Class-level Methods and Inheritance

Part 1Alice

Animated Actions Some actions are more naturally associated with a specific class of objects rather than with the overall world.

ExamplesA person walking

A wheel rolling

A fish swimming

Class-level Methods

We can write our own methods to add abilities/functions to a specific class of objects

This will result in a class-level rather than a world-level method

The next few slides will demonstrate how to build class-level methods

An example (building technique)

How can we create a skate method for ice skater objects?

We need to:

(1) tell Alice to associate the new method (the one we are about to write) with an ice skater, and

(2) write a new method to animate the ice skater skating.

Demo: The solution

First, to associate the animation with the ice skater

• select the iceSkater tile in the Object Tree

•select the methods tab in the details area

•click on the "create new method" button

Storyboard for skate

The slide actions each require several motion instructions, so we will break these two actions down into smaller pieces… this technique is called stepwise refinement

Skate:Do together move skater forward 2 meters Do in order slide on left leg slide on right leg

Refined storyboard for skate

Refinement of slideLeftDo in order Lift right leg and turn upper body forward Lower right leg and return body upright Skate:

Do together 1) move forward 2 meters 2) Do in order slideLeft slideRight

Refinement of slideRightDo in order Lift left leg and turn upper body forward Lower left leg and return body upright

Writing the code

The next step is to translate the design into program code.For the slideLeft method, a possible translation is:

Design step InstructionLift the right leg turn the right thigh forwardTurn upper body forward turn the abs forward Lower the right leg turn the right thigh backward

Return the body upright turn the abs backward

Demo

Demonstration of slideLeft

slideRight

skate

Correspondence of design to code

Skate:Do together move skater forward 2 meters Do in order slide on left leg slide on right leg

Question

Writing the methods to make an ice skater perform a skating motion is an intricate task.

Most likely, you would like to reuse these new methods in other worlds.

How can you make the skate method available for an ice skater in a different world?

Answer: Save out as a new class

1) Rename iceSkater as cleverSkater.

2) Save out as a new class. Alice saves the new class as CleverSkater.a2c

Inheritance

The CleverSkater class inherits all the properties and methods from the original IceSkater class, and also

has the newly defined methods (skate, slideLeft, slideRight)

In other programming languages, the concept of creating a new class based on a previously defined class is called inheritance.

Using CleverSkater

An instance of the CleverSkater class can be added to a new world – use File|Import.

Benefits of Inheritance

Inheritance supports the reuse of program code -- allows the programmer to write code once and use it again later in different programs.

sharing code with others in a team project

Assignment

Read Chapter 4-3 Class-level methods and Inheritance

Read Tips & Techniques 4 Visible and Invisible Objects