Carleton University COMP1406/1006, Summer 2013 Tutorial 2

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1 Carleton University COMP1406/1006, Summer 2013 Tutorial 2 Tuesday July 16 and Thursday July 18 When you have completed this tutorial, you should 1. Be comfortable with basic inheritance and overriding methods. 2. Be comfortable with static attributes and methods. 3. Have more fun with Java! 0: HP 4155 [reminder] Here are some helpful reminders about using the Windows machines in HP In particular, be careful of where you save your data while working on the school machines. Lost data on the school machines because you did not move the data to your Z drive, dropbox, google drive, etc., before you logged off will not be accepted as an excuse for late or missed assignments. 1: Inheritance 101 When a class (the child class) inherits from another class (the parent class), all of the attributes and methods (except the constructors) that the parent class are automatically also in the child class. Create a Java class called Parent. You class should have two attributes, name and x (a String and an int), and one method public int add(int n){...} which adds the input n to the integer attribute, x, of the object. Create another class, called Tut2, with a main method to test your class. Instantiate some Parent objects, and use the add() method. Something like Parent p = new Parent(); Parent q = new Parent(); p.x = 2; q.x = 5; 1

2 System.out.println(p.x + q.x); System.out.println(p.add(10)); System.out.println(p.add(10)); Create a Java class called Child that extends the Parent class from above. Leave your Child class empty. Modify your Tut2 class by first duplicating the entire body of the main method (so it appears twice) and then changing all instances of the word Parent with Child in the copied part. Run your program. Notice that there is no difference bwteeen using a Child or Parent object, Since we did not add anything to the Child class, it is essentially identical to the Parent class. Each have the same attributes and the same add method. Now add an attribute (integer y) to the Child class. Experiment in your Tut2 class with this new attribute (access it, change its value, print it to standard output, etc). Notice that only Child objects can use this y attribute. What happends if you try to access this attribute from a Parent object? 2: Inheritance 102: Basic Inheritance and Overriding Methods All Java classes, except Object, extends exactly one other class (either explicitly using the extends keyword or implicitly). The next few problems will give you practice working with basic inheritance and overriding methods. All classes have Object as an ancestor. As a result, every class has a common set of methods (the methods in Object). Look at the API for the Object class (found here) and see what methods are there. An important method from Object is the tostring() method. This returns a String that is a text representation of the object. It is often very useful to override this method so that we can display useful (i.e., human readable) information about an object. Write a Java class called Book, that has several attributes: name, author, number of pages and year the book was published. Create an appropriate constructor and override the tostring() method so that the following code Book cat = new Book("The Cat in the Hat", "Dr.Seuss", 61, 1957); System.out.println(cat); will display The Cat in the Hat, by Dr.Seuss. Published in 1957 You will override the tostring() method in this way for many of the classes that you will write in Java. 2

3 When overriding the tostring() method, make sure that the String that is returned does not contain a newline character at the end unless this is something you specifically want. (Think about why you don t want a newline.) Write a Java class called Electronic that extends the Book class that you just wrote. This class will be for electronic books (which are also books). Because Electronic extends Book, the Elecronic class has all of the attributes and methods (not including the constructors) that Book has. In your class create one constructor (with 4 inputs) just as you did for Book. In fact, just copy and paste the code you used for your Book constructor. Next, add a main method that simply creates an instance of Electronic and tries to print it (just as you did with Book). You should get an error similar to Error: /Users/.../Electronic.java:1: cannot find symbol symbol : location: constructor Book() class Book Why did this happen? When a class inherits from another class (the parent), Java will automatically call the parent s zero-argument constructor before executing any code that you have written. Since Book has no zero-argument constructor, you get the error. There are two ways to stop Java from automatically calling the parent class s zero-argument constructor. If the first line of your constructor has an explicit call to any of the parent class s constructors, using the super() method, the compiler will only execute the specified parent constructor. The other way is if the first line of your constructor explicitly calls another of the classes constructors using the this() method. (We won t talk about this second method now.) For our Electronic class, in order to do this correctly, our constructor might look like public Electronic(String name, String author, int pages, int year){ } super(name, author, pages, year); Our constructor simply calls the 4-argument constructor from the parent Book class. We specify which constructor to use by simply giving the correct input arguments to the super() method. We could add more lines of code after the call to super() so that our constructor does more things if needed. This is called constructor chaining. Each time you call a constructor, it calls a constructor from the parent class so that initialization of all the things (attributes) you get from the parent class happen automatically. When the parent s constructor is called, it will call a constructor of its parent class, and so on, until we reach Object. Modify your Eletronic class so that it overrides the tostring() method that it inherits. 3

4 (Note: it inherits the overridden version from Book.) Override it so that when you print out an Electronic object, it looks like The Cat in the Hat, by Dr.Seuss. Published in [electronic] Notice that most of the output is identical to what the Book version would have given you. Instead of duplicating the code that is the same, wouldn t it be nice if we could just call Book s version of the method and then append (concatenate) the [electronic] to the end of it? In Java, we can call a parent s method (even if we override it in the child class, and even call it in code that we are writing to override it!). In a child class in Java, we can access methods from its parent class using super. Calling super.tostring(), for example, will call the tostring() method from the super class. (In our case, the Book class). Override the tostring() method in Electronic to incorporate Book s version of tostring(). You can only use super to access a class s parent class. You can cannot do something like super.super.tostring() to try and access a grandparent s method. 3: Fun with Static In a class we have attributes and methods. If we only consider non-static attributes and methods, we can view a class as a cookie cutter or a template that we use to create objects (the cookies). Each object has its own attributes and methods, as specified by the class. For example, if a Student class has a name (String) attribute, then every Student object has its own name attribute once it is instantiated (created). Static attributes and methods are different. These belong to the class (Student for example) and not to individual objects of the class. In fact, they exist even if there are no instances of the class. (That is, we did not create any objects of the class using new). We have already seen static methods. The main method used to run a program is static. All of the method in the Math class are static. We call methods from the Math class like Math.sin(0.2) without ever creating a Math object. Think about how this is different than accessing the methods from the Random class (from Tutorial 1). Static attributes are similar. They are variables that we can access from a class without needing any objects of that class (think of E and PI from Math class). They can also be variables that objects of the class share with each other. Write a class called Bunnies. The class will have the following attributes /* attributes */ private static int[][] field; public int xpos; public int ypos; public double life; 4

5 The field attribute will be shared by all the bunnies. It is private, because only bunnies will have access to it. Your Bunnies class should have a method public int initfield(int length, int width, double prob) that initializes the field. It creates a length X width array of integers. Each array element with be a 3 with probability prob and a 0 with probability 1-prob. The method returns how many array elements are 1. In your class, you will have a main method that runs a simple simulation about bunnies. First, initialize the playing field and then create an array of bunny objects, with random locations. (Their location: xpos and ypos correspond to field[xpos][ypos]) Your simulation will consist of several time steps. At each time step, you let each bunny move to a new place in the field (that is adjacent to the current position). For example, xpos and/or ypos will change by 1 (plus or minus). Once the bunnies move, they will eat food if food exists in the current position. If the current value of element of field is greater than 0 then food exists. When a bunny eats food, you decrement the value of the field array element. Each time step that bunny does not eat, it life value decreases by 0.1. Each time a bunny eats, its life value increases by Once a bunny s life value is less than or equal to zero, it is not longer living. Run your simulation for many steps. How many steps does it take until all the bunnies have died? Add a mechanism for food to replenish itself in some way on the field. (Assuming food is grass...random grass growth? Random rain which makes grass grow everywhere? Grass regrows after 5 time steps of not having grass?) Add some logic to the bunnies. When they take a step, allow them to look at what is around it and let them decide (very basic AI) where to move to. Add a visualize method that gives a visualization of your field and where the living and dead bunnies are located. 4: More Coding Didn t finish all the question from Tutotial 1? Still want to write more Java code? Try these. Write a class called DogYears that mimics the dog years conversion calculator found at Use the formula given on the webpage to do the calculations. 5

6 If a single command line argument is given (assume it is a double) use it as the input year. If no command line arguments are given, the program should prompt the user and allow for user input (use Scanner) to read in the input year. Based on the input year, whether from command line or user input, calculate both years just as the calculator in the link does. Write a class called PrimeCount which takes a single command line argument as input, call it n, and computes the number of prime numbers less then or equal to n. Recall that a prime number that is evenly divisible by only 1 and itself. Assume that n 1. Write a class called Lagrange4 which uses one command line argument (a positive integer, which we ll call n). Your class will find four non-negative integers a, b, c, d such that n = a 2 + b 2 + c 2 + d 2 When the four numbers are found, your class will display these numbers to standard out. See s_four-square_theorem for more information about this. 6