Because you can extend at most one other class (recall from Part 1 that Java doesn’t support class-based multiple inheritance), you’re not forced to explicitly extend Object; otherwise, you ...

When to use inheritance in Java When to use composition in Java Differences between inheritance and composition Method overriding with Java inheritance Using constructors with inheritance Type ...

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 ...