Animation Mrs. C. Furman

Animation  We can animate our crab by switching the image between two pictures.  crab.png and crab2.png

Greenfoot images  We create new images by using the GreenfootImages class.  new GreenfootImage(“crab2.png”);  This file must be saved in the scenario’s images folder.

How Do Actors Get Images?  Images are assigned to classes when they are created.  Every object created from the class will receive a copy of the image.  This image can be changed  Each actor can decide to change it’s image.

Exercise  Check the documentation of the Actor class. There are two methods that allow us to change an actor’s image.  What are they called, and what are their parameters?  What do they return?

this.setImage (new GreenfootImage (“crab2.png”));  We can use the above method to set the image to something new.  Two things are happening…  A new GreenfootImage is created  This new image is being set as the image for the actor.

Setting image in Two Lines  It is better for us to separate the previous statement into 2.  This way we can better switch between the 2 images.

Storing Created Images  We will need to store the 2 images that we will be switching between  We will need variables.

Variables  They are named memory locations.  They are used to store values or objects.  We will be creating a type of variable called an instance variable, or field  Instance Variables are a bit of memory that belongs to an object. As long as the object exists and can be referenced, the instance variables exist.

Identifiers  When we name variables we have to follow these rules.  Choose names that: 1.Start with a letter 2.Contain only letters, numbers, underscores or the $ symbol 3.Can not be a keyword

Variable Identifiers  Variable names (identifiers) should clearly describe what we are storing. Use image1, rather than x.

Access Specifiers  There are 3 kinds of access specifiers, we will discuss 2 of them  public – other objects have access to these value and methods  private – only this class has access to the values and methods.

Instance Variables  For the most part, we will make our instance variables private.  Our methods up until this point have been public

Data Types  We also need to know data types when declaring a variable  There are 4 primitive types you need to know: int, double, boolean, char  Everything else is stored in an object.

int  stores positive and negative whole numbers and 0

double  stores real numbers (decimal numbers and integers)

boolean  true or false

char  a single character (letter, digit, symbol)  char literal – a character enclosed in a single quote.  char variables can be assign char literals or numbers

Storing our Image  Our image is not a primitive, it is a class  The type of our instance data is GreenfootImage  So…

What’s the line?  private GreenfootImage image2;  Before we add this to Crab, right click on Crab and select inspect  This will tell us what variables are already in the Crab object.  What variables do you find?

Look at Crab Code  Do you see WALKING_SPEED declared in Crab?  Where is it?  Why is it showing up in Crab?

Where do we put instance variables???  Instance variables are always listed first in a class.  Right after the class header and before the Constructors and methods.  This is a convention that Java programmers follow

Add Instance Variables to Crab  Above the act method add: private GreenfootImage image1; private GreenfootImage image1; private GreenfootImage image2; private GreenfootImage image2;  Recompile and right click to inspect the class again. Are these new variables added?

Null  What are image1 and image2 set to?  Null – means that the variables have been declared, but NO objects have been created.  We will need to create GreenfootImage objects to be store in image1 and image2

Assignments  = This is the assignment operator in Java  The variable goes on the LEFT always, and the value being assigned goes on the RIGHT.  This should not be confused with == which we use to compare two things to see if they are equal

Assignments  variable = expression;  image1 = new GreenfootImage (“crab.png”);  image2 = new GreenfootImage (“crab2.png”);  We need to put this code into a Constructor method of Crab  Constructors main job is to give values to instance data.

Create a Constructor  Constructors have the same name as the class  Constructors have NO return type

Above Act and After instance Data write… public Crab() { image1 = new GreenfootImage (“crab.png”); image1 = new GreenfootImage (“crab.png”); image2 = new GreenfootImage(“crab2.png”); image2 = new GreenfootImage(“crab2.png”); this.setImage (image1); this.setImage (image1);}

Compile and Inspect  Inspect Crab again  What are the value of image1 and image2?

if..else statements  We have looked at if statements..  if (something is true) { do something } { do something } What if the something is false???

if.. else  if..else provides us with something to do if the condition is false. if (condition) { execute this if condition is true } else { execute this if condition is false }

What Do we want to do?? if (the current image is image 1) { change the image to image2 } else { change the image to image1 } The above is pseudocode.

.equals( )  When we want to see if 2 objects are the same, we must use.equals()  this.getImage ().equals (image1) Current Imageimage1 variable

GreenfootImgage getImage()  This method returns the current image if (this.getImage().equals(image1)) { this.setImage(image2); this.setImage(image2);}else{ this.setImage (image1); this.setImage (image1);}

method toggleImage  Write a new method in Crab called toggleImage()  What is the return type of this method?  The body should be our if.. else statement  Be sure to call this.toggleImage() in the act() method.  Compile and run… what happens?

toogleImage method public void toggleImage() { if (this.getImage().equals(image1)) if (this.getImage().equals(image1)) { this.setImage(image2); this.setImage(image2); } else else { this.setImage (image1); this.setImage (image1); }