Programming Using C# QUEEN S UNIVERSITY BELFAST. Practical Week 7

Transcription

1 Programming Using C# QUEEN S UNIVERSITY BELFAST Practical Week 7

2 Table of Contents PRACTICAL EXERCISE TASK 1: Zoo Park (Without Inheritance)... 2 TASK 2: Zoo Park with Inheritance... 5 TASK 3: Different Animals make different noises... 8 TASK 4: Multiple inheritance Exercise 2: Testing your knowledge TASK 1: WHAT S THE OUTPUT? TASK 2: Overloading EXERCISE 3: Zoo Interface TASK 1: Zoo Keeper TASK 2: Accepting Input QUEEN S UNIVERSITY BELFAST

3 PRACTICAL 7 Practical 6 introduced you to Windows Forms. This allows you to create more exciting programs with a more intuitive user interface than the Console window. This practical builds on the concepts of objects and in particular introduces inheritance. Inheritance is the concept that when a class of objects is defined, any subclass that is defined can inherit the definitions of one or more general classes. EXERCISE 1 The first part of this exercise will set up a Zoo program without the use of inheritance. We will then move on to create it using inheritance and you should see the benefits of being able to control how your objects behave. TASK 1: Zoo Park (Without Inheritance) This task requires you to create a console application which will print the details of several animals in a zoo. Step 1: From the Visual Studio File menu, select New (or press Ctrl+Shift+n), then click on Project. From the New Project window that opens select C#, Console Application and name it e.g. Prac07Ex01Task01. Step 2: Right Click on Program.cs in the Solution Explorer and Rename to ZooPark.cs and select Yes in the followed popup. Step 3: As well as the ZooPark class we need to create an Animal class. The ZooPark class is where you will create the animal objects and print out the details to the console. Right click on the Project in the Solution Explorer, select Add and click Class to add a new class. Name it Animal and click Add. NOTE: The Animal class will be a blue print for the animal objects and will contain the states and behaviours relevant to all animals. We will now add several variables and methods to the Animal class. Step 4: Let s declare the attributes (states) the animal will have e.g. name, age etc. declared in the Animal class, see Figure 1. This is Figure 1 Step 5: Now we will add a constructor to the Animal class that will allow you to create an instance of the Animal class with the required details, see Figure 2. Figure 2 2 QUEEN S UNIVERSITY BELFAST

4 Step 6: We have enough set up to be able to create Animal objects in our ZooPark class. Let s now navigate to the ZooPark.cs class and enter the code as per Figure 3. Figure 3 Step 7: We will know add methods to the Animal class that will allow each animal to carry out several common functions: Sleep(): a method to make the animal lie down and take a nap Eat(): a method to make the animal eat MakeNoise(): a method to allow the animal to make a sound Return to the Animal.cs class and enter the code as per Figure 4. Figure 4 Step 8: Our class now allows the animals to eat, sleep and make a noise. However, what not all animals make the same noise and they don t eat the same food. Therefore we need more methods added, see Figure 5. Figure 5 3 QUEEN S UNIVERSITY BELFAST

5 As you can see the class is becoming quite cluttered and contains methods that will not be used by all animals. For example, wolves will not make an eagle noise or a lion noise but the animal class contains methods to allow this. We will now use inheritance to create a base Animal class which will give the animals everything they have in common e.g. Name, Make a Noise and then create sub classes that inherit from the base class and contain more specific behaviours. This is best described using UML diagrams. Figure 6 provides a UML for the Zoo. Figure 6 4 QUEEN S UNIVERSITY BELFAST

6 TASK 2: Zoo Park with Inheritance We will create the Zoo using inheritance in this part of the practical. This should enable you to see the benefits of being able to control how your objects behave both generically and specifically. Step 1: From the Visual Studio File menu, select New (or press Ctrl+Shift+n), then click on Project. From the New Project window that opens select C#, Console Application and name it e.g. Prac07Ex01Task02. Step 2: Navigate to your Solution folder using Windows Explorer. Open the Task01 folder and copy the Animal.cs and ZooPark.cs files and paste them into the Task 02 folder (again using Windows Explorer). Step 3: In Visual Studio right click on the project in the Solution Explorer window and select Add>Existing Item. This should open the Task 02 folder and you should see the Animal.cs and ZooPark.cs files. Click on Animal.cs and the click the add button. Step 4: Repeat this for the ZooPark.cs file. Finally delete the Program.cs file as this will cause a conflict both this file and ZooPark.cs hold a Main method and C# doesn t know which one to execute. Your Solution Explorer should now reflect the changes, see Figure 7. Delete this file! Figure 7 Step 5: Open the Animal.cs class and change the namespace to reflect the Project name i.e. Task 02. Then repeat this for the ZooPark.cs class. Step 6: Navigate back to the Animal.cs class. Remove all methods except for eat(), sleep() and makenoise() (you should not delete the constructor). Note: The Animal.cs class is the base class which contains all of the generic states and behaviours that each animal will have. However, the zoo contains several different animals including Tigers, Eagles and Wolves. We will now have to create a class for each specific animal but it will be able to inherit generic states and behaviours from the Animal base class. Step 7: Create a Tiger class that inherits from the base Animal class. To do this, add a new class to your project called Tiger.cs which will create a blank class, see Figure 8. Figure 8 5 QUEEN S UNIVERSITY BELFAST

7 NOTE: We mentioned that you should inherit the generic states and behaviours from the base class (i.e. the Animal.cs class). This will mean that we can apply any generic attributes or functions but in the sub class we can then define more specific attributes and functions (refer to the UML in Figure 6). The syntax is as follows: Step 8: Therefore to allow the Tiger to inherit from the Animal class, change the class definition to reflect Figure 9. Figure 9 Step 9: Let s navigate back to the ZooPark.cs class where you will see the three Animal objects created. Comment out the tonytiger Animal object; this has been set up using the base class but we now want it to be of the specific subclass Tiger. Now create an instance of the Tiger class to see if it does in fact inherit from the Animal class, see Figure 10. Figure 10 Note: we currently have no constructor in the Tiger and therefore we do not need to specify values for any attributes. Step 10: If you try to access the public methods and variables of the Animal object and the Tiger object, you can see that Tiger has inherited all of the public methods and attributes, see Figure 11. Figure 11 Step 11: Now let s create the constructor for the Tiger. It will inherit the attributes from the base class and also set a species and a number of stripes. Return to the Tiger.cs class and add a constructor, see Figure QUEEN S UNIVERSITY BELFAST

8 Figure 12 Note: We pass in the values for name, diet, location, weight, age, colour, species and numstripes. The first six are immediately passed to the base class to set up the appropriate generic instance variables. The species and numstripes parameters are then used within the Tiger constructor to set up the specific instance variables relating to the tiger. Step 12: Return to the ZooPark.cs class and amend the creation of the Tiger object, see Figure 13. Figure 13 Step 13: Create classes for a Wolf and an Eagle that all inherit from the base Animal class, as per the details in the UML, see Figure 6. Alter the creation of the two remaining Animal objects so that they are now an Eagle object and a Wolf object, see Figure 14. Figure 14 Note: Now you have subclasses for each animal that all inherit from the base Animal class. We also have specified each subclass of animal, in that they all take the attributes from the base class but the Tiger and Eagle has then more specific states. The idea of inheritance implements the IS-A relationship. For example, Tiger IS-A animal and Wolf IS-A Animal. Step 14: We now need to add some methods which are unique to each animal subclass. We will use the Eagle as an example. Therefore, open the Eagle class and add methods, see Figure 15. Figure 15 Note: If you try to access the public methods and variables of an Eagle object, you can see that Eagle has inherited all of the public methods and attributes of Animal but also has its own specific methods and attributes as shown below. 7 QUEEN S UNIVERSITY BELFAST

9 TASK 3: Different Animals make different noises Tigers roar, wolves howl and eagle s whistle. Each of the classes that inherit from Animal will a have makenoise() method, but each of those methods will work a different way and have different code. When a subclass changes the behaviour of one of the methods that it inherited, we say that it overrides the method. Therefore, when you have a subclass that inherits from a base class, it must inherit all of the base behaviours but you can modify them in the subclass so they are not performed exactly the same way i.e. they become specific. Step 1: Continue working on your project from Task 2 (Exercise 1). To allow the base methods to be overridden, the base methods must be marked as virtual. The virtual keyword designates a method that is overridden in subclasses. Open the Animal.cs class and mark the makenoise() method as virtual as shown below: Figure 16 Step 2: Since the base makenoise() method is now virtual, we can override this method in the Tiger class to allow the Tiger object to ROAARRRR when the makenoise() method is called. To do this, create a makenoise() method in the Tiger class and include the override keyword in the method declaration, see Figure 17. Figure 17 Step 3: Return to the ZooPark.cs class and call the makenoise() method on a Tiger object and an Animal object, see Figure 18. Figure 18 Step 4: Save your work (Ctrl+Shift+S), run the program by clicking the start button on the toolbar or use the shortcut Ctrl+F5, see Figure 19. Figure 19 Step 5: Add an override method for makenoise() in the Eagle and Wolf subclasses to perform the correct animal noise e.g. Wolf howls, Eagle whistle. Step 6: We will now override the eat() method in the base class and then for the Tiger subclass to allow the tiger to eat the correct amount of meat. First amend the eat() method as virtual in the base Animal class, see Figure QUEEN S UNIVERSITY BELFAST

10 Figure 20 Step 7: Now create an eat() method in the Tiger class and include the override keyword in the method declaration, see Figure 21. Figure 21 Step 8: Return to the ZooPark.cs and call the eat() method on a Tiger object, Wolf object and an Animal object to view which method is executed, see Figure 22. Figure 22 Step 9: Save your work (Ctrl+Shift+S), run the program by clicking the start button on the toolbar or use the shortcut Ctrl+F5, see Figure 23. Figure 23 Note: As we have not yet overridden the eat() method in the Wolf subclass it looks to the parent (or base class). Therefore, when we call the eat() method on the Wolf object it returns An animal eats. Step 10: Override the eat() method in each subclass to allow each animal to eat the correct food and the correct quantity e.g. Wolf eat 10lbs of meat, Eagles eat 1lb of fish. Step 11: Test the eat() method for each of the objects. Save your work (Ctrl+Shift+S), run the program by clicking the start button on the toolbar or use the shortcut Ctrl+F5. Step 12: Time to be creative so decide on a new method that the animal will perform. Add a new method in the Animal class that can be overridden and then override the method in the subclasses of the animals that can perform this action. Step 13: You can also complete the other methods that are part of the subclasses with appropriate information and test them. 9 QUEEN S UNIVERSITY BELFAST

11 TASK 4: Multiple inheritance So the Zoo Park is starting to take shape but could we make it better. At the moment Tiger inherits from the base class Animal but if a Lion joined our Zoo than since they are both Big Cats they will have some common methods and attributes. Why don t we make a Feline class that inherits from Animal and make the Tiger inherit from the Feline class instead of the Animal class which will contain all of the common methods and attributes specific to Cats and also all from the Animal base class. Figure 24 illustrates an update UML. Figure 24 Step 1: Add the new Feline class that inherits from the base Animal class, see Figure 25. Figure QUEEN S UNIVERSITY BELFAST

12 Step 2: We will now need to alter the Tiger class as it needs to inherit from the Feline (rather than Animal) class. Amend the code in Tiger.cs as per Figure 26. Figure 26 Step 3: Add the common Feline attributes and methods to the feline class, as per the UML in Figure 24. Step 4: Return to ZooPark.cs and call a method of the Tiger class, the Feline class and the base Animal class, see Figure 27. Figure 27 Step 5: Save your work (Ctrl+Shift+S), run the program by clicking the start button on the toolbar or use the shortcut Ctrl+F5, see Figure 28. Figure 28 Note: The sleep() method is executed in the Feline class for the tiger object but for all other objects the method in the base class is executed. The eat() method is executed in the Tiger class. Step 7: The Zoo has a new arrival and it is a Lion. Now add a new Lion class that inherits from the Feline. Step 6: The Zoo has another new arrival and it is a Penguin. Now add a new Bird class that inherits from the base Animal class as there are now two birds that will share some common attributes and methods e.g. wingspan and bird species. Step 7: You will also need a new Penguin class that inherits from the Bird class you should also change the Eagle class to inherit from the Bird class instead of directly from the Animal class. Think of methods and attributes that only birds would have and methods and attributes that are unique to each type of bird. For example, you could add a Fly method in to the Bird class and in each subclass state if that bird can fly or not by overriding the method. Step 8: Remember to fully test your program using the ZooPark.cs file. 11 QUEEN S UNIVERSITY BELFAST

13 Exercise 2: Testing your knowledge TASK 1: WHAT S THE OUTPUT? You should try to work out the code in Figure 29 on paper and then transfer the code into Visual Studio and run it to see what output you actually get. Remember that if you don t get the answer you expect to check both your workings on paper and the code Figure 29 Part A: What will the following code display given the code in Figure 29? 12 QUEEN S UNIVERSITY BELFAST

14 Part B: What will the following code display given the code in Figure 29? TASK 2: Overloading Overloading is what happens when you have two methods with the same name but different signatures. Step 1: From the Visual Studio File menu, select New (or press Ctrl+Shift+n), then click on Project. From the New Project window that opens select C#, Console Application and name it e.g. Prac07Ex02Task02. Step 2: Add two methods that have the same name but the parameter list in the method header is different, see Figure 30. Figure 30 Note: We now have two methods that can be called by passing in one variable or two and print out the variables to the Console. Step 3: Call each method using different parameters, test your code and view the Console, see Figure 31. Figure 31 At compile time, the compiler works out which one it's going to call, based on the compile time types of the arguments and the target of the method call e.g. if the method call contains a string and an int, call the method that prints both. C# generally does not allow two methods in the same class to have the same name unless the number or type of the parameters differs. 13 QUEEN S UNIVERSITY BELFAST

15 EXERCISE 3: Zoo Interface The exercise will use Windows Forms Applications to demonstrate inheritance and to further enhance the use of GUI s. We will continue to use the Zoo example. TASK 1: Zoo Keeper Step 1: From the Visual Studio File menu, select New (or press Ctrl+Shift+n), then click on Project. From the New Project window that opens select C#, Windows Form Application and name it e.g. Prac07Ex03Task01. Step 2: Right click on Form1.cs in the Solution Explorer and rename to ZooKeeper.cs. Step 3: Using the Properties window, change the title of your form to something meaningful e.g. Zoo Keeper. Step 4: Navigate to your Solution folder using Windows Explorer. Open the Exercise 1 Task 2 folder and copy the Animal.cs, Feline.cs, Bird.cs, Tiger.cs, Wolf.cs, Eagle.cs, Lion.cs and Penguin.cs files and paste them into the Exercise 3 Task 1 folder (again using Windows Explorer). Step 3: In Visual Studio right click on the project in the Solution Explorer window and select Add>Existing Item. This should open the Task 01 folder and you should see all the files you have just copied. Click on them all while holding down the CTRL key on the keyboard and the click the add button. Step 4: Update the namespace reference for these files. Step 5: Create the GUI illustrated in Figure 32 using the toolbox to add elements to the form e.g. Labels, Buttons and text boxes. You can reposition any element on the screen by clicking and dragging the element to the desired location. You should also remember to name each of the elements appropriately Figure 32 Note: To check that the code will work as designed we will test if, when we press one of the buttons the Current Information text box is populated. We will then move on to take user input. 14 QUEEN S UNIVERSITY BELFAST

16 Step 6: As the instance variables are private we will need accessor and mutator methods to access and change information held in these variables. Refer to practical 4 and create these methods for the Animal.cs class. Step 7: Double click on the Eagle Button to create the click event method automatically and add the following code. For this to work at this stage we will need to create an instance of the Eagle, see Figure 33. Figure 33 Step 8: Save your work (Ctrl+Shift+S), run the program by clicking the start button on the toolbar or use the shortcut Ctrl+F5, see Figure 34. Figure QUEEN S UNIVERSITY BELFAST

17 TASK 2: Accepting Input Step 1: Continue working on the same project, right click on the Project in the Solution Explorer and select Add -> Windows Form. Name the Form EagleInut and click Add. Create a user interface similar to Figure 35. Remember to name all of the input fields appropriately. Figure 35 Step 2: Navigate to Form1.cs and create a new object reference to the EagleInput form, see Figure 36. Figure 36 Step 3: We will not update the code for the Eagle button (in the Form1.cs) so that when clicked it opens the new EagleInput form, see Figure 37. Figure 37 Step 8: Save your work (Ctrl+Shift+S), run the program by clicking the start button on the toolbar or use the shortcut Ctrl+F5. Test the program by clicking on the Eagle button the Eagle Input form should open. We will now implement the Submit Details button to actually create a new Eagle. 16 QUEEN S UNIVERSITY BELFAST

18 Step 9: Navigate to the EagleInput.cs [Design] file and double click on the Submit Details button. When this button is clicked the details will be read from each field in the form and a new Eagle object will be created. The information will then be printed in a pop up window, see Figure 38. Figure 38 Step 8: Save your work (Ctrl+Shift+S), run the program by clicking the start button on the toolbar or use the shortcut Ctrl+F5. Test the program by clicking on the Eagle button the Eagle Input form should open, enter details and click on the Submit button. Step 9: Now add the remaining functionality to this program for the rest of the animals. 17 QUEEN S UNIVERSITY BELFAST