Golang understanding interface

24 December 2017

本文简单总结自Understanding Go Interfaces，对应的PPT。

In object-oriented programming, a protocol or interface is a common means for unrelated objects to communicate with each other.

concrete types

描述了内存布局

通过方法将行为绑定到数据上

type Number int
func (n Number) Positive() bool {
    return n > 0
}

abstract types

描述了行为

定义了方法集

type Positiver interface {
    Positive() bool
}

为何使用interface？

1、编写通用的算法

// A type, typically a collection, that satisfies sort.Interface can be
// sorted by the routines in this package. The methods require that the
// elements of the collection be enumerated by an integer index.
type Interface interface {
	// Len is the number of elements in the collection.
	Len() int
	// Less reports whether the element with
	// index i should sort before the element with index j.
	Less(i, j int) bool
	// Swap swaps the elements with indexes i and j.
	Swap(i, j int)
}

2、隐藏实现细节

3、提供拦截点

type headers struct {
    rt http.RoundTripper
    v map[string]string
}

func (h headers) RoundTrip(r *http.Request) (*http.Response, error) {
    for key, value := range h.v {
        r.Header.Set(key, value)
    }
    return h.rt.RoundTrip(r)
}

func main() {
    c := &http.Client{
        Transport: headers{
            rt: http.DefaultTransport,
            v: map[string]string{"foo": "bar"},
        },
    }

    res, err := c.Get("http://golang.org")
}

The bigger the interface, the weaker the abstraction. - Rob Pike

方法集越大，抽象程度越低。事物的抽象，是从众多的事物中抽取出共同的、本质性的特征。抽象程度越高，涵盖的事物也就越多，相应的方法集也会越少。

Be conservative in what you do/send, be liberal in what you accept from others. - Robustness Principle

在方法定义和实现的时候应当遵行的原则：对于内部所做的/返回的尽量保守，对于从外部接收的尽量开放。

Return concrete types, receive interface as parameters.

Golang interface与其他语言的interface不同点主要在于Golang interface是隐式实现的，而其他语言需要显示的implement。

GoLang 21