What is the function-as-interface pattern in Go and how does it enable flexible APIs? What is interface pollution and how do you avoid it in Go? What is the difference between fmt.Stringer and fmt.GoStringer? How does Go's standard library use interface layering for I/O transformation? When should you use reflection instead of interfaces for type-agnostic code in Go? How does thinking about interfaces as 'behaviours, not data' guide better design? What is the interface upgrade (optional interface) pattern in Go? What is the difference between embedding a struct and embedding an interface inside a struct? Are interface values in Go safe to use concurrently? How do you combine table-driven tests with interface mocks in Go? How is context.Context an interface and how does its design demonstrate Go best practices? How does Go embody the Interface Segregation Principle (ISP)? How do type aliases and type definitions differ in relation to interface satisfaction in Go? How does embedding propagate interface satisfaction through multiple levels? What is the zero value of an interface type and how does it differ from a zero-value struct? How do you use wrapper types to adapt existing types to satisfy interfaces in Go? Walk through a complete Go OOP design: payment processing without inheritance. What are the edge cases in Go interface value equality that trip up experienced developers? What is sync.Locker and how is it used in real-world Go concurrency code? Summarise the key rules and best practices for Go interfaces that interviewers test.
Show more question and Answers...

GoLang Concurrency Mastery Interview Questions

Comments & Discussions

Interviews Questions

About Javapedia.net

Javapedia.net is for Java and J2EE developers, technologist and college students who prepare of interview. Also this site includes many practical examples.

This site is developed using J2EE technologies by Steve Antony, a senior Developer/lead at one of the logistics based company.

Follow Javapedia updates

contact: javatutorials2016[at]gmail[dot]com

Kindly consider donating for maintaining this website. Thanks.

Copyright © 2026, javapedia.net, all rights reserved. · .

DMCA.com Protection Status