Liskov Substitution (L)

 1class Rectangle {
 2    void setWidth(int w){ this.w = w; }
 3    void setHeight(int h){ this.h = h; }
 4    int area(){ return w * h; }
 5}
 6class Square extends Rectangle {
 7    void setWidth(int w){ this.w = w; this.h = w; }   // ⚠ also sets height
 8}