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.).其中有提到一個比較經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;}}按照自然規則，square是特殊的rectangle（width==height），但是在用兩個class來表述時是不符合里氏替換規則的，因為square的setter不滿足里氏替換規則里的Postconditionscannotbeweakenedinasubtype。那么到底怎樣設計才行呢？