Open a new Eclipse (or using your favorite Java editor) Java Project Chapter9Exercise and create a class Animal. Give the Animal a single private int attribute hunger to hold how hungry the Animal is, a constructor that takes no arguments and sets this attribute to zero, and a method getHunger that returns it. Then write an abstract method talk that takes no arguments and returns nothing. (Recall that to make a method abstract, you put abstract at the beginning of the method signature (before public) and put a semicolon at the end of the signature instead of an opening brace.)

Try to compile this. You will get an error message about not having overridden the abstract method talk. This error comes because the class Animal has an abstract method, but it is not declared to be an abstract class. Add abstract between public and class at the top of the file. This tells the computer that you intend Animal to be an abstract class. (Eclipse tries to help you with errors as you write code; pay attention to these hints and messages; dont just simply correct code without understanding what these errors are pointing to!)

Now check that Animal compiles. It does, but because Animal is an abstract class, you are not able to actually create Animal objects. For that, we need to create a subclass.

Create a class Zebra. Make it a subclass of Animal by adding extends Animal to the end of the line giving the class name. Write a constructor for Zebra that takes no arguments. The entire body of this method should be super(); This calls the constructor for Animal. That constructor then sets its attribute hunger to 0. (Actually, Java will automatically call the superclass constructor for us, but its a good habit to explicitly include a call to a superclass constructor in each subclass constructor.)

If you try to compile at this point, you again get the error message about not having overridden the talk method. Oops! By inheriting from Animal, we promised to implement a talk method. Add a method talk that takes no arguments and returns nothing, just as promised in the Animal class. Make this method print the string The Zebra quietly chews..

After writing talk, the project should compile. Create a Zebra object (you can do this by writing a main method either in the Zebra class or in a client class; the latter is preferred as it is not a good practice to add main method to a pure Java class). Even though we did not declare any attributes within the code for Zebra, you see that it has the int attribute inherited from Animal. Note the methods are able to invoke on the Zebra object talk and getHunger. Call both methods to verify that talk prints out the message about chewing and that getHunger returns 0.

There wouldnt be any reason to write the abstract class Animal if we planned to write only one subclass. Write another subclass of Animal called Lion. As we did with Zebra, give it a constructor that takes no arguments and calls the Animal constructor. Then write a talk method that just prints the message Roar!. Verify that this compiles and that both getHunger and talk work as intended in the Lion.

So far the hunger attribute does not do very much because it never changes from 0. Add a method timePasses to Animal that increases the hunger attribute by one. (The idea is that this is called each unit of time so the animals gradually get hungrier over time.) Lions are not content to quietly get hungrier, though. Override the timePasses method in Lion with a method that increases hunger by 1 as above, but also prints the message The Lion paces hungrily. if its new value is at least 3. Note that you will not be able to access hunger directly because it is a private attribute of Animal. One solution is to change the access restrictions on hunger (e.g. make it public), but better is to access the attribute indirectly. To increase hunger, call the the timePasses method of Animal (using super.timePasses();). Then, use the getHunger method to read the value of hunger.

Now that the animals can get hungry, we should have a way to feed them. Add a method feed to Animal that sets hunger back to 0. Compile and create some animals to make sure they get hungrier and that hungry lions start pacing.

As a last step for the animals, lets write a toString method. This is the method that is called automatically when an object is printed (or a String representation is needed for some other reason). It must take no arguments and return a String. The classes already inherit such a method from the Object class (which you can verify by looking in inherited from Object in the menu of methods). Since this toString method doesnt give a very useful string, lets override this method in Zebra and Lion with methods that return the class of the animal. (So the method in Zebra returns Zebra and the one in Lion returns Lion.) Because the signature of your toString method needs to exactly match the signature of the one youre overriding, you will need to explicitly make it public:

public String toString()

Once you complete this method, again compile, create some animals, and ensure that this works before proceeding.

Now lets write some code to use our animal classes. Create a new class called Zoo. This one should not inherit from Animal since it doesnt have an is a relationship with Animal. To start with, give your Zoo a single attribute called cage which stores an Animal. Give it a constructor that takes an Animal object and stores it in this attribute. Then write a method print that prints the message The zoo contains a followed the animals type. (Since weve written a toString method, you can print an Animal object as if it were a String.) Printing the Zoo should produce a message such as the following:

The zoo contains a Lion

A zoo with only one animal isnt going to attract many visitors. Therefore, we want to expand the Zoo class so that it can accommodate multiple Animal objects. Rename cage to cage1 and add cage2 and cage3. Remove the argument to the constructor and just have it set all these variables to null. (That means that the variable doesnt reference any object.) Then create methods to set each of these (call them putInCage1, putInCage2, and putInCage3); the methods take an Animal object and set the appropriate variable. Then modify print to print any of these that are not null in a format such as

The zoo contains the following: Lion

Zebra Since you dont want to print a null reference, youll need to check each of them before printing it:

if(cage1 != null) { //print first animal

} if(cage2 != null) { //print 2nd animal

} ...

Now, write Zoo methods timePasses, allTalk, and feedAll that call the corresponding method for each (non-null) animal in the zoo. Then, add another subclass of Animal; youll find this quite easy since the only methods you have to write are the constructor, talk, and toString.

To submit this weekly exercise, gather all *.java source-code files that have been created for the above classes (in Eclipse, you can find these files in src folder under the folder that has been created for this Java project), place them in a folder, zip/compress and attach the folder.