面向对象方法有许多特征，如软件系统是由对象组成的；(23)；对象彼此间仅能通过传递消息互相联系；层次结构的继承。

(102)

(103)

(100)

(101)

Inside　the　computer,　the　operation　system(of　another　software　module　called　a　data　communication(71))must　decide　which　program　will　get　control　next.　Each　program　remember,　is　associated(72).　　a　particular　front　-end　port.　If　a　program's　data　have　not　yet　reached　its　front　-　end　buffer,　there　is　no　point　giving　that　program　control.　Thus　the　data　communication　monitor　sends　its　own(73)signal　to　the　front　-end　devices　next　port　is　polled.　Note　that　the(74)computer　speed.　The　computer　never　has　to　wait　for　a　terminal　or　a　communication　line.　Instead,　it　is　the　expressive　front　-　end　processor　that　waits　for　the　slower　system(75)to　respond.

(97)

(98)

(95)

(96)

Software　engineering　is　the　science　and　art　of　specifying,　designing,　implementing,　and(66)with　economy,　timeliness　and　elegance,　programs,(67)and　operating　procedures　whereby　computers　　can　be　made　useful　to　humanity.

This　definition　implicitly　sets(68)the　basic　goal　of　software　engineering　research　and　development:　　The　basic　goal　of　software　engineering　is　to　facilitate　specification,　design,　implementation　and　evolution　　of　programs,　documentation　and　operating(69)which　are　useful　to　humanity　in　an　economic,(70)and　elegant　manner(using　the　appropriate　science　and　art).