1. Java Graphics
CS 2511

2. Swing Graphics
Empty Swing containers have no visual appearance except for a background color
Every JComponent must have a paintComponent method that is called when the component is first made visible or needs to be redrawn for some reason
The JPanel component is a lightweight container, making it suitable as a drawing area
A common way to do graphics is to extend the JPanel class and override the paintComponent method

3. The paintComponent Method
Called by the JVM when this component needs to be redrawn
A single argument, the component's graphics context (class: Graphics), is passed when paintComponent is called
A Graphics object contains:
the component on which it draws
the current color and font
location origin
clipping information
and more

4. Graphics vs. Graphics2D The Graphics class has limitations:
Cannot use real number coordinates
Cannot draw dotted, dashed, or variable-width lines
Cannot easily draw complex curves or fill complex shapes
Cannot use textures or gradient colors to fill shapes
The newer Graphics2D class extends Graphics and provides these capabilities
All GUI components use a Graphics2D object but paintComponent passes a Graphics object for backward compatibility

5. General Approach public class MyPanel extends JPanel {
// instance variables
public MyPanel() {
// public constructor }
// public methods
// private helper methods
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D)g;
// drawing messages sent to g2d
...

6. The paintComponent Method
super.paintComponent(g) is called first to ensure that painting responsibilities defined in JPanel are carried out
You should not call paintComponent directly; it is called by the JVM when it needs to
You can indirectly call paintComponent on a component by using component.repaint()

7. Some Basic Graphics Methods
void setColor(Color color)
void setFont(Font font)
void drawString(String text, int x, int y)
(x,y)is the coordinate of the lower left corner of the drawn string's leftmost character

8. Graphics Example import javax.swing.*; import java.awt.*;
public class GraphicsPanel extends JPanel {
public GraphicsPanel() {
setPreferredSize(new Dimension(200,200));
setBackground(Color.magenta); // panel color }
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2D = (Graphics2D)g;
g2D.setColor(Color.blue); // drawing color
g2D.setFont(new Font("Helvetica", Font.BOLD, 24));
g2D.drawString("Hello World", 25, 25);

9. Graphics Example (cont'd)
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class MainFrame extends JFrame {
public MainFrame() {
setSize(new Dimension(500,300));
setLocation(100,100);
addWindowListener(new WindowAdapter () {
public void windowClosing(WindowEvent e) {
dispose();
System.exit(0); }
});
getContentPane().setLayout(
new FlowLayout(FlowLayout.CENTER));
GraphicsPanel gp = new GraphicsPanel();
getContentPane().add(gp);
setVisible(true);
public static void main(String[] args) {
new MainFrame();

10. Display

11. Notes On The Example
GraphicsPanel extends JPanel so that the paintComponent method can be overridden
If you forget to call super's paintComponent method, you can get pixels from another desktop frame as background garbage
The background color is associated with the panel; the paint color with the Graphics2D object
The MainFrame class extends JFrame and an instance of it is created in the main method

12. Drawing Shapes
You can draw any object that implements the java.awt.Shape interface.
Example: suppose g2D is a Graphics2D object:
Shape s = ...;
g2D.draw(s);
The Java library supplies a number of classes
that implement the Shape interface type.

13. Inheritance Hierarchy of Geometric Shape Classes

14. Line Shapes
java.awt.geom.Line2D is an abstract class with two concrete subclasses that are also inner classes:
Line2D.Double
Line2D.Float
A Line2D object represents a line segment in (x,y) coordinate space.
To create a line segment, first create its endpoints using the java.awt.geom.Point2D class

15. Line Example import java.awt.geom.*; ...
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2D = (Graphics2D)g;
Point2D.Double corner1 = new Point2D.Double(100, 50);
Point2D.Double corner2 = new Point2D.Double(50, 150);
Point2D.Double corner3 = new Point2D.Double(150, 150);
Line2D.Double side1 = new Line2D.Double(corner1, corner2);
Line2D.Double side2 = new Line2D.Double(corner2, corner3);
Line2D.Double side3 = new Line2D.Double(corner3, corner1);
g2D.draw(side1);
g2D.draw(side2);
g2D.draw(side3); }

16. An Easier Way: Polygons
java.awt.Polygon implements the Shape interface.
Specify the x and y coordinates of a closed polygon's
vertices with the following constructor:
Polygon(int[] xpoints,
int[] ypoints,
int npoints)
import java.awt.*;
...
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2D = (Graphics2D)g;
Polygon triangle = new Polygon(new int[] {100, 50, 150},
new int[] {50, 150, 150},
3);
g2D.draw(triangle); // produces same triangles as before }

17. Rectangular Shapes Abstract subclasses of RectangularShape:
Rectangle2D, RoundRectangle2D
Ellipse2D, Arc2D
Each has concrete .Double and .Float subclasses
Each constructor requires x,y coordinate of upper left corner of bounding rectangle, and also the rectangle's width and height
Use draw to draw an outline of the shape in the current color.
Use fill to fill the shape with the current color.

18. Additional Parameters for Rounded Rectangles
arcWidth
(x,y)
arcHeight
height
width

19. Example public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2D = (Graphics2D)g;
Rectangle2D.Double rect1 =
new Rectangle2D.Double(25,25,100,100);
g2D.setColor(Color.black);
g2D.draw(rect1);
RoundRectangle2D.Double rect2 =
new RoundRectangle2D.Double(50,50,100,100,80,30);
g2D.setColor(Color.green);
g2D.fill(rect2);
Ellipse2D.Double rect3 =
new Ellipse2D.Double(75,75,100,80);
g2D.setColor(Color.blue);
g2D.fill(rect3); }

20. New Display

21. Additional Parameters for Arcs
width
-270 90
(x,y)
±180
height
270
-90
angularExtent
startAngle
Closure type for the arc: OPEN, CHORD, or PIE

22. Arc Example public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2D = (Graphics2D)g;
Arc2D.Double arc =
new Arc2D.Double(25,25,150,100,0,120,Arc2D.PIE);
g2D.setColor(Color.black);
g2D.fill(arc);
arc = new Arc2D.Double(25,25,150,100,120,120,Arc2D.PIE);
g2D.setColor(Color.green);
arc = new Arc2D.Double(25,25,150,100,240,120,Arc2D.PIE);
g2D.setColor(Color.orange); }

23. Arc Display

24. Arc Example Modified public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2D = (Graphics2D)g;
Arc2D.Double arc =
new Arc2D.Double(25,25,150,100,0,120,Arc2D.CHORD);
g2D.setColor(Color.black);
g2D.fill(arc);
arc =
new Arc2D.Double(25,25,150,100,120,120,Arc2D.CHORD);
g2D.setColor(Color.green);
new Arc2D.Double(25,25,150,100,240,120,Arc2D.CHORD);
g2D.setColor(Color.orange); }

25. New Arc Display

26. More Features Other things you can do with Shapes:
Check for containment of points and rectangles
Get bounding rectangle
Check for intersection with another bounding rectangle
Other things you can do with 2D graphics contexts:
Change stroke properties
Draw images
Perform rotations, scalings, and translations