Transcript Document

Greenfoot
Computer Programming
Lesson 2
Created by NW 2012 – please note all copyright on images used is property of copyright holder. Note: some of the more complicated descriptions are taken
from http://www.greenfoot.org/doc/ so credit to them too!
Objectives
• Develop awareness of Java code.
• Be able to apply learning to programming
Greenfoot.
• Understand how to detect Actors and create
methods.
Created by NW 2012 – please note all copyright on images used is property of copyright holder. Note: some of the more complicated descriptions are
taken from http://www.greenfoot.org/doc/ so credit to them too!
Chapter 2: The First Program: Little Crab
• We will use the little-crab scenario for this chapter.
• Once we open our scenario, you should notice that
there is the usual Actor class, an Animal class, and
the Crab class.
• Animal and Crab inherit instructions from the Animal
class. A Crab is an Animal, and an Animal is an
Actor!
• When we first use this program the crab does
nothing!
Chapter 2: The First Program: Little Crab
• Making the crab move – Open the editor to look at the source
code.
• We look at the act method. The first line of the method is
called the signature of the method.
• The lines of code between the braces is called the body.
• We can put the command - > move( ) ; in the body to make
the crab, well, let’s see… move !
• move( ) ; -> is technically called a method call.
• We can also make the crab turn!
• Use the turn method call
Chapter 2: The First Program: Little Crab
• use turn(5);
• The number 5 in the parentheses specifies how many degrees
the crab could turn. This is called the parameter.
• The degree value is specified out of 360 degrees, so we can
use 0 and 359.
• In the wombat and leaves scenario, the wombats were limited
to the edge of the world. The wombats really just got stuck
on the edge of the world.
• The crab will reach the world edge and turn around.
Chapter 2: The First Program: Little Crab
• The method that will help us to help the crab find the edge of
the world is boolean atWorldEdge( ) - The return type of
boolean will return a value of true or false when the crab gets
to the edge of the world.
• When we look at the act method for crab, we see the
reserved word if (remember Rodney the Robot – selection
structure : ) ). This will test to see if the crab is at the edge of
the world or not! If it is, it will turn!
Last lesson….
Last lesson we learned how to program the simple movement of an Ant. (our actor).
Now our ant is getting hungry after all his twisting and turning so lets give him something
to eat!
1. Right click on “ACTOR” and
choose “new subclass….”
Choose a food you
would like your ant to
eat – don’t forget to
give it a name.
Now add this to your ANT code
Change “Hamburger” to
whatever you called you
food – don’t forget Java is
case SeNsAtIvE!!!
You noticed nothing happens any different…..YET!
Add this:
Change “Hamburger” to
whatever you called you
food – don’t forget Java is
case SeNsAtIvE!!!
PLAY!
Now add your ANT and its food into the world and play. Note what happens when your
ant easts it food!
Now we have played its time to tidy up!
• When you change your code it
is called Refactoring.
• We are going to refactor our
code so that it is easier to
read – at the moment the top
half deals with moving our ant
and the bottom half its food.
• It will be easier if we split
them up – this may be
important later on if we need
to change the code.
Save the world
It’s a bit of a pain when our game is over and we need to rebuild the world every
time.
Lets save it!
Create your layout of how you would like your game to start. Then RIGHT CLICK
on the world and choose “save the world” – this adds code automatically into
our class.
Now after we have played the
game – and click RESET our
world reappears.