Interfaces in Go

An interface type is a method set. If a type contains methods of the interface, it implements the interface. 

A type can implement multiple interfaces. For instance all type implement the empty interface.

An interface may use a  interface type name  in place of a method specification. This is called embedding interface.

type ReadWriter interface {	
  Read(b Buffer) bool	
  Write(b Buffer) bool
}
type File interface {	
  ReadWriter  // same as adding the methods of ReadWriter	            
  Close()
}

The empty interface

A type of empty interface can be used to declare a generic data type. 

E.g. Implementing a linked list in Golang.

A linked list is a linear data structure where each element is a separate object. Linked list elements or nodes are not stored at contiguous location; the elements are linked using pointers.

We could declare the linked list node struct as follows

type Node struct {
    Next *Node
    Data interface{}
}

//creates a New Node
func NewNode(d interface{}) *Node {
    nn := Node{Data: d, Next: nil}
    return &nn
}

//Append : Adds a node to linked list
func (n *Node) Append(d interface{}) {
    nn := NewNode(d)

    if n == nil {
        n = nn
    } else {
        m := n
        for m.Next != nil {
            m = m.Next
        }
        m.Next = nn
    }
}

 This allows us to use the same struct to hold data of different types.

Here I use int as the data type for Node.Data

n := NewNode(0) // int Data
n.Append(3)
n.Append(9)
for m := n; m != nil; m = m.Next {
    fmt.Println(m.Data)
}

I could also use string as the data type using the same struct that I defined above.

n1 := NewNode("a") //string Data
n1.Append("b")
n1.Append("c")

Here is the full example:

https://play.golang.org/p/fo_sSndTsmc

I also have examples of using the empty interface when Unmarshalling JSON in my other post: Custom Unmarshal JSON in Go

Useful interfaces in Go stdlib

Error Interface

The error type is an interface that has a method Error.

type error interface {    
 Error() string
}

The most commonly used implementation of the error interface is the errorString type in the errors package.

// errorString is a trivial implementation of error.type errorString struct {    
 s string
}
func (e *errorString) Error() string {    
 return e.s
}

Handler Interface

The Handler Interface in the net/http package requires one method ServerHTTP

type Handler interface {	
       ServeHTTP(ResponseWriter, *Request)
}

Within that same package you will find HandlerFunc implements the Handler interface. 

type HandlerFunc func(ResponseWriter, *Request)     

// ServeHTTP calls f(w, r).  
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {  	     
  f(w, r)  
}

The HandlerFunc makes it possible for us to pass in any function to make it a Handler. All we would have to do is wrap it in  HandlerFunc. 

http.Handle("/", http.HandlerFunc(indexHandlerHelloWorld))null

We can have also have our own struct that has fields and methods and implements the Handler Interface by defining the ServeHTTP method as a member of the struct. 

Stringer Interface

The fmt package has a stringer interface. It can be implemented by a type that declares the String method. 

type Stringer interface {        
       String() string
}

If a type implements the stringer interface, a call to fmt.Println or fmt.Printf of the variable of type will use that method. 

E.g. If you want  to print a struct in a formatted fashion with key names and values, the struct needs to implement the Stringer interface

type Family struct {
    Name string
    Age  int
}

func (f *Family) String() string {
    return fmt.Sprintf("Name: %s\t Age: %d", f.Name, f.Age)
}
func main() {
family := []Family{
        {"Alice", 23},
        {"David", 6},
        {"Erik", 2},
        {"Mary", 32},
    }

    for _, i := range family {
        fmt.Println(&i)
    }
}

https://play.golang.org/p/F7rNPyClwG4

The fmt package has other interfaces like Scanner, Formatter and State.

References

https://golang.org/ref/spec#Interface_types