Download public void paintComponent(Graphics g)

Swing Graphics
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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
The paintComponent Method
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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:
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the component on which it draws
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the current color and font
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location origin
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clipping information
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and more
Graphics vs. Graphics2D
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The Graphics class has limitations:
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Cannot use real number coordinates
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Cannot draw dotted, dashed, or variable-width lines
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Cannot easily draw complex curves or fill complex
shapes
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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
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
...
}
The paintComponent Method
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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()
Some Basic Graphics Methods
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void setColor(Color color)
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void setFont(Font font)
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void drawString(String text, int x,
int y)
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(x,y)is the coordinate of the lower left corner of the
drawn string's leftmost character
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);
}
}
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();
Display
Notes On The Example
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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
Drawing Shapes
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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.
Inheritance Hierarchy of Geometric
Shape Classes
Line Shapes
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java.awt.geom.Line2D is an abstract class
with two concrete subclasses that are also inner
classes:
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Line2D.Double
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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
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);
}
...
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 triangls as before
}
Rectangular Shapes
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Abstract subclasses of RectangularShape:
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Rectangle2D, RoundRectangle2D
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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
Additional Parameters for Rounded
Rectangles
arcWidth
(x,y)
arcHeight
height
width
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);
}
New Display
Additional Parameters for Arcs
width
(x,y)
-270
90
0
±180
angularExtent
270
-90
height
startAngle
Closure type for the arc: OPEN, CHORD, or PIE
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);
g2D.fill(arc);
arc = new
Arc2D.Double(25,25,150,100,240,120,Arc2D.PIE);
g2D.setColor(Color.orange);
g2D.fill(arc);
}
Arc Display
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);
g2D.fill(arc);
arc =
new Arc2D.Double(25,25,150,100,240,120,Arc2D.CHORD);
g2D.setColor(Color.orange);
g2D.fill(arc);
}
New Arc Display
More Features
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Other things you can do with Shapes:
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Check for containment of points and rectangles
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Get bounding rectangle
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Check for intersection with another bounding
rectangle
Other things you can do with 2D graphics
contexts:
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Change stroke properties
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Draw images
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Perform rotations, scalings, and translations