COMP 105 Introduction to Computer Science

Kalamazoo College Computer Science Program Fall 2008

Lab 2: Inheritance and Class Level Methods

Introduction

The purpose of this lab is to practice working with class level methods.

Exercises

• Litter Collector *

  The goal of this exercise is to create a character that specializes in cleaning up untidy Alice worlds by stacking up all of the objects in one place. The first step is to select any character from the Alice gallery that has some sort of movable appendage that could be used to pick things up and throw them. We will code the cleaning animation in several steps:

  • Step 1: Picking Up Litter

    The first step is to give your character a class level method called "pickup" that moves the character to an object and performs the pick-up animation. This method should take a single parameter specifying the object to be picked up. Don't worry about making your animation perfectly realistic. Anything that creates the impression of an object being picked up will suffice. Remember to add appropriate comments for this method. These should describe what the method does, and what its parameter is.

  • Step 2: Throwing the Litter

    Once your character can pick up litter, it needs to be able to throw the litter on the trash pile. Create a new class level method called "throw" that animates a throwing motion and moves the litter object to its destination. This method should take two parameters: one specifying the litter object to be thrown, and one specifying the object that the litter should be placed on top of. Remember to add appropriate comments for this method.

    Hint: One way to create the effect of one object being thrown on top of another is to use the "move to" method along with the "position" option. Once the position option has been selected you will see some code that says: "offset by = vector3(0,0,0)." You can create a specific offset using the world level function "right, up, forward". This is located at the very bottom of the list of world level functions. It probably won't be possible to perfectly position one object on top of another. Do your best.

  • Step 3: Putting the Pieces Together

    Now that we have methods for picking up objects and throwing them, writing a "cleanUp" method that combines those two steps should be a breeze. The "cleanUp" method should take two parameters: a litter object, and an object that the litter should be placed on top of.

    Test your completed method by programming your character to make a stack of litter. The first call to "cleanUp" will place object2 onto object1, the second call will place object3 onto object2, and so on.

    Once you have tested your character in a simple world, save the character so that it can be used in other Alice worlds. You can do this by right clicking the character and selecting the "save object..." option.

    Further test your newly created class by opening an existing world and creating an animation in which the character stacks several objects on top of each other using the "cleanUp" method. You can bring your character into the new world by using the "Import" option under the file menu. Make sure that the "cleanUp" method works well for litter and target items of various sizes.

    Remember to add appropriate comments for this method. You should also make sure you have added your name, date, and a program description at the beginning of the program.

    When you are finished, print your code and hand it in.

• Heavy Litter **

  You may feel that the "clean" method from the previous exercise looks unrealistic because it results in tall stacks of precariously balanced objects. It might be slightly more realistic if we changed the animation so that every time an object is stacked on top of another object, the top object squashes the bottom one. Create a new class level method "cleanWithSquash" that implements this.

  Notice that the first steps in the "cleanWithSquash" animation are exactly the same as the "clean" animation. Rather than re-writing all of that code, your "cleanWithSquash" method should include a call to your "clean" method as its first step.

  You can animate the squashing effect for the bottom object by using the resize method, and selecting "dimension -> top to bottom" under the "more..." menu. You'll need to do some math to determine how far the top object should move down when the bottom object is squashed.

• Extras **

• Depending on how you animated the throw, it is likely that the speed of the litter as it travels through the air depends on how far away the thrower is from the target. Change your code so that the object moves at the same speed no matter how far it is thrown. Hint: distance = rate * time.
• Add a third parameter to the "clean" method that specifies how fast the litter should be thrown in meters/second.
• Modify the code so that the thrower yells out the name of the object that is being thrown.