Crab World

Crab World

Games and SimulationsO-O Programming in Java

The Walker School

The Walker School – Games and Simulations - 2010

Crab World


What are the objects in this world?

Have students play the working game for a few minutes.

What are the classes in this world?

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.


Movement


MethodCall

Turning


Parameter

What happens to the crab if you use a negative number?

Standard Java Class


Method Signature

ClassSignature

Import (Library) Statements

Constructors(not shown)

Instance Variables(not shown)

Functions and MethodsA function is a named sequence of statements that performs a desired operation. This operation is specified in a functiondefinition.

NAME( LIST OF PARAMETERS ):STATEMENTS

There can be any number of statements inside the function, but they have tobe 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.


Method Call


MethodCall

Method Body

This conditional statement uses a built in function to turn an object if it hits the edge of the world.

Function (or Method) CallThe 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);


Dealing with Screen Edges


ConditionalStatement

Conditionals are Decision Statements


Are if … thenstatements

Only executed if something is True.

Condition

Instruction(s)

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


http://www.youtube.com/watch?v=KUwt0K4K4_o

API Views

Documentation View

Source CodeView

Contains1. Class Tree2. Author3. Constructors4. Methods


Adding Random Behavior


What percentage of time will the crab turn 5 degrees?

Operator

OperatorsOperators are special symbols that represent computations like addition and multiplication. The values the operator uses are called operands.

When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence.

Assignment (=)Comparison (==, <=, <=, etc.)Logical (and, or, not)


Common Operators

• +• -• *• >• <• <= or >=

• ==• !=• ||• &• %


What do each of these operators do?

Order of Operation

The acronym PEMDAS is a useful way to remember the order of operations

The order of evaluation depends on the rules of precedence.

PEMDASParentheses ( )ExponentiationMultiplication and Division

Operators with the same precedence are evaluated from left to right.


Dot Notation


Dot Notation

Used when calling a method defined in another class.

Class-name.method-name (parameters);


Explain the behavior being expressed here.

Adding a New Subclass


1. Right click onclass.2. Choose newsubclass.

1. Name the world.2. Choose a new

object.

Inheritance


The Crab class inherits all of the methods of the Animal class, while theAnimal Class inherits all of the methods of the Greenfoot Actor class.

Subclass

Class Diagram Relationships


A “crab” is-a “animal”

What do crabs eat? or What eats a crab?

Creating New Methods


Comments

Method Statement

New methods need to be called in the act() method.

Imports the behavior from the java.lang.Class to the Worm class.

Find this behavior in the Java API?

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?

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().


One Possible Solution


Remember to call the method in the act() method.

Adding Keyboard Control


static boolean isKeyDown(String key)

ClassMethod

ReturnsEither T or F

Expects aString

AnyKey

Method Call


• 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.




Adding Sound


Challenge: Search the internet for other sound files and add a new sound to the Crab class when it east a worm.

Watch – Making Things Move (Part II)http://www.youtube.com/watch?v=QWruHXIPDIk&feature=related


Mover “Helper” Support Class

For more advanced students.

http://www.youtube.com/watch?v=QWruHXIPDIk&feature=related

Ending the Game


In what class is thismethod called?

Creating New Objects


Java keyword “new” allows us to create new objects of any of the existing classes.

Java Keywordshttp://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?

http://en.wikipedia.org/wiki/List_of_Java_keywords

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.


Parameter List


Allows us to pass information to the constructor.

Arguments and ParametersArguments 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.


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.


• 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.




Adding a Win-State(counting worms)


Instance Variables – Step #1


An instance variable is memory that belongs to an object.

Create variables to hold the number of worms.

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.

Increment the Count – Step #3


Crab Win-State, Play Winner


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 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?




Adding a Second Player



• 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.


Crab Keyboard Controls Solution


Lobster Keyboard Controls Solution


World Population Solution


Places to Hide



• 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.


Add a Rock

Adds a rock to the world for the crab to hide behind.


Random Number of Rocks

Remember to comment out the firstRock object. Otherwise you will get the random number +1 rock.


Adding a Penalty


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.


Adding A Win Message

Done after Piano Project


Adding a Win MessageStep #1: Import the necessary Java libraries.


Adding a Win Message (cont.)

Step #2: Create the method.


Adding a Win Message (cont.)


Step #3: Cast the method in the CrabWinState().

Documents

Crab World