Chapter 1 - Getting to know Greenfoot - PowerPoint Presentation

Slide1

Chapter 1 - Getting to know Greenfoot

Bruce Chittenden

And modified by Mr. LeeSlide2

Start Menu | All Programs

Greenfoot | GreenfootSlide3

1.1 Getting Started

Q:\

Assignments\

Mr. Lee\

Scenarios\Copy “wombats”folder to “MyDocuments”.Open the scenarioin Greenfoot.Slide4

Which leads to the question:

“What are wombats?”

Wombats are native Australian mammals.Slide5

The World: where programs will run, where we will see things happen.

World

Class

Diagram

Execution

ControlsSlide6

Class Diagram

: This is the little beige boxes and arrows on the right. We’ll come back to them later.

World

Class

Diagram

Execution

ControlsSlide7

Execution Controls: includes the Act, Run, and Reset buttons, and the speed slider. More on them later.

World

Class

Diagram

Execution

ControlsSlide8

1.2 Objects and Classes

We’ll discuss the Class Diagram first. It shows us the classes involved in this scenario. Here, we have 5 classes:

World

,

Wombat World

,

Actor

,

Wombat

, and

Leaf

.

The classes

World

and

Actor are will always be there – they come with the

Greenfoot system.The classes are specific to the wombat scenario. They may be different or not exist at all in other scenarios.

Let’s start by looking at the Wombat class. It stands for the general concept of a wombat. So this class describes all wombats.

Once we have a class in

Greenfoot

(or Java), we can create

objects

from it.Slide9

Right Click on Wombat

Click New Wombat()

Drag to World

Here’s how you create objects from the

Wombat

class. Try it out!

You’ll get a small picture of a wombat object, which you can move around the screen with your mouse.

Now place it in the World by clicking somewhere in the world.

Once you have a class, you can create as many objects from it as you like.Slide10

Exercise 1.1

: Create some wombats in the world.

Make some leaves too.Slide11

There’s a shortcut to placing objects: shift-clicking into the world.

Make sure the leaf class is selected, then hold down the Shift key and click into the world. What happens?

Extra Credit

: Make a journal entry in your notebook about worlds, classes, or objects.Slide12

Once we have placed some objects into the world, we can interact with these objects by

right-clicking

on them. The

object menu will pop up.

The object menu shows all the operations the object can perform (how the object can

act)

. In this case, we see what the wombat can do. Two other functions, Inspect and Remove

, will be discussed later.

1.3 Interacting with Objects

Invoke the Move method

Right Click on the WombatSlide13

In Java, these operations an object can perform are called

methods

. (Notice that they all end with parentheses.) We can

invoke a method by clicking on it in the menu.

Invoke the Move method

Right Click on the Wombat

Exercise 1.2

: Invoke the move() method. What does it do? Try it a few times. Invoke the

turnLeft

() method. Finally, place two wombats into the world and make them face each other.Slide14

Let’s look closer at the object menu. The

move

and

turnLeft methods are listed as:

We see more than just the method names. There is the word

void

at the beginning, and round brackets at the end. These are symbols that tell us what data goes into the method and what comes back from it.

The word at the beginning is called the

return type

. It tells us what the method returns to us when we invoke it. The word

void means “nothing”

in this context: methods of

void

return type do not return any information. They just carry out their actions, and then stop.

1.4 Return types

void move()

void

turnLeft

()Slide15

Any word other than void tells us the type of information returned by the method.

In the wombat’s menu we also see the return types:

int

– short for “integers” (what are some examples?)

boolean

– one only two possible values will be returned: “true” or “false”

Methods with return type

void

are like commands for our wombats. If we invoke

turnLeft

(), the wombat turns left.

Methods with any other return type are like questions: do what you have to do, but give us an answer when you’re done.Slide16

Consider the

canMove

method.

When you invoke it, you will see a result similar to the one below.

Here, the important information (data) is the word “true” returned by the method call. In effect, we just asked the wombat, “

Can you move?

” And the wombat answers, “Yes!” (true)

Exercise 1.3

: Invoke the

canMove

() method on any of your wombats. What does it usually return? Can you make the method return

false

?Slide17

Try out another method with a return value:

Using this method, we can find out how many leaves this wombat has eaten.

Hint if you are stuck: Can you make the wombat eat some leaves?

Int

getLeavesEaten

()

Exercise 1.4

: Create a new wombat and invoke the

getLeavesEaten

method. It will always return 0. Can you create a situation in which the method returns some positive number?Slide18

The other bit in the object menu are the round brackets after the method name.

The parentheses hold the

parameter list

, which tells us whether the method needs

any information

to run, and if so, what kind of info. If I tell you to jump, you would ask, “how high?”

If the parentheses have nothing in between, e.g. turnLeft(), the parameter list is empty. In other words, it expects no parameters.

The entire description of each method shown in the object menu, including the return type, method name, and parameter list, is called the

method signature

.

1.5 ParametersSlide19

Let’s try out the

setDirection

method. We can see it has int

direction

as its parameter list, which

tell us that the method expects one parameter of type int, which specifies a direction.Let’s invoke the method, and you will see the following dialog box.

The comment near the top of the dialog box gives more details: the direction parameter should be between 0 and 3.Slide20

Exercise 1.5

: Invoke the

setDirection

(

int

direction) method. Provide a parameter value and see what happens to the wombat. Which number corresponds to what direction? Make a list on paper.

What happens when you type a number greater than 3? How about a number that is a word (three) or not an integer (e.g. 2.3)?Slide21

Exercise 1.6

: Place a wombat and a good number of leaves into the world. Then, invoke a wombat's act() method several times. What does this method do? How does it differ from the move method

?

Exercise

1.7

:

Still with a wombat and some leaves in the world, click the Act

button in the executive controls near the bottom of the

Greenfoot

window. What does this do?

Exercise 1.8

: Add a few wombats to the world. What is the difference now between clicking the

Act

button and invoking the act()

method on the wombats?Exercise 1.9

Click the Run button. What does it do?

1.6-1.9 Execution ControlsSlide22

Exercise 1.6Slide23

Exercise 1.6

Wombat Moves toward the LeavesSlide24

Exercise 1.6

Wombat Moves to LeavesSlide25

Example 1.7

Wombat Eats LeafSlide26

Exercise 1.8Slide27

Exercise 1.8

The >Act Execution Control Affects All the WombatsSlide28

Exercise 1.9

Wombat Runs Around the Edge of the WorldSlide29

Exercise 1.9

act()

method

If we’re sitting on a leaf, eat the leafOtherwise, if we can move forward, move forwardOtherwise, turn left

Leaves do nothing when called to act.

The

act() method is fundamental to Greenfoot objects. We will see it in every chapter. All Greenfoot classes/objects have this method.Slide30

Execution Controls, recapped

The

Act

button is equivalent to invoking the

act()

method of every object exactly once. (Refer to the previous slide.)

The

Run

button is equivalent to continuously clicking the

Act

button. You may have noticed that when clicked, the

Run

button becomes a

Pause

button, which can stop the whole show.

The

slider

to the right of the buttons sets the speed.Slide31

A Brave New World

You can erase objects from the world by right-clicking them and choosing

Remove

.

But if the world becomes cluttered, and you want to start all over, there’s a shortcut: discard the world and create a new one.

This is usually done via the

Reset

button.

You will get a new, empty world. The old one has been discarded along with all objects in it – you can have only one world at a time.Slide32

Invoke a world method

We have seen objects in the world have methods you can invoke via a pop-up menu. The world itself is also an object, with methods you can invoke.

Above the world display is a title that shows the name of the world—”

WombatWorld

” in this case. Right-click

this title to see the

world’s menu.Slide33

One method is

populate()

.

Reset

the world, then try it out. Note the empty parameter list.Slide34

The other method is

randomLeaves

, which places some leaves at random locations in the world.

Note the words in the parenthesis—called the

parameter

:

int

howMany

It means you must supply additional information when you invoke the method.

int

means a whole number is expected, and

howMany

suggests that you can specify how many leaves to place in the world.Slide35

Invoke the

randomLeaves

method. A dialog will pop up asking for a value for the parameter. What would you enter to get the same number of leaves as the picture below?Slide36

1.8 Understanding the Class Diagram

At the top of the diagram, you can see two classes called

World

and

WombatWorld

, connected by an arrow.

The World class is in every

Greenfoot

scenario—it’s built-in. The class under it,

WombatWorld

in this case, represents the specific world for this scenario.

The arrow shows an

is-a

relationship:

WombatWorld

is

a

World

.

We can also say that

WombatWorld

is

a

subclass

of

World

.Slide37

Below the world classes, we see a second group of classes linked by an arrow.

Each class represents its own objects. Reading from the bottom, we see

Leaves

and

Wombats

. Both

are Actors in the world.Again, we have subclass

relationships. For example,

Leaf

is

a

subclass

of

Actor

. We

can also say that Actor is a superclass of Leaf and

Wombat.What is a superclass and what is a subclass in the diagram to the left?

Animal

Mammal

Bonobo

HumanSlide38

1.11 Summary

In this chapter, we saw what Greenfoot scenarios can look like and how to interact with them.

We have seen how to create objects, and how to communicate with these objects by invoking their methods.

Some methods are commands to objects, while other methods return information about the object.

Parameters are used to provide additional information to methods, while return values pass information back to the caller.

Objects were created from their classes.Slide39

Concept Summary