Liskovsubstitution（里氏替換）是oop五原則（SOLID）之一，wikipedia上的介紹如下Substitutabilityisaprincipleinobject-orientedprogramming.Itstatesthat,inacomputerprogram,ifSisasubtypeofT,thenobjectsoftypeTmaybereplacedwithobjectsoftypeS(i.e.,objectsoftypeSmaybesubstitutedforobjectsoftypeT)withoutalteringanyofthedesirablepropertiesofthatprogram(correctness,taskperformed,etc.).其中有提到一個(gè)比較經(jīng)典的違反此原則的案例，那就是square和rectangle，先看代碼：publicclassRectangle{privatedoublewidth;privatedoubleheight;publicvoidsetWidth(doublewidth){this.width=width;}publicvoidsetHeight(doubleheight){this.height=height;}publicdoublegetHeight(){returnthis.height;}publicdoublegetWidth(){returnthis.width;}publicdoublegetPerimeter(){return2*width+2*height;}publicdoublegetArea(){returnwidth*height;}}publicclassSquareextendsRectangle{publicvoidsetWidth(doublewidth){this.width=width;this.height=width;}publicvoidsetHeight(doubleheight){this.height=height;this.width=height;}}按照自然規(guī)則，square是特殊的rectangle（width==height），但是在用兩個(gè)class來(lái)表述時(shí)是不符合里氏替換規(guī)則的，因?yàn)閟quare的setter不滿足里氏替換規(guī)則里的Postconditionscannotbeweakenedinasubtype。那么到底怎樣設(shè)計(jì)才行呢？