Methods & Classes in Alice

Transcription

1 Part A Creating Custom Methods for an Object Alice provides primitive methods with each class to allow the programmer to accomplish some basic tasks such as movement. Some classes also include custom methods which provide actions that are customized to that particular class. For example, the Penguin class provides wing_flap, walk, and jump, and each custom method is written specifically for the Penguin. These methods would not be suitable for some other classes, such as the IceSkater or the Pyramid. In addition, if a class does not provide a method you need, you can write your own custom method for an object of that class. Methods that are part of a class are called class-level methods. These methods cannot be used by any other objects in the world their scope is limited to the class where they were created. Tutorial Creating a class-level method 1. Start Alice and create a new world. Add an instance of the Pterodactyl class (Animals). 2. Select the pterodactyl object and choose the methods tab in the details panel (lower left corner). Click on the create new method button. In the New Method dialog box, enter flapwings as the name of the method, and click OK. A new tab for the flapwings method should appear at the top of the Method Editor, as shown. Since the flapwings method belongs to the pterodactyl object, the full name of the method, using dot notation, is pterodactyl.flapwings. Page 1 of 5

2 3. Complete the flapwings method by creating the instructions shown. 4. In order to test the new method, it is necessary to call the method from world.my first method. Switch method tabs and add the pterodactyl.flapwings method, then run your animation. Recall that an object is an instance of a class. The class is the blueprint, while the object is the physical (or in the case of Alice, graphical) representation of the blueprint. Adding a custom method is a modification to the object only. If you add another instance of the same class, your custom method will not appear in the second object. Page 2 of 5

3 5. Add a second pterodactyl (Animals) to your world. Notice that your second object, which is a new instance of the pterodactyl class, does not have the flapwings method. Part B Saving an Object to a New Class Quite often, when you add a new, class-level method to an object, you will want to create additional objects just like it, each with the new method. To do that, you must save the object to a new class, so that your changes are also saved. The new class will contain all of the custom methods you have added. When you create any new instances of your custom class, each will also have your custom methods. Tutorial Saving an Object to a New Class 1. Change the name of the pterodactyl object to something different, such as flyingpterodactyl. You can change the name of an object by right-clicking on the name in the object tree, and then selecting rename from the menu that appears. 2. Right-click the tile for the flyingpterodactyl object in the object tree and select the save object... menu option. Notice that the name of the file for the new class has already been suggested as FlyingPterodactyl.a2c. Browse to a location on your system where you wish to save the object and then click Save. Page 3 of 5

4 3. Save the world, and then start a new Alice world. 4. Now we will create an instance of the flyingpterodactyl class. Since this is a custom class, it is not located in the standard Alice galleries. Instead, you must browse for your custom file. From the File menu, select Import. Browse to the location on your system where you saved FlyingPterodactyl.a2c, select this file, and click Import. 5. Move the flyingpterodactyl object slightly and add a second instance of the object. Notice that each object already has the flapwings custom method. 6. Test your world by having each flyingpterodactyl flap their wings. Page 4 of 5

5 Inheritance & Reusing Code In the real world, there are many examples of objects which are specialized versions of other, more general objects. For example, an insect is a general specification for a type of organism that has certain properties and characteristics, such as having six legs. Because grasshoppers and bees are insects, they have the same general characteristics of all insects (e.g., six legs). In addition, they have very special characteristics of their own (e.g., grasshoppers can chirp, while some bees can sting). This concept of generalization and specialization can also be applied in programming. Our new class, FlyingPterodactyl, has all of the methods and properties of the Pterodactyl class, plus we added the flapwings method. In programming terms, we would say that FlyingPterodactyl has inherited all of the methods and properties from the Pterodactyl class. Inheritance allows programmers to reuse useful code in many programs without having to rewrite the code every time. Note: As a programming language/environment, Alice is considered to be awkward and weak when it comes to inheritance. The concept is generally correct, but as you move on to more traditional programming languages, you will find inheritance to be even more useful and powerful. Keeping Methods Independent of Other Objects When you are creating a new, class-level custom method in Alice, keep in mind that the objects in your particular program will not necessarily be present in future programs. Thus your class-level methods should not refer specifically to another object, nor should they use methods from other objects. Page 5 of 5