Think Java:

How to Think Like a Computer

Scientist

5.1.2

by Allen B. Downey

Chapter 5:

Introduction to GridWorld

Agenda

• History and motivation

• Getting started with GUI controls

• Coding in GridWorld

• Hint for Ex1

• Hint for Ex2

• Hint for Ex3

• Start Assignment 5

• Review Practice Test Solutions

• Questions on Assignment 3 and 4

2

3

GridWorld is a Case Study for HS AP Exam

• Give students a larger program to study and extend– context to apply inheritance and polymorphism

• Serve as an example of good design and coding practices– and a context for discussion of issues

• An early study showed that students who had a case study performed better on the AP exam than those who didn't– women perform better than men usually on case study

questions

4

Goals of GridWorld

• Provide open-ended framework for more advanced coding projects

• Be rich enough for a variety of exam questions

• Engage students in a more dynamic problem domain

• Help illustrate more abstract object-oriented programming concepts

GridWorld Demo

• 2D Arcade Game

• Objects: Bug

– Flower

– Rock

• Controls to run/stop

• Ability to add or

modify objects

Setting Up in BlueJ

• First, download the zip file linked from the lab5 handout. Extract this and place on your desktop.

• In BlueJ, choose Tools > Preferences > Libraries

– Then ADD, Navigate to GridWorldCode folder

– Choose gridworld.jar

– Shut down BlueJ and restart

Start Lab 5

• Explore GridWorld's GUI interface by following the instructions on page 1 and 2 of the lab5 handout.

Agenda

• History and motivation

• Classes, Objects and Methods

• Coding in GridWorld

• Hint for Ex1

• Hint for Ex2

• Hint for Ex3

• Start Assignment 5

• Review Practice Test Solutions

• Questions on Assignment 3 and 4

8

Classes, Objects and Methods

• Objects are software entities

that model real world things

• Objects have attributes

– like color (red), direction—facing N,

– health status, etc.

• Objects have methods to allow you to see attributes (getColor, getLocation…) and modify attributes (move, turn, …)

Classes, Objects and Methods

• These are the methods available for Bug objects:

Classes, Objects and Methods

• These are the methods available for Flower objects:

Classes, Objects and Methods

• These are the methods available for Rock objects:

Objects vs Classes

• A class is like a blueprint or design used for creating objects.

• One class can generate thousands of objects.

• "Class" is kind of like "Data Type"

• "Object" is kind of like "Variable"

Agenda

• History and motivation

• Classes, Objects and Methods

• Coding in GridWorld

• Hint for Ex1

• Hint for Ex2

• Hint for Ex3

• Start Assignment 5

• Review Practice Test Solutions

• Questions on Assignment 3 and 4

14

Coding in GridWorld

• We are using a complex software framework already developed for us

• Very typical of real software development

• Have to import software packages we are using

– are in gridworld.jar

– Documentation

Objects are created in code using newfollowed by a constructor method

• To make an object, we use the new operator

Referring to and Modifying Objects

• Some objects can be created anonymously and passed to other methods, like:

•world.add(new Bug());

•world.add(new Rock());

• But if we want to access an object, we must store it in a variable of that class

•Bug redbug = new Bug();

•redBug.move();

Hints on Exercise 1

• We can invoke move() directly on redBug

– redBug.move();

• Or we can pass redBug to a method and have it implement more complex commands– moveBug(redBug);

Hints on Exercise 1

public static void moveBug(Bug someBug, int n)

{

if (n>0)

{

if ( someBug.canMove() )

{

someBug.move();

// invoke moveBug again with n-1

}

}

}

Hints on Exercise 2

import info.gridworld.actor.Actor; // add this at top

• ActorWorld world =

new ActorWorld (new UnboundedGrid<Actor>());

If you finish Lab 5 early, start working on Assignment 5