Crab World
Games and Simulations
O-O Programming in Java
The Walker School
The Walker School – Games and Simulations - 2010
Crab World
Have students play the working game
for a few minutes.
What are the objects in this world?
What are the classes in this world?
The Walker School – Games and Simulations - 2010
Object Oriented Programming
The discipline of object-oriented programming is based
on methods, subroutines that are attached to objects or
object classes. In the compilation technique called threaded
code, the executable program is basically a sequence of
subroutine calls.
The Walker School – Games and Simulations - 2010
Movement
Method
Call
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Turning
Parameter
What happens to the crab if you use a negative number?
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Standard Java Class
Import (Library)
Statements
Class
Signature
Instance Variables
(not shown)
Constructors
(not shown)
Method
Signature
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Functions and Methods
A function is a named sequence of statements that performs a
desired operation. This operation is specified in a function
definition.
NAME( LIST OF PARAMETERS ):
STATEMENTS
There can be any number of statements inside the function, but they have to
be indented from the left margin.
Creating a new function gives you an opportunity to name a group of
statements. Functions can simplify a program by hiding a complex
computation behind a single command and by using English words in
place of arcane code. Creating a new function can make a program
smaller by eliminating repetitive code.
The Walker School – Games and Simulations - 2010
Method Call
This conditional statement uses a built in function to turn an
object if it hits the edge of the world.
Method
Body
Method
Call
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Function (or Method) Call
The value or variable, which is called the argument of the function, has
to be enclosed in parentheses. It is common to say that a function
“takes” an argument and “returns” a result. The result is called the
return value.
turn (speed);
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Dealing with Screen Edges
Conditional
Statement
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Conditionals are Decision Statements
Are if … then
statements
Condition
Only executed if something is True.
Instruction(s)
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Conditional Types
• Branched
• Chained
• Nested
…gives use the ability to check conditions and
change the behavior of the program accordingly
if (method())
{
statement;
statement;
}
Watch – Making Things Move (Part I)
http://www.youtube.com/watch?v=KUwt0K4K4_o
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API Views
Source Code
View
Documentation
View
Contains
1. Class Tree
2. Author
3. Constructors
4. Methods
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Adding Random Behavior
What
percentage of
time will the
crab turn 5
degrees?
Operator
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Operators
Operators are special symbols that represent computations like addition and
multiplication. The values the operator uses are called operands.
Assignment (=)
Comparison (==, <=, >=, !=, etc.)
Logical: &&, ||, ! (and, or, not)
When more than one operator appears in an
expression, the order of evaluation depends
on the rules of precedence.
The Walker School – Games and Simulations - 2010
Common Operators
•
•
•
•
•
•
+
*
>
<
<= or >=
•
•
•
•
•
==
!=
||
&&
%
What do each of these operators do?
The Walker School – Games and Simulations - 2010
Order of Operation
The order of evaluation depends on the rules of precedence.
PEMDAS
Parentheses ( )
Exponentiation
Multiplication and Division
Operators with the same precedence are evaluated from left
to right.
The acronym PEMDAS is a useful way to
remember the order of operations
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Dot Notation
Used when calling a method defined in another class.
Dot
Notation
Class-name.method-name (parameters);
The Walker School – Games and Simulations - 2010
Explain the behavior being expressed here.
The Walker School – Games and Simulations - 2010
Adding a New Subclass
1. Right click on
class.
2. Choose new
subclass.
1. Name the world.
2. Choose a new
object.
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Inheritance
Subclass
The Crab class inherits all of the methods of the Animal class, while the
Animal Class inherits all of the methods of the Greenfoot Actor class.
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Class Diagram Relationships
What do crabs eat? or What eats a crab?
A “crab” is-a
“animal”
The Walker School – Games and Simulations - 2010
Creating New Methods
Find this behavior
in the Java API?
Comments
Method
Statement
Imports the behavior from the
java.lang.Class to the Worm class.
New methods need to be
called in the act() method.
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Commenting New Methods
Comments
Each time you add a new method, you should include a
comment explaining what it does. Your comments should be
clear and concise, free of spelling and grammatical errors. Think
of yourself having to read another programmers code. Would
you want it to be easy to read?
The Walker School – Games and Simulations - 2010
Programming Challenge
• Add a new actor to your program, say a
lobster or a human that might eat the crab.
You will need to copy the act method from the
crab to the lobster, and other methods, such
as lookForWorm(), which you will need to
change to lookForCrab().
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One Possible Solution
Remember to call the method in
the act() method.
The Walker School – Games and Simulations - 2010
Adding Keyboard Control
static boolean isKeyDown(String key)
Method Call
Class
Method
Returns
Either T or F
Expects a
String
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Any
Key
Programming Challenge
• Add additional keyboard controls to make the
crab move up or down. What other keyboard
functionality could you add? For example, you
could make the crab jump back 10 pixels when
it encounters a lobster if you hit the “j” key.
The Walker School – Games and Simulations - 2010
One Possible Solution
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Adding Sound
Challenge: Search the internet for other sound files and add a new sound
to the Crab class when it east a worm.
The Walker School – Games and Simulations - 2010
Watch – Making Things Move (Part II)
http://www.youtube.com/watch?v=QWruHXIPDIk&feature=related
Mover
“Helper”
Support Class
For more
advanced
students.
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Ending the Game
In what class is this
method called?
The Walker School – Games and Simulations - 2010
Creating New Objects
Java keyword “new” allows us to create new objects of
any of the existing classes.
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Java Keywords
http://en.wikipedia.org/wiki/List_of_Java_keywords
Search the list; What is one other
keyword that we have used already
and what does it mean?
The Walker School – Games and Simulations - 2010
Constructors
• A special kind of method that is always
automatically executed whenever an
instance of a class is created.
• A constructor has no return type (static
or void) between the keyword “public”
and the name.
• The name of the constructor is always
the same as the name of the class.
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Parameter List
Allows us to pass information to the constructor.
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Arguments and Parameters
Arguments are values that control how the function does its job.
Some functions can take more than one
argument.
Values that are passed from one function to
another get assigned to variables called
parameters.
The Walker School – Games and Simulations - 2010
Adding Objects Automatically
To populate a world automatically add
characters to the “World” class.
You can add each object individually, or
you can create a method that adds
actors randomly.
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Programming Challenge
• Combine what you have learned about
random behavior and adding new objects to
create a method that would add a random
number of worms up to 30 and a random
number of lobsters up to 3.
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One Possible Solution
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Adding a Win-State
(counting worms)
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Instance Variables – Step #1
Create variables to hold
the number of worms.
An instance variable is memory that belongs to an object.
The Walker School – Games and Simulations - 2010
Assignment – Step #2
Create an assignment
and initiate the variable
to 0 at the beginning of
the game.
An assignment is a statement that enables us to store
something into a variable. It is written with an equal = symbol.
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Increment the Count – Step #3
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Crab Win-State, Play Winner
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Lobster Win-State, Play Winner
Execution always begins at the first statement of the program.
Statements are executed one at a time, in order from top to
bottom. This is referred to a the Flow of Execution.
The Walker School – Games and Simulations - 2010
Programming Challenge
• The current state of your games allows a
player to win if their crab eats 8 worms. How
would you make sure there were at least 8
worms added to each new game, if you were
using a random number generator?
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One Possible Solution
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Adding a Second Player
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Programming Challenge
• Add a second player where one of you plays
the crab and another plays the lobster. Each
time the game starts, there will be one crab
and one lobster. You will need to add keyboard
controls for each character. Make sure the key
controls are on opposite sides of the
keyboard. The number of worms at the
beginning of the game can be random.
The Walker School – Games and Simulations - 2010
Crab Keyboard Controls Solution
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Lobster Keyboard Controls Solution
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World Population Solution
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Places to Hide
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Programming Challenge
• Add places for the crab to hide from the lobster,
such as a rock or a beach house. Your choices
are to add one rock all the time, or to add a
random number of rocks. Probably, shouldn’t be
more than 3 objects at a time. Also, how would
you make sure that there is always at least one
object for the crab to hide behind.
The Walker School – Games and Simulations - 2010
Add a Rock
Adds a rock to the world for the crab to hide behind.
The Walker School – Games and Simulations - 2010
Random Number of Rocks
Remember to comment out the firstRock object.
Otherwise you will get the random number +1 rock.
The Walker School – Games and Simulations - 2010
Adding a Penalty
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Extended Programming Challenge
• Create a penalty for the crab and the lobster.
If the crab touches the edge of the world, then
another worm is added to the world and his
score goes down by 1. If the lobster touches
the edge of the world, then a point is added to
the crab’s score.
The Walker School – Games and Simulations - 2010
Adding A Win Message
Done after Piano Project
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Adding a Win Message
Step #1: Import the necessary Java libraries.
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Adding a Win Message (cont.)
Step #2: Create the method.
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Adding a Win Message (cont.)
Step #3: Cast the method in the CrabWinState().
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