di_2.go

package main

import "log"

// An interface for an abstraction between structs
type StringWriter interface {
        Write(string)
}

// LogWriter now implements StringWriter
type LogWriter struct {
        Counter	int
}
func (w *LogWriter) Write(msg string) {
        log.Println(msg)
	w.Counter++
}

// Watcher can rely on the abstraction rather than 
// the implementation of LogWriter
type Watcher struct {
	Writer StringWriter
}

// Then this method calls the StringWriter's method
// which can be mapped to just about any struct with
// any implementation of Write.
func (w *Watcher) Notify(msg string) {
        w.Writer.Write(msg)
}