There are four classes that we’ll meet today that will allow us to do real graphics (meaning, drawing shapes and lines) in Java:

• The class PaintPanel and the interface Painter give you a place to draw and an interface for drawing. These two classes allow you to do simple graphics without having to deal with some features of the Java language that are not covered in CSCI 235.
• Class Graphics is used by a Painter to actually do the drawing. You can find more features of this class in section 18.3 of your textbook.
• Class Polygon is used by Graphics to describe polygons.

PaintPanel

A PaintPanel is a window component, just like JLabel or JButton. It can be added to a window. It is simply an area where the shapes will appear. The methods that we’ll be concerned with are

• PaintPanel(int width, int height) – The constructor which allows the programmer to specify the size, in pixels.
• int width() and int height() – methods for finding out the width and height, in pixels, of the panel.
• void setPainter(Painter) – A method for attaching a Painter object to this panel, which will do the actual drawing; more about this below.
• void repaint() – Refresh this panel (after, presumably, changes have been made).

Painter

This interface contains one method, void paint(Graphics g). Objects of classes that implement this interface are attached to a PaintPanel. When the paint() method is called, the Painter draws shapes and lines on the paint panel by calling appropriate methods on the Graphics object, more on that below.

The interaction works like this: Any time a change has been made which affects the graphical display, we need to call repaint() on the PaintPanel. This will automatically call the paint() method of the Painter object attached to the PaintPanel, giving the appropriate Graphics object as a parameter.

This also happens (ie, the paint() method of the Painter is automatically called) when the window containing the PaintPanel is first made visible.

Graphics

An instance of Graphics controls the drawing of a certain GUI component; in our examples it will always be a PaintPanel. When the following methods are called on the Graphics object, then lines, shapes, etc, appear on the PaintPanel. Coordinates act like coordinates in the first quadrant of the plane, except that the origin is in the upper lefthand corner, not the lower lefthand corner.

• setColor(Color c)
  changes the “pen color”, ie, specifies the color of the next thing drawn.
• drawRect(int x, int y, int width, int height)
  draw (the outline of) a rectangle with one corner at point (x,y) and having size width by height. If width and height are both positive, then the given pair of coordinates will specify the upper lefthand corner.
• fillRect(int x, int y, int width, int height)
  like the previous except that the rectangle is filled in.
• drawOval(int x, int y, int width, int height)
  fillOval(int x, int y, int width, int height)
  like drawRect() except for an oval. Note that (x,y) still refers to the upper lefthand “corner” of the oval, which really means the upper lefthand corner of the smallest rectangle that frames the oval. If width and height are equal, then this draws a circle.
• drawPolygon(Polygon p)
  fillPolygon(Polygon p)
  like the above but for a polygon.
• drawLine(int x1, int y1, int x2, int y2)
  draw a line from (x₁,y₁) to (x₂,y₂).

Polygon

This class is used to represent a polygon which can be passed to the fillPolygon() method. We’ll only need the constructor:

Polygon(int[] xpoints, int[] ypoints, int npoints)