A Tour of GO (4) -- Methods and Interfaces

1. GO does not have class. However, we can define methods on types.

• A method is a function with a special receiver argument.
• The receiver appears in its own argument list between the func keyword and the method name.
  func (v Vertex) Abs() float64 {
  return math.Sqrt(v.Xv.X + v.Yv.Y)
  }
• Remember: A method is just a function with a receiver argument. This one functions the same as the previous one.
  func Abs(v Vertex) float64 {
  return math.Sqrt(v.Xv.X + v.Yv.Y)
  }
• Important: You can only declare a method with a receiver whose type is defined in the same package. That includes the built-in type. However you can use
  type MyFloat float64
• Pointer receivers:
  func (v *Vertex) Scale(f float64) {
  v.X = v.X * f
  v.Y = v.Y * f
  }

  • Method with pointer receivers can modify the value to which the receiver points. ( GO is passing by value by default)

  • Method and pointer indirection. Method will be called automatically even v is a value not a pointer, but function will not.
    var v Vertex
    /* Method /
    v.Scale(5) // OK
    p := &v
    p.Scale(10) // OK
    / function */
    ScaleFunc(v) // Compile error!
    ScaleFunc(&v) // OK

  • The equivalent thing happens int the reverse direction. The method takes value as receiver can also takes pointer while function cannot.

  • Using pointer as receiver is encouraged. In general, all methods on a give type should have either value or pointer receivers, but not a mixture of both.

2. Interfaces

• An interface type is defined by a set of methods.
  type Abser interface {
  Abs() float64
  }
• A type implements an interface by implementing the methods.
• Stringer. A Stringer is a type that can describe itself as a string. The fmt pkg look for this interface to print values.
  type Stringer interface {
  String() string
  }

3. Error

• The error type is a built-in interface similar to fmt.Stringer.
  type error interface{
  Error() string
  }

4. Readers

• The IO package specifies the io.Readerinterface, which denote the read end of a stream of data.

5. HTTP servers
   *Package http serves HTTP request using any value that implements http.Handler:
6. Image

• Package image defines the Image interface.