Simple graphics in Java

There are four classes/interfaces that we can use 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.

The first two of these (PaintPanel and Painter) are not standard Java. But you can feel free to copy them and use them in your own programs. You should always be able to find the source for these two files in the directory /cslab/class/csci235/javalib as well as via a link from the class web page.

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.

Whenever we want the panel to be drawn, we will call repaint() on the PaintPanel. That automatically calls the attached Painter’s method paint() with the appropriate Graphics object.

The panel’s repaint() method is also automatically called whenever the panel needs to be drawn; for example, when it is made visible for the first time, when it is shown after being minimized, or when it is uncovered by a window that hid all or part of it.

Consult sections 18.3–4 of the textbook for additional information about the following.

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.
  (See the documentation for java.awt.Color for names of contants and other ways to get color values.)
• 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 need only the constructor:

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