設最底層的基類為Shape，Shape有周長和麵積，圓Circle和橢圓Ellipse繼承於Shape，設定計算周長的介面為circum()，計算面積的介面為area()。繼承圖表如下：

# part1 -------------------------------------------------------------------
##定義基類Shape
setClass("Shape",slots = list(name="character"
 
))

##定義圓形類Circle,繼承Shape
setClass("Circle",contains = "Shape",slots = list(radius="numeric"),
         prototype = list(radius=1))
##驗證radius大於0
setValidity("Circle",function(object){
    if([email protected] <= 0) stop("Radius is negative")
})

##定義計算面積的介面
setGeneric("area",function(obj,...) standardGeneric("area"
 
))

##實現面積介面
setMethod("area","Circle",function(obj,...){
    print("Area Circle Method")  
    pi * [email protected]^2
})


##定義周長的介面
setGeneric("circum",function(obj,...) standardGeneric("circum"))

##實現周長介面
setMethod("circum","Circle",function(obj,...){
    2 * pi *[email protected]
})


##建立兩個圓的例項---------------#
 

c1 <- new("Circle",name="c1")
c2 <- new("Circle",name="c2",radius=5)
# c1; c2

##計算兩個圓的面積
area(c1)
area(c2)

##計算兩個圓的周長
circum(c1)
circum(c2)


# part2 -------------------------------------------------------------------
##定義橢圓類Ellipse, 繼承Shape
setClass("Ellipse",contains = "Shape",slots = list(radius="numeric"),
         prototype = list(radius = c(1,1)))

##驗證橢圓的半徑
setValidity("Ellipse",function(object) {
  if (length([email protected]) != 2 ) stop("It's not Ellipse.")
  if (length(which([email protected]<=0))>0) stop("Radius is negative.")
})

##計算橢圓的面積
setMethod("area","Ellipse",function(obj,...){
  print("Area Ellipse Method")  
  pi * prod([email protected])
})

##計算橢圓的周長
setMethod("circum","Ellipse",function(obj,...){
  cat("Ellipse Circum: \n")  
  2*pi * sqrt(([email protected][1]^2 + [email protected][2]^2)/2 )
})


##建立橢圓的兩個例項---------------------#
e1 <- new("Ellipse",name="e1"); 
e2 <- new("Ellipse",name="e2",radius=c(5,1))


##計算兩個橢圓的面積
area(e1)
area(e2)

##計算兩個橢圓的周長
circum(e1)
circum(e2)

　　效果如下：

　　方法二：當橢圓的長半徑和短半徑相等時，radius的兩個值相等，形成的圖形為圓形。利用這個特點，我們就可以重新設計圓與橢圓的關係。即橢圓是圓的父類，圓就是橢圓的子類。程式碼如下：
　　//S4a.R　　

# part1 -------------------------------------------------------------------
##面向物件
##Shape --> Ellipse -->Circle
##定義基類Shape
setClass("Shape",slots = list(name="character",shape="character"))
##定義獲取圖形的種類介面
setGeneric("getShape",function(obj,...) standardGeneric("getShape"))
##實現獲取圖形的種類介面
setMethod("getShape","Shape",function(obj,...){
    cat([email protected],"\n")
})


##定義橢圓類Ellipse,繼承Shape
setClass("Ellipse", contains = "Shape",slots = list(radius="numeric"),
         prototype = list(radius=c(1,1),shape="Ellipse"))
##定義圓形Circle, 繼承Ellipse
setClass("Circle",contains = "Ellipse",slots = list(radius="numeric"),
         prototype = list(radius=1, shape="Circle"))

##定義area介面
setGeneric("area",function(obj,...) standardGeneric("area"))
##定義circum介面
setGeneric("circum",function(obj,...) standardGeneric("circum"))

##area介面的Ellipse實現
setMethod("area","Ellipse",function(obj,...){
    cat("Ellipse Area: \n")
    pi*prod([email protected])
})

##area介面的Circle實現
setMethod("area","Circle",function(obj,...){
    cat("Circle Area: \n")  
    pi*[email protected]^2
})


##circum介面的Ellipse實現
setMethod("circum","Ellipse",function(obj,...){
    cat("Ellipse Circum: \n")
    2*pi*sqrt(([email protected][1]^2 + [email protected][2]^2)/2)
})

##circum介面的Circle實現
setMethod("circum","Circle",function(obj,...){
    cat("Circle circum: \n")
    2*pi*[email protected]
})

##建立例項
e1 <- new("Ellipse",name="e1",radius=c(2,5))
c1 <- new("Circle",name="c1",radius=2)

##計算面積
area(e1)
area(c1)

##計算周長
circum(e1)
circum(c1)

##獲取圖形的型別
getShape(e1)
getShape(c1)