Interface

• Abstract Type

type Dense interface {
    Density() float64
}

• Defines Behaviours - set of method signatures.

func IsDenser(a, b Dense) bool {
    return a.Density() > b.Density()
}

result = IsDenser(&oxygen, &hydrogen)
result = IsDenser(&oxygen, &hydrogen)

Interface values

• Dynamic type
• Dynamic value
• 

d.Density()

• Method call through an interface must use dynamic dispatch

• Compiler would have generated code to obtain the address of the method from the type descriptor, then make an indirect call to that address.

• The receiver argument for the call is a copy of the interface's dynamic value

Encapsulate (封裝)

• Interface enables us to encapsulate the logic within user defined data type.

Abstraction

• Interface provides abstraction for higher level functions with guarantee on befaviour of the underlying concrete type.

Exercise

Density

original

https://go.dev/play/p/CDEqy-OtLzw

package main

import "fmt"

// Metal - mass and volume information
type Metal struct {
    mass   float64
    volume float64
}

// Density - return density of metal
func (m *Metal) Density() float64 {
    // density = mass/volume
    return m.mass / m.volume
}

// IsDenser - compare density of two objects
func IsDenser(a, b *Metal) bool {
    return a.Density() > b.Density()
}

func main() {
    gold := Metal{478, 24}
    silver := Metal{100, 10}

    result := IsDenser(&gold, &silver)
    if result {
        fmt.Println("gold has higher density than silver")
    } else {
        fmt.Println("silver has higher density than gold")
    }
}

Use Interface

導入interface 讓兩個object可直接比較 https://go.dev/play/p/WRCBuDeubxQ

package main

import "fmt"

// Metal - mass and volume information
type Metal struct {
    mass   float64
    volume float64
}

// (Encapsulate) Density - return density of metal
func (m *Metal) Density() float64 {
    return m.mass / m.volume
}

// Gas - measurements for Gas
type Gas struct {
    pressure      float64
    temperature   float64
    molecularMass float64
}

// (Encapsulate) Density - return density of liquid
func (g *Gas) Density() float64 {
    var density float64
    density = (g.molecularMass * g.pressure) / (0.0821 * (g.temperature + 273))
    return density
}

// (Absttraction) IsDenser - compare density of two objects
func IsDenser(a, b Dense) bool {
    return a.Density() > b.Density()
}

// Dense - interface
type Dense interface {
    Density() float64
}

func main() {
    gold := Metal{478, 24}
    silver := Metal{100, 10}

    result := IsDenser(&gold, &silver)
    if result {
        fmt.Println("gold has higher density than silver")
    } else {
        fmt.Println("silver has higher density than gold")
    }

    oxygen := Gas{pressure: 5,
        temperature:   27,
        molecularMass: 32}

    hydrogen := Gas{pressure: 1,
        temperature:   0,
        molecularMass: 2}

    result = IsDenser(&oxygen, &hydrogen)

    if result {
        fmt.Println("oxygen has higher density than hydrogen")
    } else {
        fmt.Println("hydrogen has higher density than oxygen")
    }
}

// Formula for density of metal
// density = (mass)/(volume)

// Formula for density of Gas
// density = ((molecular mass) * (pressure)) / ((gas constant) * (temperature in kelvin))