इंटरफ़ेस परिभाषित करना और लागू करना

package main
import "fmt"

type Greeter interface{ Greet() string }

type English struct{}
func (e English) Greet() string { return "Hello" }

type Spanish struct{}
func (s Spanish) Greet() string { return "Hola" }

func say(g Greeter) { fmt.Println(g.Greet()) }

func main() {
    say(English{})
    say(Spanish{})
}

package main
import "fmt"

func printAny(v any) { fmt.Printf("%T: %v\n", v, v) }

func main() {
    printAny(42)
    printAny("hello")
    printAny([]int{1,2,3})
}

package main
import "fmt"

type Animal interface{ Sound() string }
type Dog struct{}
func (d Dog) Sound() string { return "woof" }

func main() {
    var a Animal
    fmt.Println(a == nil) // true
    a = Dog{}
    fmt.Println(a == nil) // false
    fmt.Println(a.Sound())
}

package main
import "fmt"

type Reader interface{ Read() string }
type Writer interface{ Write(string) }
type ReadWriter interface{ Reader; Writer }

type File struct{ content string }
func (f *File) Read() string       { return f.content }
func (f *File) Write(s string)     { f.content = s }

func main() {
    var rw ReadWriter = &File{}
    rw.Write("hello")
    fmt.Println(rw.Read())
}

package main
import "fmt"

type Stringer interface{ String() string }

type Point struct{ X, Y int }
func (p Point) String() string { return fmt.Sprintf("(%d,%d)", p.X, p.Y) }

func main() {
    var s Stringer = Point{3, 4}
    fmt.Println(s.String())
}

package main
import "fmt"

type MyErr struct{}
func (e *MyErr) Error() string { return "oops" }

func bad() error {
    var e *MyErr // nil pointer
    return e     // returns non-nil interface!
}

func main() {
    err := bad()
    fmt.Println(err == nil) // false — common gotcha
}

package main
import "fmt"

type EmailSender interface{ Send(to, body string) error }

type MockSender struct{ Sent []string }
func (m *MockSender) Send(to, body string) error {
    m.Sent = append(m.Sent, to)
    return nil
}

func notify(s EmailSender, addr string) { s.Send(addr, "hello") }

func main() {
    mock := &MockSender{}
    notify(mock, "a@b.com")
    fmt.Println(mock.Sent) // [a@b.com]
}

package main

type Logger interface{ Log(string) }

type FileLogger struct{}
func (f *FileLogger) Log(s string) {}

func NewFileLogger() *FileLogger { // returns concrete
    return &FileLogger{}
}

func process(l Logger) {} // accepts interface

func main() { process(NewFileLogger()) }