Use Java's extends keyword to derive a child class from a parent class, invoke parent class constructors and methods, override methods, and more.

Compare inheritance and composition, the two fundamental ways to relate Java classes, then practice debugging ClassCastExceptions in Java inheritance.

This looks like the Liskov substitution principle that Uncle Bob Martin described: Substitute a derived class (Square) for a base class (Rectangle). From the standpoint of the compiler, the above code ...