Exercises

Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for a small fee from www.BruceEckel.com.

1. Add a new method in the base class of Shapes.java that prints a message, but don’t override it in the derived classes. Explain what happens. Now override it in one of the derived classes but not the others, and see what happens. Finally, override it in all the derived classes. Add a new type of Shape to Shapes.java and verify in main( ) that polymorphism works for your new type as it does in the old types.
2. Change Music3.java so that what( ) becomes the root Object method toString( ). Try printing the Instrument objects using System.out.println( ) (without any casting). Add a new type of Instrument to Music3.java and verify that polymorphism works for your new type.
3. Modify Music3.java so that it randomly creates Instrument objects the way Shapes.java does.
4. Create an inheritance hierarchy of Rodent: Mouse, Gerbil, Hamster, etc. In the base class, provide methods that are common to all Rodents, and override these in the derived classes to perform different behaviors depending on the specific type of Rodent. Create an array of Rodent, fill it with different specific types of Rodents, and call your base-class methods to see what happens. Modify Exercise 6 so that Rodent is an abstract class. Make the methods of Rodent abstract whenever possible.
5. Create a class as abstract without including any abstract methods and verify that you cannot create any instances of that class. Add class Pickle to Sandwich.java.
6. Modify Exercise 6 so that it demonstrates the order of initialization of the base classes and derived classes. Now add member objects to both the base and derived classes and show the order in which their initialization occurs during construction.
7. Create a base class with two methods. In the first method, call the second method. Inherit a class and override the second method. Create an object of the derived class, upcast it to the base type, and call the first method. Explain what happens.
8. Create a base class with an abstract print( ) method that is overridden in a derived class. The overridden version of the method prints the value of an int variable defined in the derived class. At the point of definition of this variable, give it a nonzero value. In the base-class constructor, call this method. In main( ), create an object of the derived type, and then call its print( ) method. Explain the results.
9. Following the example in Transmogrify.java, create a Starship class containing an AlertStatus reference that can indicate three different states. Include methods to change the states.
10. Create an abstract class with no methods. Derive a class and add a method. Create a static method that takes a reference to the base class, downcasts it to the derived class, and calls the method. In main( ), demonstrate that it works. Now put the abstract declaration for the method in the base class, thus eliminating the need for the downcast.

    [32] For C++ programmers, this is the analogue of C++’s pure virtual function.
    

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