Day 6 Some Basic Graphics

Transcription

1 Introduction Day 6 Some Basic Graphics The preceding days have focused on basic Java syntax and those concepts fundamental to object-oriented programming: encapsulation, inheritance and polymorphism. We now change directions a bit and move into a how-to phase. In the following section, we will describe the classes and methods necessary to set up a simple GUI (graphical user interface) and do some graphics programming. Finally, we use a few graphics methods and a little recursion to draw a simple fractal, Sierpienski s gasket. Java Graphics Terms We begin with a short vocabulary of graphics terms: A Component is anything that is displayed on the screen, e.g., windows, buttons, text boxes, labels etc. All component classes extend the Component class. Components are sometimes called widgets. A Container is an object that can hold components. A container is a component that can contain other objects. The class Container is a subclass of Component so a Container is-a Component. A Window object is a window without a title bar. A Frame is a window with a title bar and border. To create a user interface you must first create a frame (or applet) to hold the components of the interface. A frame contains sub-elements called Panes. Most of the time you will use a window pane, called the content pane. The content pane is where you normally add components. A Panel is an invisible container that holds user-interface components. Normally you add components such as buttons to a panel and then you add the panel to the content frame. Component hierarchy The Abstract Windows Toolkit and Swing Java provides two libraries that contain the classes for graphics programming: the original Abstract Windows Toolkit (AWT) and the newer Swing library. AWT was the original class library that Sun provided for graphics programming. While programs designed with AWT components (buttons, text fields, menus etc.) run on any platform (e.g., Windows, Unix) such components used the interface elements of the particular platform. In other words, a button on a Windows machine may not look or behave the same way as a button on a Unix machine or an Apple. GUI programs designed on one platform did not always work well on another. The more modern Swing library paints the components on the screen so that the look and feel of a GUI is consistent from platform to platform. Swing does not replace AWT; in fact, Swing uses many AWT classes. Swing does provide new user interface components (e.g., buttons, textboxes, check boxes etc.) which are platform independent. The following chart compares Swing and AWT. 199

2 AWT in java.awt package each component was mapped to a corresponding platform-dependent interface called a peer. platform specific and prone to platform specific bugs. components may look different on different platforms. Components have the look of a particular platform. Swing in javax.swing package no platform dependent peers. Components have a Java look. code written in Java. components are called lightweight. components are all prefixed with J, e.g., JButton, JLabel. components are called heavyweight. The Java component hierarchy has the following form: 200

3 Component Class Methods: Every component in the hierarchy inherits the following methods whose purposes are evident from their names. void setsize(int width, int height) void setlocation(int xcoord, int ycoord) // move the top left corner of the component to point (x, y) of the container. //The upper left corner of a container has position (0, 0) and (x, y) is the point located x //pixels to the right of and y pixels down from (0, 0). void setbounds(int x, int y, int width, int height) // moves component to position (x, y) and resizes void setenabled(boolean x) // e.g., a button can be greyed0ut void setvisible(boolean x) void setname(string name) int getheight() int getwidth() int getx() //gets x-coordinate of the component int gety() // gets y-coordinate String getname() boolean isenabled() boolean isvisible() //Methods of the Frame Class: void settitle(string title) void setresizable(boolean x) void show() void hide() void toback() // sends window to the back void tofront() //sends window to the front 201

4 Example: The following program will create a frame with title I ve been framed. You should note that the upper left corner of the screen has coordinates (0, 0) and that the screen point (x, y) is located x pixels to the right of and y pixels down from (0, 0). Consequently, a call to setbounds (x, y, width, height) by a JFrame object places the upper left corner of the frame at position (x, y) relative to the upper left corner of the screen. Also, notice the close() method in main(). import javax.swing.*; public class MyVeryFirstFrame extends JFrame public MyVeryFirstFrame () settitle("i've been framed!"); setbounds(267,200,267,200); public MyVeryFirstFrame (String title) settitle(title); setbounds(267,200,267,200); public class GUI1 JFrame frame = new MyVeryFirstFrame ("------I've been framed!----"); frame.show(); frame.setresizable(false); public class GUI0 //alternate version JFrame frame = new JFrame(); frame.settitle("i ve been framed!"); frame.setbounds(250,250,200,200); frame.setvisible(true); frame.setresizable(true); 202

5 Centering a Frame You can use methods from the Toolkit class (in AWT) to center a frame. Simply instantiate a Toolkit object and invoke the following methods: Toolkit getdefaulttoolkit() //static method returns Toolkit object for the current system Dimension getscreensize() //not static The Dimension class has two accessible fields: int height int width Thus if d is a Dimension object, d.height and d.width represent the height and width of the screen. The following program centers a frame relative to the screen. Example: import java.awt.*; import javax.swing.*; public class AnotherFrameClass extends JFrame public AnotherFrameClass() settitle("i am framed!"); setbounds(267,200,267,200); public AnotherFrameClass(String title) settitle(title); setbounds(0,0,300,300); public class GUI2 JFrame frame = new AnotherFrameClass("------I am framed!----"); frame.show(); frame.setresizable(false); Toolkit tk = Toolkit.getDefaultToolkit(); Dimension d = tk.getscreensize(); int width = 300; int height = 300; System.out.println("Screen: "+d.width+" "+d.height); System.out.println("Top corner: " + ( d.width-width)/2+ " "+ (d.height-height)/2); frame.setbounds((d.width-width)/2, (d.height-height)/2,width,height); 203

6 The picture below may be a bit deceiving because the frame is placed in the middle of the screen and not the DOS window. 204

7 The window (or its subclasses Frame /JFrame) is a top-level container. You cannot add components directly to a frame. You must first add the components to the ContentPane and then add the ContentPane to the frame. A Frame is called a top-level Container and a ContentPane an intermediate level Container. Adding a Button to a Frame How to add components to the content pane: To add components to the content pane, invoke the getcontentpane() method in JFrame. This method returns the content pane as a Container object. The method add(component x) (in the Container class), adds the component to the calling container. Example: The following example will add a simple button to a frame. A button is a member of the JButton class: JButtonClass Constructors: JButton() JButton(String text) //Other methods in JButton void settext(string text) String gettext() void setenabled(boolean x) We can now add a button. public class GUI3 JFrame frame = new AnotherFrameClass("------I am framed----"); Container contentpane = frame.getcontentpane(); JButton button = new JButton("Hello"); contentpane.add(button); frame.show(); 205

8 Example: Here we add two buttons--- or do we? public class GUI3a JFrame frame = new AnotherFrameClass("------I am framed!----"); Container contentpane = frame.getcontentpane(); JButton button1 = new JButton("Hello"); JButton button2 = new JButton("Bye"); contentpane.add(button1); contentpane.add(button2); frame.show(); 206

9 Aren t there supposed to be two buttons? What happened to the Hello button? Well, yes, there are two buttons, but the second was placed on top of the first. This is not a particularly desirable action. However, Java provides layout managers to help with the placement of components in a frame (content pane). Containers use layout managers (from the LayoutManager classes) to arrange their components. In the previous example, the default LayoutManager placed both buttons in the ContentPane in the center and one on top of the other. However, there are several LayoutManager classes available so that you can have a bit more control over the placement of components. A LayoutManager object will help with placement of components. The first and simplest of the Layout Managers is the FlowLayoutManager. The FlowLayoutManager arranges components in a container left to right, in the order in which they are added to the container. Constructors of FlowLayout Class FlowLayout() constructs a FlowLayout manager with centered alignment, i.e., components will be centered in a Container. FlowLayout(Alignment) constructs a FlowLayout manager with specified alignment: CENTER, LEFT, or RIGHT FlowLayout(Alignment, int x, int y) constructs a FlowLayout manager with specified alignment. Parameters x and y represent horizontal and vertical space between components. The alignment fields of the FlowLayout class, CENTER, LEFT, and RIGHT are accessed as FlowLayout.CENTER etc. 207

10 Method of the Container class needed to layout components setlayout(layoutmanager m) sets the layout manager for this container For example: If cp represents the content pane for some frame then cp.setlayout (new FlowLayout()); causes components to be added to the content pane using a FlowLayout object. Example: In the following example 10 buttons are added to the content pane of a frame. The buttons are left justified. public class GUI4 JFrame frame = new AnotherFrameClass("------I am framed----"); Container contentpane = frame.getcontentpane(); contentpane.setlayout(new FlowLayout(FlowLayout.LEFT)); for(int i = 0; i<10; i++) contentpane.add(new JButton("Button "+i)); frame.show(); 208

11 Here is a different version with center alignment: Java and Object Oriented Programming public class GUI4a JFrame frame = new AnotherFrameClass("------I have been framed----"); Container contentpane = frame.getcontentpane(); JButton button; contentpane.setlayout(new FlowLayout(FlowLayout.CENTER)); for(int i = 0; i<10; i++) button = new JButton("Button "+i); contentpane.add(button); frame.show(); 209

12 BorderLayout Manager The BorderLayout manager divides the window into five areas: NORTH WEST SOUTH EAST CENTER Constructors: BorderLayout() BorderLayout(int horizontalgap, int verticalgap) Component method: add(component, region) Constants: NORTH WEST SOUTH EAST CENTER Example: public class GUI5 JFrame frame = new AnotherFrameClass("------I am framed----"); Container contentpane = frame.getcontentpane(); JButton button; contentpane.setlayout(new BorderLayout()); contentpane.add(new JButton("Dewey"), BorderLayout.NORTH); contentpane.add(new JButton("Hughie"), BorderLayout.EAST); contentpane.add(new JButton("Louie"), BorderLayout.WEST); frame.show(); 210

13 GridLayout Manager The GridLayout manager arranges the components in a grid/table/matrix like fashion, left to right, top to bottom, row by row. Constructors GridLayout(int rows, int cols) GridLayout(int rows, int columns, int x, int y) x and y are the horizontal and vertical gaps between components Gridlayout() one column in a single row Example: public class GUI6 JFrame frame = new AnotherFrameClass("------I am framed----"); Container contentpane = frame.getcontentpane(); JButton button; contentpane.setlayout(new GridLayout(3,2)); for(int i = 1; i <= 6; i++) button = new JButton("Button " + i); contentpane.add(button); frame.show(); 211

14 Here is a grid with gaps between the buttons: Java and Object Oriented Programming public class GUI6a JFrame frame = new AnotherFrameClass("------I am framed----"); Container contentpane = frame.getcontentpane(); JButton button; frame.setbounds(267,200,367,200); contentpane.setlayout(new GridLayout(2,3,20,20)); for(int i = 1; i <= 6; i++) button = new JButton("Button " + i); contentpane.add(button); frame.show(); 212

15 Panels A panel is an invisible container used for grouping components. A panel may contain sub-panels as well as other components. A panel is an intermediate level container. A panel may have a layout manager. For example, you might place five buttons on panel1 using a flow layout manager, and on panel2 you might arrange four textboxes using a grid layout manager. Now, you can place these two panels or groups of components on the content pane using, say, the BorderLayout manager. The program in the following example places buttons on three different panels and then places the three panels on the content pane. public class GUI7 JFrame frame = new AnotherFrameClass("Using Panels"); JButton button; Container contentpane = frame.getcontentpane(); // constructors for 3 panels JPanel p1= new JPanel(); JPanel p2 = new JPanel(); JPanel p3 = new JPanel(new FlowLayout()); // p1 uses a GridLayout manager p1.setlayout(new GridLayout(3,2)); for (int i = 1; i <= 6; i++) button = new JButton(" "+i); p1.add(button); //p2 uses a BorderLayout manager p2.setlayout(new BorderLayout()); p2.add(new JButton("East"), BorderLayout.EAST); p2.add(new JButton("West"), BorderLayout.WEST); //p3 uses a FlowLayout as set in its constructor for (int i = 1; i <= 4; i++) button = new JButton("Button "+ i); p3.add(button); //contentpane uses a Border Layout manager contentpane.setlayout(new BorderLayout()); contentpane.add(p1, BorderLayout.WEST); contentpane.add(p2, BorderLayout.EAST); contentpane.add(p3, BorderLayout.SOUTH); frame.show(); 213

16 . Here is another form of the same program: public class NewFrame extends JFrame public NewFrame()// default constructor JButton button; Container contentpane = this.getcontentpane();//this is not necessary JPanel p1 = new JPanel(); JPanel p2 = new JPanel(); JPanel p3 = new JPanel(); p1.setlayout(new GridLayout(3,2)); for (int i = 1; i <= 6; i++) button = new JButton(" "+i); p1.add(button); p2.setlayout(new BorderLayout()); p2.add(new JButton("East"), BorderLayout.EAST); p2.add(new JButton("West"), BorderLayout.WEST); p3.setlayout(new FlowLayout()); for (int i = 1; i <= 4; i++) button = new JButton("Button "+ i); p3.add(button); contentpane.setlayout(new BorderLayout()); contentpane.add(p1, BorderLayout.WEST); contentpane.add(p2, BorderLayout.EAST); contentpane.add(p3, BorderLayout.SOUTH); public class GUI7a JFrame frame = new NewFrame(); frame.setbounds(267,200,267,200); frame.settitle("using Panels"); frame.show(); 214

17 Placing Components on a Frame Without a Layout Manager A layout manager is a convenience but not a necessity. You may place components on a frame without a layout manager. To do this you must first set the Layout of your container to null. The following program places buttons onto a frame without using a layout manager. public class AnotherFrameClass extends JFrame public AnotherFrameClass() settitle("my First Frame"); setbounds(250,250,200,200); public AnotherFrameClass(String title) settitle(title); setbounds(0,0,300,300); public class GUI8 JFrame frame = new AnotherFrameClass("NO LAYOUT MANAGERS"); Container contentpane = frame.getcontentpane(); contentpane.setlayout(null); // not even the default one JButton button1 = new JButton("Moe"); button1.setbounds(30,30,70,50); // inherited from Container contentpane.add(button1); JButton button2 = new JButton("Larry"); button2.setbounds(30,200,70,30); contentpane.add(button2); JButton button3 = new JButton("Curley"); button3.setbounds(200,115,70,70); contentpane.add(button3); frame.show(); 215

18 The paint and paintcomponent Methods Almost all containers and components have a paint (or paintcomponent) method. The paintcomponent method is used for JComponents (e.g., JPanel) otherwise the paint method is used. (Note: JFrame is not a JComponent.) The paint(component) method for a particular component draws the component on the screen. The paint(component) method is called automatically whenever the component has to be drawn on the screen. For example, if a component is resized the paint method is called. Do not invoke the paint(component) method directly. The paint(component) method for each component knows how to draw that component. Thus the paint method for a JFrame knows how to draw a frame. Nonetheless, all frames are not equal. Frames come in various shapes, sizes and colors. Each frame (or other component) must supply a bit more information (e.g., a location) to the paint method. This information is passed to the paint method via a parameter a Graphics object: void paint(graphics g) This parameter, g, supplies information about how to draw the particular component, where g is an instance of a Graphics subclass class. The Graphics class. Graphics is an abstract class. Every component that can be drawn on the screen has an associated Graphics object. Thus a button has an associated Graphics object, so does a panel, a frame etc. A program does not directly create a Graphics object with a constructor. Java takes care of that. A component s Graphics object encapsulates the graphics content of a component (i.e., the dimensions of the component, the location, the color, the font etc.) which is all the information necessary for drawing the component. 216

19 When the paint method is (automatically) invoked, the Graphics object of the calling object is passed to paint. For example, if a JFrame frame, must be painted, then frame.paint(g) is (automatically) invoked. The object g is the Graphics object associated with frame -- the graphics context of frame. The object g contains information used by the paint method to draw the JFrame frame on the screen. If it is necessary to repaint a component, the Component class supplies a method, repaint(). The repaint() method calls paint(graphics g). The Graphics class also has a number of methods. Four of the most commonly used methods of the graphics class are: void drawstring(string message, int x, int y) // (x, y) is starting position. void setcolor(color c) void setfont(font f) void drawimage(image img, int x, int y, ImageObserver observer) where an Image object is an image (e.g.,.jpg or.gif file), x and y designate the position of the image, and observer is the object on which the image is drawn, usually this. (In subsequent examples, you will see how to load an image.) Notice the parameters, Color and Font. Color and Font both extend Object: The Color Class Constructor: Color(byte r, byte g, byte b) //where r, g, and b are in the range Example: Color c = new Color(255, 0, 0); //c is a red color object Color c = new Color(100,100,100); The class also has a number of class constants available: red, white, blue, green, yellow, black, cyan, magenta, pink, orange, gray, lightgray, and darkgray. These colors are accessed with the class name, e.g., Color.red. The component class, which is the parent of all components, has methods: setbackground(color c) setforeground(color c). So, any component can invoke these methods as component.setbackground(color.red); or as: or Color c = new Color(255,0,0); component.setbackground(c); component.setbackground(new Color(255,0,0); 217

20 The Font class Constructor: Font(String name, int style, int size) Example: Font font = new Font( Courier, Font.BOLD, 12); style should be a class constant: Font.PLAIN, Font.BOLD, Font.ITALIC, with values 0,1,2. The constants may be joined Font.BOLD+Font.ITALIC (value 3) etc. So for example, to make a font both bold and italic instantiate the font as or less intuitively as Font font = new Font( Courier, Font.BOLD+Font.ITALIC, 12); Font font = new Font( Courier, 3, 12); //since BOLD+ITALIC = 1+2 = 3 Methods: public String getname() public int getsize() public int getstyle() public boolean isplain() public boolean isitalic() public boolean isbold() Drawing on Panels To draw on a panel, extend the JPanel class by overriding the paintcomponent(graphics g) method. Example: In the code below, NewPanel is-a JPanel. However, NewPanel has its own enhanced version of paintcomponent. When NewPanel is drawn, the customized paintcomponent method will automatically be invoked. import java.awt.*; import javax.swing.*; public class NewPanel extends JPanel public void paintcomponent(graphics g) super.paintcomponent(g); g.setcolor(color.yellow); Font font= new Font("Sans Serif", Font.BOLD, 16); g.setfont(font); setbackground(color.red); //inherited from Component g.drawstring("alas, poor Yorick, I knew him well", 50,50); 218

21 public class GUI1 public static void main(string args[]) JFrame frame = new JFrame("Painting Panels"); frame.setbounds(0,0,400,200); Container cp = frame.getcontentpane(); NewPanel p = new NewPanel(); cp.add(p); frame.show(); Here the Graphics object associated with this component has a context where the background color is red, the font is bold, San Serif, yellow and 16 point. Notice, the paintcomponent of the super class is called first: super.paintcomponent(g); Example: Displaying an image The following example displays the picture (familiar to baby boomers) stored in the file peterpan1.jpg. You should note that the statement Image picture = new ImageIcon("peterpan1.jpg").getImage(); obtains the image (an object) that is stored in the file peterpan1.jpg. Image is an abstract class, ImageIcon is a subclass of Image, and getimage() is a method of the ImageIcon class. public class PeterPan extends JPanel public void paintcomponent(graphics g) super.paintcomponent(g); Image picture = new ImageIcon("peterpan1.jpg").getImage( ); g.drawimage(picture, 0, 0, this); 219

22 public class ShowPicture public static void main(string args[]) JFrame frame = new JFrame("I m Flyin "); frame.setbounds(0,0,300,300); Container cp = frame.getcontentpane(); PeterPan p = new PeterPan(); cp.add(p); frame.show(); Output: 220

23 Drawing Shapes The following methods of the Graphics class can be used to draw shapes. void drawline(int startx, int starty, int endx, int endy) void drawoval(int x, int y, int width, int height) //Draws an ellipse within the boundary of the rectangle specified void drawrect(int x, int y, int width, int height) void fillrect(int x, int y, int width, int height) void draw3drectangle(int x, int y, int w, int h, boolean raised) void drawstring(string message, int x, int y) // (x, y) is the starting position void filloval(int x, int y, int width, int height) void drawarc(int x, int y, int w, int h, int startangle, int spanangle) 221

24 Example: The following example uses the methods of Graphics to draw not-so-happy Yorick. import java.awt.*; import javax.swing.*; public class Face extends JPanel public void paintcomponent(graphics g) super.paintcomponent(g); g.setcolor(color.black); Font font= new Font("Comic Sans Serif", Font.BOLD, 16); g.setfont(font); setbackground(color.white); // a method of Component g.drawoval(50, 50, 200, 200); // face g.filloval(100,100, 25,25); //left eye g.filloval(150,100, 25,25); // right eye g.drawline(125,135,100,160); // upper nose g.drawline(100,160, 120,160); // lower nose g.drawarc(75, 175, 100, 40, 350, 200); //mouth g.drawstring("alas, poor Yorick, I knew him well", 20,20); public class GUI7a public static void main(string args[]) JFrame frame = new JFrame("Painting Panels"); frame.setbounds(0,0,300,300); Container cp = frame.getcontentpane(); Face p = new Face(); cp.add(p); frame.show(); 222

25 The Polygon class The data of the Polygon class consists of a list of coordinates (x, y). Each ordered pair defines the vertex of a polygon and each successive pair represents a line segment that is an edge of a polygon. An edge also joins the first and last pairs. Constructors: Polygon() //empty polygon Polygon( int[] xarray, int[] yarray, int numpoints) Methods: drawpolygon(int[] x, int[] y, int num); drawpolygon( Polygon p); fillpolygon(int[] x,int[] y, int num); addpoint(int x, int y); Example: This simple program draws a parallelogram on a panel. import javax.swing.*; import java.awt.*; class PolygonPanel extends JPanel public void paintcomponent(graphics g) super.paintcomponent(g); g.setcolor(color.magenta); setbackground(color.white); Polygon parallelogram = new Polygon( ); parallelogram.addpoint(100, 100); parallelogram.addpoint(200, 100); parallelogram.addpoint(225, 20); parallelogram.addpoint(125, 20); g.drawpolygon(parallelogram); 223

26 public class GUI8 JFrame frame = new JFrame("Polygon"); Container cp = frame.getcontentpane(); cp.add(new PolygonPanel()); frame.setbounds(0,0, 300, 200); frame.setvisible(true); Here is another version of the same program: public class Polygon2Panel extends JPanel public void paintcomponent(graphics g) super.paintcomponent(g); g.setcolor(color.magenta); int[ ] x = 100, 200, 225, 125; int[ ] y = 100, 100, 20, 20; Polygon parallelogram = new Polygon(x, y, 4); g.drawpolygon(parallelogram); public class GUI9 JFrame frame = new JFrame("Drawing Polygons"); Container cp = frame.getcontentpane(); cp.add(new Polygon2Panel()); frame.setbounds(100,100, 300,300); frame.setvisible(true); 224

27 Example: Recursive drawing Java and Object Oriented Programming Below is a picture of what is called Sierpinski s gasket. To generate Sierpinski s gasket, begin with an equilateral triangle. Now, find the midpoint of each side and form three more triangles as in the diagram, discarding the triangle in the center. 225

28 Repeat the process giving 9 triangles. Continue forever producing 1, 3, 9, 27, 81, 243, 729,... triangles. The Sierpinski s gasket is the set of points that remain if the process is carried out indefinitely. The amazing thing about the Sierpinski s gasket is that Sierpinski s gasket has zero area, despite containing an infinite number of points. The next program draws Sierpinski s gasket on a panel. The program first calls the paintcomponent method that, in turn, makes the first call to the recursive method sier, which does the actual drawing. The recursion stops when the depth of recursion has exceeded

29 import javax.swing.*; import java.awt.*; public class Sierp extends JPanel int x1,y1,x2,y2,x3,y3,depth; // coordinates of a triangle public Sierp(int a, int b, int c, int d, int e, int f, int dep) x1 =a; y1 =b; x2 =c; y2 =d; x3 =e; y3 =f; depth = dep; public void sier(int x1, int y1, int x2, int y2, int x3, int y3, int depth, Graphics g) // set up an draw a triangle Polygon triangle = new Polygon(); triangle.addpoint(x1,y1); triangle.addpoint(x2,y2); triangle.addpoint(x3,y3); g.drawpolygon(triangle); // recursively draw three triangles using one original point and two midpoints if (depth > 0) sier(x1,y1, (x1+x2)/2, (y1+y2)/2, (x1+x3)/2, (y1+y3)/2, depth-1,g); sier((x1+x2)/2, (y1+y2)/2, x2,y2, (x3+x2)/2, (y3+y2)/2, depth-1,g); sier((x1+x3)/2, (y1+y3)/2, (x3+x2)/2, (y3+y2)/2, x3,y3, depth-1,g); public void paintcomponent(graphics g) super.paintcomponent(g); g.setcolor(color.black); setbackground(color.white); // starts the recursion sier( x1, y1, x2, y2, x3, y3, depth, g); 227

30 public class GUI9 JFrame frame = new JFrame("Triangle"); Container cp = frame.getcontentpane(); // get a panel and pass the panel coordinates a triangle //as well as the maximum depth of recursion Sierp sp = new Sierp(210,410,10,10,410,10,10); cp.add(sp); frame.setbounds(0,0, 450,450); frame.setvisible(true); The output: 228

31 Example: A somewhat simpler recursive program produced the following megaphone. As in the previous example, the paintcomponent() method of the panel makes the first recursive call. Note that the paintcomponent() method itself is not recursive. Why? import javax.swing.*; import java.awt.*; public class Circles extends JPanel int x,y,w, h; // position (x, y), width and height public Circles(int a, int b, int c) //constructor x = a;//horizontal position y = b;//vertical position w = c;//width h = c; public void draw(int x1, int y1, int w, int h, Graphics g) g.drawoval(x, y, w, h) ; //draw a circle //recursively call draw adjusting location and dimensions if (w > 0) draw(x+10, y+10, w-10, h-10, g); public void paintcomponent(graphics g) super.paintcomponent(g); g.setcolor(color.black); setbackground(color.white); draw( x, y, w, h, g); 229

32 public class GUI10 JFrame frame = new JFrame("Circles"); Container cp = frame.getcontentpane(); //instantiate a panel width and height are both 400 Circles cir = new Circles(10, 10, 400); cp.add(cir); frame.setbounds(0,0, 450, 450); frame.setvisible(true); 230