What is it

Subclasses should be substitutable for their base classes.

An object (such as a class) may be replaced by a sub-object (such as a class that extends the first class) without breaking the program

• if S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program

When we use inheritance we assume that the child class inherits everything that the superclass has.

• The child class extends the behavior but never narrows it down.

class Test {
   static void getAreaTest(Rectangle r) {
      int width = r.getWidth();
      r.setHeight(10);
      System.out.println("Expected area of " + (width * 10) + ", got " + r.getArea());
   }
   public static void main(String[] args) {
      Rectangle rc = new Rectangle(2, 3);
      getAreaTest(rc);

       // squares test would fail
      Rectangle sq = new Square();
      sq.setWidth(5);
      getAreaTest(sq);
   }
}