OVERLOADING METHODS AND CONSTRUCTORS: The Ball Class

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Phillip Willis
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1 OVERLOADING METHODS AND CONSTRUCTORS: The Ball Class Create a Ball Demo program that uses a Ball class. Use the following UML diagram to create the Ball class: Ball - ballcolor: Color - ballwidth, ballheight: Integer - mybrush: SolidBrush - xpos, ypos: Integer New() New(xPos: Integer, ypos: Integer) New(xPos: Integer, ypos: Integer, width: Integer, height: Integer) + Color(col: Color): Color + DrawBall(g: Graphics) + Height(bHeight: Integer): Integer + Move() + Move(xPixels: Integer, ypixels: Integer) + Width(bWidth: Integer): Integer + X(ballX: Integer): Integer + Y(ballY: Integer): Integer The Ball class will be responsible for knowing its own location (i.e. x- and y-position), its dimensions, and its colour. We are going to implement this responsibility by providing the Ball class with variables that will store each of these values. We are going to declare the variables as follows: Public Class Ball Private mybrush As SolidBrush Private col As Color Private xpos, ypos As Integer Private ballwidth, ballheight As Integer End Class IMPLEMENTING CONSTRUCTORS We are going to create a constructor that does not take any parameters and initializes our class fields or variables with default values. It s going to look something like this: The Ball Class Page 1 of 9

2 Public Sub New() col = Color.Blue mybrush = New SolidBrush(col) xpos = 0 ypos = 0 ballwidth = 30 ballheight = 30 Since we are using a no-arg constructor for the Ball class and no parameters are being passed to the constructor, we need to assign default values for the Ball object that will be created. In the above example, we are going to set the x- and y-position of the ball at 0, 0, set the dimension to 30 x 30, and set the colour to blue. Now we can create our Ball object in our program by writing the following lines of code in our Ball Demo program: Public Class BallDemo Private g As Graphics Private myball As Ball Private Sub BallDemo_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load g = Me.CreateGraphics() myball = New Ball Private Sub BallDemo_Paint(sender As Object, e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint myball.drawball(g) End Class Before we can run the program, we need to create a method called DrawBall() because this is where we will be writing the code to draw our Ball object onto the form. In order to draw the ball, however, we need the Graphics object that is being used in the program. So our DrawBall() method in the Ball class would look like this: The Ball Class Page 2 of 9

3 Public Sub DrawBall(ByVal g As Graphics) g.fillellipse(mybrush, xpos, ypos, ballwidth, ballheight) The responsibility of drawing the Ball object should fall on the Ball class because we the information required to draw the ball is in the class itself, not the program. However, in order to draw the Ball object, the class needs the Graphics object that is being used in the program. This is why the Graphics object needs to be passed to the DrawBall() method. When you run the program, you should see a Ball object at the top-left corner of the form: OVERLOADING CONSTRUCTORS One way to overload the constructor and provide more flexibility to the Ball class is to allow the user the ability to indicate where to position the ball when it s created. So we can create a second constructor that allows the user to pass two integers representing the x- and y-positions where they may want the ball to initially be positioned: Public Sub New(ByVal x As Integer, ByVal y As Integer) xpos = x ypos = y ballwidth = 30 ballheight = 30 col = Color.Blue mybrush = New SolidBrush(col) The Ball Class Page 3 of 9

4 When you initialize a Ball object using this constructor, you will need to pass the x-position and the y- position of the ball. So now we can create a Ball object in our program as follows: Public Class BallDemo Private myball As Ball Private Sub BallDemo_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load myball = New Ball(100, 200) End Class Let s create a third constructor that allows the user to specify the location of the Ball object and its dimensions. The constructor would look something like this: Public Sub New(ByVal x As Integer, ByVal y As Integer, ByVal width As Integer, ByVal height As Integer) xpos = x ypos = y ballwidth = width ballheight = height col = Color.Blue mybrush = New SolidBrush(col) IMPLEMENTING PROPERTIES Now that we have our class variables and constructors in place, it s time to create properties for our Ball class. Public Property X As Integer 'This property will set and return the x-position of the Ball Public Property Y As Integer The Ball Class Page 4 of 9

5 'This property will set and return the y-position of the Ball Public Property Color As Color 'This property will set and return the color of the Ball Public Property Width As Integer 'This property will set and return the width of the Ball Public Property Height As Integer 'This property will set and return the height of the Ball IMPLEMENTING METHODS Now let s add a method that will make the ball move. We will simply call the method Move(). Public Sub Move() xpos += 5 ypos += 3 The purpose of the Move() method is to move the x- and y-position of the ball object a specific number of pixels. In our example, we are going to increase the x-position of the ball by 5 pixels and increase the y-position of the ball by 3 pixels. OVERLOADING METHODS Let s create a second Move() method that allows programmers to specify how many pixels along the x- and y-axis to move the ball. The Ball Class Page 5 of 9

6 Public Shadows Sub Move(ByVal xpixels As Integer, ByVal ypixels As Integer) xpos += xpixels ypos += ypixels USING THE Ball CLASS IN A PROGRAM Now that we have factored all of the responsibility for the Ball out of the program itself, we can use the Ball class within our Ball Demo program. We will start by declaring an instance of the Ball as a member of our program: Public Class BallDemo Private g As Graphics Private myball As Ball Private Sub BallDemo_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load g = Me.CreateGraphics() myball = New Ball(0, 0, 50, 50) Private Sub DrawingImages_Paint(sender As Object, e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint myball.drawball(g) Private Sub tmrball_tick(byval sender As System.Object, ByVal e As System.EventArgs) Handles tmrball.tick myball.move() Me.Refresh() The Ball Class Page 6 of 9

Private Sub btnstart_click(byval sender As System.Object, ByVal e As System.EventArgs) Handles btnstart.click If btnstart.text = "START" Then btnstart.text = "STOP" tmrball.start() Else btnstart.

7 Private Sub btnstart_click(byval sender As System.Object, ByVal e As System.EventArgs) Handles btnstart.click If btnstart.text = "START" Then btnstart.text = "STOP" tmrball.start() Else btnstart.text = "START" tmrball.stop() End If End Class When the program launches and you click the START button, the ball will start moving horizontally to the right and vertically to the bottom of the screen. The problem, of course, is that the ball is going to eventually disappear from the screen. To solve this problem, we will need to first create two variables that will keep track of the direction along the x-axis and the direction along the y-axis that the ball will be travelling. Create two variables in your program that will keep track of the direction the ball will be traveling: Private xdir, ydir As Integer Set the variables to a default value of your choice. I will set the x-direction and y-direction to 4 and 3 respectively when the program loads: Private Sub BallDemo_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load myball = New Ball(0, 0, 50, 50) The Ball Class Page 7 of 9

8 xdir = 5 ydir = 3 Next, we will need to add code that will check if the ball has banged into any of the walls (top, bottom, left or right). In order to do this, we will need to write the code inside the timer s Tick procedure as follows: Private Sub tmrball_tick(byval sender As System.Object, ByVal e As System.EventArgs) Handles tmrball.tick myball.move(xdir, ydir) 'Check if the ball hits the left or right wall If myball.x + myball.width >= Me.Width - 20 Or myball.x <= 0 Then xdir = -xdir End If 'Check if the ball hits the top or bottom wall If myball.y <= 0 Or myball.y + myball.height >= Me.Height - 40 Then ydir = -ydir End If Now when you run the program, the ball will check if it hits any of the four walls and reverse its position accordingly. When checking if the ball reaches the right wall, we need to get the x-position of the ball and add the width because we want to check if the right side of the ball hits the wall. If we didn t add the width, we would be checking if the left side of the ball hits the wall. xpos xpos + width When checking if the ball reaches the bottom wall, we need to do the same thing we did when checking if the ball hits the right wall with the only difference being that we need to use the y-position of the ball and the height of the panel. The Ball Class Page 8 of 9

9 ypos ypos + height The Ball Class Page 9 of 9

DRAWING AND MOVING IMAGES

DRAWING AND MOVING IMAGES Moving images and shapes in a Visual Basic application simply requires the user of a Timer that changes the x- and y-positions every time the Timer ticks. In our first example,