Transcript Objects First With Java

Greenfoot
Programming
simulations and
games in Java
Lecture 4. Greenfoot
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Pablo Romero, Department of Informatics
Lecture plan
Greenfoot in action
 The Wombats scenario
 Modifying the code

* Greenfoot Designed and implemented at the University of Kent and
Deakin University. Copyright held by Poul Henriksen and Michael
Koelling. Slides based on those by Eueung Mulyana
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Greenfoot
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Greenfoot
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Looking at the classes
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The Wombat class
import necessary packages
class header;
Wombat is a subclass
from Actor
data (constants and
variables)
class constructor and
methods
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Constructor and leaf methods
constructor  initialising
direction and leavesEaten
is there any Leaf object
in my position?
this method is inherited from
the superclass Actor
remove that Leaf object
update the variable
leavesEaten
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if you find leaves, eat !
The act method
if you don‘t find leaves, but
you can move forward, then
move!
if you don‘t find leaves and
you also cannot move, then
turn to the left!
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The turnLeft method
change direction 90 degrees
to the left
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The setDirection method
this method is inherited from
the superclass Actor
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The move method
if object reaches one of
the borders, do nothing!
the object moves
forward;
depends on
its direction
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setLocation() is
inherited from Actor
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The canMove method
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The canMove method
these methods are
inherited from Actor
new coordinate
if the object moves
forward;
depends on
its direction
checks if the object
reaches the edges of the world
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The WombatWorld class
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Constructor
populate
randomLeaves
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The WombatWorld constructor
calls the World constructor
sets the background.
the method is inherited
from the superclass World
cell.jpg
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The populate method
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The randomLeaves method
Create a Leaf object at a random
position (x,y); repeat howMany
times
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So far

Greenfoot environment to program
simulations and games in java
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World and Actor are system classes
The wombats program
Leaf
 Wombat (getLeavesEaten, foundLeaf,
eatLeaf, setDirection, turnLeft, canMove,
move, act)
 WombatWorld (populate, randomLeaves)
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Exercise

How would you change the behaviour
of Wombat objects so that instead of
always turning left, they could turn to
other directions in a random way?
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GreenFoot. getRandomNumber(int limit)
Generates a random number
between 0 and limit
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Exercise

Create a new method, turnRandom(),
that will turn to the left a random
number of times. This method will
replace the method turnLeft() in the
act() method of Wombats. Use the
method of the greenfoot library
GreenFoot. getRandomNumber(int
limit) that produces a random integer
between zero and limit-1.
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The method turnRandom
Generate a random
number between 0 and 3
call turnLeft()
turns times
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The updated method act
if you find leaves, eat !
if you don‘t find leaves, but
you can move forward, then
move!
if you don‘t find leaves and
you also cannot move, then
turn to a random direction!
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Before and after
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to set the image of an object
programmatically
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Before and after
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Adding Rocks
class image for
the Rock class
New classes can
be added easily
 Can import images
for classes
 However wombats
can go through
rocks
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images available in the
wombats
scenario folder
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check all Rock objects
in front of you;
if there is no Rock object,
return true (i.e. okay it‘s
clear!) , otherwise return
false (i.e. you cannot move!)
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Summary
Greenfoot environment to program
simulations and games (greenfoot.org)
 Provides an action loop
 World subclasses set up the world
 Actor subclasses define the characters of
the game / simulation
 Sophisticated behaviour can be
implemented in Java
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