1. Games and Simulations O-O Programming in Java The Walker School
Crab World
Games and Simulations
O-O Programming in Java
The Walker School
The Walker School – Games and Simulations

2. 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?
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3. 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.
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4. Movement
Method
Call
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5. Turning
Parameter
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What happens to the crab if you use a negative number?

6. Standard Java Class Import (Library) Statements Class Signature
Instance Variables
(not shown)
Constructors
(not shown)
Method
Signature
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7. Functions and Methods NAME( LIST OF PARAMETERS ): STATEMENTS
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.
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8. 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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9. 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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10. Dealing with Screen Edges
Conditional
Statement
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11. Conditionals are Decision Statements
Are if … then
statements
Condition
Only executed if something is True.
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Instruction(s)

12. Conditional Types if (method()) { statement; } Branched Chained Nested
…gives use the ability to check conditions and change the behavior of the program accordingly
Branched
Chained
Nested
if (method()) {
statement; }

13. Watch – Making Things Move (Part I) http://www. youtube. com/watch
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14. API Views Documentation Source Code View View Contains Class Tree
Author
Constructors
Methods
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15. Adding Random Behavior
What percentage of time will the crab turn 5 degrees?
Operator
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16. Operators Assignment (=) Comparison (==, <=, >=, !=, etc.)
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.
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17. Common Operators + - * > < <= or >= == != || && %
What do each of these operators do?
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18. Order of Operation PEMDAS Parentheses ( ) Exponentiation
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.
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The acronym PEMDAS is a useful way to remember the order of operations

19. Dot Notation Used when calling a method defined in another class. Dot
Class-name.method-name (parameters);
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20. Explain the behavior being expressed here.
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21. Adding a New Subclass 1. Name the world. 2. Choose a new object.
1. Right click on
class.
2. Choose new
subclass.
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22. Inheritance
Subclass
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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.

23. Class Diagram Relationships
What do crabs eat? or What eats a crab?
A “crab” is-a “animal”
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24. Creating New Methods Find this behavior in the Java API? Comments
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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25. 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?
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26. 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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27. One Possible Solution Remember to call the method in the act() method.
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28. Adding Keyboard Control
static boolean isKeyDown(String key)
Method Call
Class
Method
Returns
Either T or F
Expects a
String
Any
Key
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29. 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.
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30. One Possible Solution
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31. Adding Sound
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Challenge: Search the internet for other sound files and add a new sound to the Crab class when it east a worm.

32. Watch – Making Things Move (Part II) http://www. youtube. com/watch
Mover “Helper” Support Class
For more advanced students.
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33. Ending the Game In what class is this method called?
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34. Creating New Objects
Java keyword “new” allows us to create new objects of any of the existing classes.
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35. 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?
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36. 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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37. Parameter List Allows us to pass information to the constructor.
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38. 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.
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39. 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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40. 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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41. One Possible Solution
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42. Adding a Win-State (counting worms)
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43. Instance Variables – Step #1
Create variables to hold the number of worms.
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An instance variable is memory that belongs to an object.

44. Assignment – Step #2
Create an assignment and initiate the variable to 0 at the beginning of the game.
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An assignment is a statement that enables us to store something into a variable. It is written with an equal = symbol.

45. Increment the Count – Step #3
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46. Crab Win-State, Play Winner
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47. 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.
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48. 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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49. One Possible Solution
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50. Adding a Second Player
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51. 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.
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52. Crab Keyboard Controls Solution
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53. Lobster Keyboard Controls Solution
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54. World Population Solution
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55. Places to Hide
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56. 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.
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57. Add a Rock Adds a rock to the world for the crab to hide behind.
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Adds a rock to the world for the crab to hide behind.

58. Random Number of Rocks Remember to comment out the firstRock object.
Otherwise you will get the random number +1 rock.
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59. Adding a Penalty
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60. 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.
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61. Done after Piano Project
Adding A Win Message
Done after Piano Project
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62. Adding a Win Message Step #1: Import the necessary Java libraries.
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63. Adding a Win Message (cont.)
Step #2: Create the method.
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64. Adding a Win Message (cont.)
Step #3: Cast the method in the CrabWinState().
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