This tutorial gives you the bare minimum you need to paint basic geometric shapes in Java using the Graphics2D functions.
Everything you will paint will be done through member functions of the class Graphics2D. The BufferedPanel window used in our applications has a member function GetBufferGraphics that returns its Graphics2D object - so called graphics context:
You see the following statement at the beginning of the functions that do the painting in the given window:
Graphics2D G = window.getBufferGraphics();
Next, you need to define at least one object for each of the three shapes (Line, Ellipse, Rectangle) you want to paint. You do this as follows:
Rectangle2D.Double R = new Rectangle2D.Double;
Ellipse2D.Double E = new Ellipse2D.Double;
Line2D.Double L = new Line2D.Double;
Each of these shapes also has a Float version, but we will not use them. The .Double indicates that the coordinates and sizes of these shapes are given as double.
To set the location and size of each object, we use the following statements:
R.setRect(left, top, width, height);
E.setFrame(left, top, width, height);
L.setLine(x1, y1, x2, y2);
Notice, that to set the location and size of an oval (ellipse), we specify the size of the rectangular frame around the ellipse.
Now we can actually paint these shapes as follows:
In addition, we can use the draw function with Ellipse or Rectangle to draw just the outlines of these shapes.
To set the colors, use the Graphics2D member function setPaint() in one of the following two forms:
// standard issue red
// pale blue that you choose
G.SetPaint(new Color(100, 100, 255);
The Color class defines the following color constants:
Color.white . . . . . . . . . new Color(255, 255, 255)
Color.lightGray . . . . . . . new Color(192, 192, 192)
Color.gray . . . . . . . . . new Color(128, 128, 128)
Color.darkGray . . . . . . . new Color(64, 64, 64)
Color.black . . . . . . . . . new Color(0, 0, 0)
Color.red . . . . . . . . . . new Color(255, 0, 0)
Color.pink . . . . . . . . . new Color(255, 175, 175)
Color.orange . . . . . . . . new Color(255, 200, 0)
Color.yellow . . . . . . . . new Color(255, 255, 0)
Color.green . . . . . . . . . new Color(0, 255, 0)
Color.magenta . . . . . . . . new Color(255, 0, 255)
Color.cyan . . . . . . . . . new Color(0, 255, 255)
Color.blue . . . . . . . . . new Color(0, 0, 255)
There are many tricks you can play with colors - we include only one that is used in the Target task of the Loops lab. In this lab, we are painting progressively larger circles, but want to reverse colors on all the existing circles. So we start with one red circle. The larger second circle is show in red, but the part that was the original first circle is converted to the original background color, which was white. When the third circle is painted, the inside turns red again, the middle band turns to white and the area outside of the previous two circles turns red.
To do this we started the task with the call
and reset to the original mode at the end by calling:
In addition there is a round rectangle and an arc.
The window in the Locomotive painting is a rounded rectangle, defined and set as follows:
= new RoundRectangle2D.Double();
Rd.setRoundRect(x + 0.65 * width, y + 0.15 * height,
0.2 * width, 0.3 * height,
0.1 * width, 0.1 * height);
The last two values in the setRoundRect function specify the width and the height of the rounded arc. The coordinates still specify the whole rectangle - without the corners rounded off.
The Arc is a part of an ellipse. Again, you need to set the Arc by specifying the location and size of the frame enclosing the ellipse from which the arc is "cut out", then specify the starting and ending angle (in degrees, though this is a bit tricky and we will not explain it here). Finally, you need to select the method for cutting out the piece of the pie from the ellipse. To do so, use one of the following constants as the last argument for the setArc function: OPEN, PIE, CHORD. The first one will give you just a curved line, the second will give you a pie piece, and the third one will connect the two end points of the arc with a straight line:
Arc2D.Double A = new Arc2D.Double();
A.setArc(left, top, width, height, 0, 90, PIE);
The code shown above will give you the top right quarter of the ellipse.
If you want to learn more about the use of Graphics2D class and creating graphics, the best reference is:
Jonathan Knudsen, Java 2D Graphics, O'Reilly, 1999.