block4.go

package encapsulationexample

// Counter defines the interface that a counter instance needs to implement.
type Counter interface{
  N() int
  Increment()
  Reset()
}

// SimpleCounter - a simple counter.
type simpleCounter struct {
   counter int
}

func NewSimpleCounter() Counter {
   return &simpleCounter{
       counter: 0,
   }
}

func (c *simpleCounter) N() int { return c.counter }

func (c *simpleCounter) Increment() { c.counter++ }

func (c *simpleCounter) Reset() { c.counter = 0 }

// CounterDemo shows how to use the `Counter` interface.
func CounterDemo() {
  var c Counter
  c = NewSimpleCounter()
  // This would cause a compiler error as the `Counter` interface does not have `counter` field.
  c.counter = 5
  // It will print out "counter value: 0" if you comment the above statement.
  fmt.Print("counter value: %d\n", c.N())
}