ObjectDraw and Objects Early

1. ObjectDraw and Objects Early Chris Nevison Barbara Wells

2. Objects Early • Introduce Programming in Modern Context • Use Graphics for Motivation • Work with Objects from Day One • ObjectDraw Library (Bruce, Danyluk, Murtaugh) • text: Java: An Eventful Approach, under development • provides tools for OO early with graphics • teaches use of API • leads to use of standard tools

3. Elevator import objectdraw.WindowController; import objectdraw.FilledRect; import objectdraw.Location; public class ElevatorSystem1 extends WindowController{ private FilledRect elevator; public void begin() { elevator = new FilledRect(100, 200, 20, 30, canvas); } public void onMouseClick(Location point) { elevator.move(0, -60); } }

4. Reading the Code: "import" • import • provides access to classes from named package (library) • objectdraw is package • library classes used are • WindowController • FilledRect • Location • Information available from Application Programming Interface (API)

5. Reading the Code: "class" • Declaration of a class begins with • public class <<name of class>> • name here is ElevatorSystem1 • keyword extends means this class is an extension of another, in this case WindowController • this means ElevatorSystem1is aWindowController • it has all the attributes and methods of a WindowController • it may add its own attributes and methods • it may override WindowController methods to change their behavior

6. Reading the Code: "private" • Every component of a class has an indication of visibility: we will use two of these throughout: "private" and "public" (later we will add a third) • private means this component can only be accessed within this class • public means this component can be accessed from any other class, by the appropriate naming conventions

7. Reading the Code: "private" • private FilledRect elevator; • private means this data field can only be used within this class specification • We will always make data fields private • FilledRect declares that the following identifier will be a reference to a FilledRect (as defined in the objectdraw package, by the import statement) • elevator is an identifier that we choose to represent an instance variable -- a variable that is supplied for every object (instance) of this class • No value has yet been supplied for this variable

8. Reading the Code: "public void begin" • begin is a method supplied by WindowController that we must override • whatever actions we specify in the body of begin will be carried when the ElevatorSystem1 code first starts • the body of the method is everything between the { and matching } • public makes this method accessible from other classes • void indicates that this method does not return any result • the ( ) are a place where information (parameters) can be given to the method -- none are given for begin

9. Reading the Code: "elevator =" • This statement is inside the { } so it is the body of the begin method and will be executed when the ElevatorSystem1 first starts • elevator refers to the declared instance variable • = is the assignment operator • the expression on the right is evaluated and assigned to the variable on the left (backwards, perhaps) • the left side must be a variable that the right side can be assigned to

10. Reading the Code: "elevator =" • new on the right is the operator that creates a new object. • FilledRect is the new object being created • (100.0, 200.0, 20.0, 30.0, canvas) is the list of actual parameters used to create the FilledRect • we can find out what actual parameters are needed by consulting constructors in the API. This constructor needs four numbers. The first two specify the x and y coordinates of the upper left corner of the FilledRect, the next two the width and height (all in pixels)

11. Reading the Code: "elevator =" • what is canvas? • a FilledRect needs a drawing canvas where is "lives" - where it is drawn and can move • canvas is a drawing canvas the is an attribute of the WindowController class that our ElevatorSystem1 inherits -- we do not need to worry about the details • So FilledRect is at the 100, 200 location of the supplied canvas, with width 20 and height 30 pixels. • Note: x (horizontal) coordinates increase left to right, y (vertical) increase top to bottom and the (0, 0) location is in the upper left of the window

12. Reading the Code, summary so far • We are using tools specified from the objectdraw package • Since our ElevatorSystem1 class extends the WindowController class, we get things like the canvas (for drawing objects) automatically • We override the begin method to indicate what happens when the ElevatorSystem1 is first started • Our begin method creates a FilledRect and assigns a reference to the instance variableelevator. The FilledRect is drawn.

13. Reading the Code: onMouseClick • onMouseClick is one of several methods for the WindowController that respond to user mouse actions, that we can override • public so it is accessible from other classes • void because it does not return any result • (Location point) indicates that this method must be supplied with a Location when it is called (somewhere in the system)

14. Reading the Code: elevator.move • We are in the body of the onMouseClick method, specifying what happens when the system calls this method in response to a user action. • elevator refers to the instance variable • the "." means access a component of that object • move is a method of the elevator object -- it is specified in the FilledRect class specification (we can get information on this from the API) • (0.0, -60.0) are the actual parameters supplied to the move method, the 0.0 is the distance moved in the x (horizontal) and -60.0 in the y (vertical) direction. In the vertical, a negative move is up.

15. Reading the Code, summary • When the code starts, a filled rectangle (FilledRect) is draw at the given position • Whenever the user clicks the mouse, the rectangle moves up, modeling an elevator • The system calls the onMouseClick method in response to outside events (the user clicking the physical mouse). This is typical of modern programs and is called event-driven programming

16. Exercise 1 • Draw a Scene • Consult the objectdraw API to find different shapes available: FilledRect, FilledOval, FramedRect, FramedOval, FramedArc, Line • Use these shapes to draw a house or another scene • All your code can be in begin method

17. Exercise 2 • Modify the ElevatorSystem1 example. • follow instructions to open the Elevator1.java file • follow instructions to open the objectdraw API • look at the API for FilledRect and Location • read the methods available, including inherited methods • Make a change in the behavior by changing the body of either the begin method or the onMouseClick method • change color, move differently, use moveTo instead of move, use the Location formal parameter • Note: for colors try Color.yellow, etc. See the API for Color in the java.awt documentation

18. Aspects of Java in this example • Use of libraries (import package code) • Declaration of a class • extend another class • inherit attributes and methods • Instance variable (private) • Override inherited methods • begin, onMouseClick • Create an object (new FilledRect) • Invoke object method (elevator.move) • supply actual parameters

19. objectdraw Context (for teachers) • WindowController extends a class called applet • An applet • can run in a browser window (or with appletviewer) • always starts with init method • WindowController calls the begin method in its init method, after doing some preliminary setup of the system • Freestanding Java programs are applications • start with a class with a main method • can also invoke graphics and user events, using the awt or swing libraries