Getting started¶

Now is a good time to start working with the AP Computer Science Case Study, which is a program called GridWorld. To get started, install GridWorld, which you can download from the College Board.

When you unpack this code, you should have a folder named GridWorldCode that contains projects/firstProject, which contains BugRunner.java.

Make a copy of BugRunner.java in another folder and then import it into your development environment. There are instructions here that might help.

Once you run BugRunner.java, download the GridWorld Student Manual.

The Student Manual uses vocabulary I have not presented yet, so to get you started, here is a quick preview:

• The components of GridWorld, including Bugs, Rocks and the Grid itself, are objects.
• A constructor is a special method that creates new objects.
• A class is a set of objects; every object belongs to a class.
• An object is also called an instance because it is a member, or instance, of a class.
• An attribute is a piece of information about an object, like its color or location.
• An accessor method is a method that returns an attribute of an object.
• A modifier method changes an attribute of an object.

Now you should be able to read Part 1 of the Student Manual and do the exercises.

BugRunner¶

BugRunner.java contains this code:

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import info.gridworld.actor.ActorWorld;
import info.gridworld.actor.Bug;
import info.gridworld.actor.Rock;

public class BugRunner
{
    public static void main(String[] args)
    {
        ActorWorld world = new ActorWorld();
        world.add(new Bug());
        world.add(new Rock());
        world.show();
    }
}

The first three lines are import statements; they list the classes from GridWorld used in this program. You can find the documentation for these classes here.

Like the other programs we have seen, BugRunner defines a class that provides a main method. The first line of main creates an ActorWorld object. new is a Java keyword that creates new objects.

The next two lines create a Bug and a Rock, and add them to world. The last line shows the world on the screen.

Open BugRunner.java for editing and replace this line:

world.add(new Bug());

with these lines:

Bug redBug = new Bug();
world.add(redBug);

The first line assigns the Bug to a variable named redBug; we can use redBug to invoke the Bug’s methods. Try this:

System.out.println(redBug.getLocation());

Note: If you run this before adding the Bug to the world, the result is null, which means that the Bug doesn’t have a location yet.

Invoke the other accessor methods and print the bug’s attributes. Invoke the methods canMove, move and turn and be sure you understand what they do.

Exercises¶

import java.awt.Color;

Color purple = new Color(148, 0, 211);

int x = bug.getLocation().getCol();