Graphics in Java

1. Graphics in Java • Graphical information appears onscreen inside frame objects • actually frame windows with title bars • To use frames, import the JFrame class from the javax.swing package.

2. Displaying Java graphics • To display graphics, frames need to • be constructed, • be given a nonzero size, and • be made visible • Frames often should • have titles • halt their creating program when they are closed • Methods and constructors to do all this are described on p. 61 of the text.

3. Coordinates and frames • Frames use the coordinate system that we saw in Section 2.4. • This system is like a 2-dimensional array of pixels. • But note that the point at position (x,y) is not in row x and column y. • instead it's in row y and column x.

4. Java components • Objects that appear in frames (component objects) • must be added explicitly to frames, and • must know how to draw themselves. • Component objects are those whose class extends the JComponent class.

5. Drawing Java components • Component objects need a public method voidpaintComponent that takes a Graphicsargument. • To draw a component object, cast the Graphicsargument to a Graphics2Dobject, then send messages to this object. • The component object needs to know how to draw its own components. • the methods to do this needn't have any special name.

6. The Graphics2D class • Methods for the Graphics2D class include • draw • setColor • fill, and • drawString • The drawand fill methods take Shape objects as arguments.

7. Shapes in Java • Instances of the Rectangle, Ellipse2D.Double, and Line2D.Double class are Shape objects. • One Ellipse2D.Double constructor has four arguments like those of Rectangle. • They specify a bounding box (or framing rectangle, or bounding rectangle) for the ellipse • One constructor for Line2D.Double has 4 arguments -- the x and y coordinates of one endpoint, then those of the other.

8. Drawing text strings in Java • The drawString method takes as arguments the string and the coordinates of the lower-left corner of its desired position • cf. Figure 23 on p. 67 of the text.

9. Colors in Java • The setColor method takes a Color object as argument and uses this color for subsequent drawing. • An instance of the Color class can be initialized to one of the constants of that class • cf. the table on p. 68 • Or a 3-argument constructor can be used for initialization • it takes the color’s red, green, and blue components as integers from 0 through 255

10. Important details for Java graphics • The Graphics, Graphics2D, Color, Ellipse2D.Double, and Line2D.Double classes should be properly imported • cf. the example on p. 69 of the text • Note that to import the latter two classes, you don’t need to specify the full class name. • To add a component to a frame object, pass the add message to the frame with the component as argument.

11. Java applets • One common application of Java graphics is to applets – programs that run inside a web browser • cf. page 6 of the text

12. Drawing complicated images • For all but the simplest drawings, it’s a good idea to have a class for each object in the drawing. • If objects have several pieces of the same type, it’s likely worth having classes for them. • cf. Section 3.8