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Java Software Solutions
Lewis and Loftus
Graphics -- Introduction
• The use of graphics is common among modern software
systems
• Java has strong API support for graphics in the
java.awt (abstract windowing toolkit) package
• Chapter 7 focuses on:
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–
–
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–
Chapter 7
the coordinate system for Java graphics
the use of color
drawing shapes such as lines, ovals, rectangles, etc.
the use of fonts
basic animation techniques
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
Lewis and Loftus
The Graphics Class
• An object of the Graphics class represents a particular
drawing surface
• It defines a graphics context in which drawn shapes will
be rendered
• The Graphics class contains methods for drawing
various shapes and controlling visual aspects like font
and color
• An applet has a graphics context, which is automatically
passed to the paint method when it is called
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
Lewis and Loftus
The Coordinate System
• A simple two-dimensional coordinate system exists for
each graphics context (or drawing surface)
• Each point on the coordinate system represents a single
pixel
• The top left corner of the area is coordinate <0, 0>
• A drawing surface has a particular width and height
• Anything drawn outside of that area will not be visible
• See Coordinates.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The Coordinate System
<0, 0>
X
x
y
<x, y>
<width-1, height-1>
Y
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
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Color
• The Color class is used to define and manage the color
in which shapes are drawn
• Colors are defined by their RGB value, which defines the
relative contribution of the primary colors red, green, and
blue
• Each drawing surface has a foreground color and a
background color
• The setColor method of the Graphics class defines
the foreground color, and the setBackground method
of the component (the applet) sets the background color
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
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Color
• The Color class contains several predefined colors,
defined as public, static constants
• See Nature.java
• Many other colors can be defined using the constructor
of the Color class
• Over 16 million colors can be defined, but humans
cannot distinguish between many similar colors
• Furthermore, the hardware of most systems has
limitations to the color options available
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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XOR Mode
• Drawing in normal mode causes shapes of the same color
to blend together
• Drawing in XOR mode causes the overlapping portions
of the shapes to be rendered in a contrasting color
• This effect can be used to "erase" a shape by redrawing it
in the same color in the same spot while in XOR mode
• See XOR_Demo.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
Lewis and Loftus
Drawing Shapes
• The Graphics class contains methods for drawing
several specific shapes:
lines, ovals, rectangles, arcs, polygons, and polylines
• Most shapes can be drawn filled or unfilled
• A line, drawn with the drawLine method, is always
one pixel wide and cannot be filled
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Ovals
• An oval is defined by its bounding rectangle:
width
height
• The methods that draw an oval take four parameters, all
integers:
drawOval (x, y, width, height)
fillOval (x, y, width, height)
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Lewis and Loftus
Ovals
• The first two parameters are the <x, y> coordinate of the
top-left corner of the bounding rectangle
• The third and fourth parameters specify the width and
height of the bounding rectangle
• The drawOval method draws an unfilled oval and the
fillOval method draws a filled (opaque) oval
• See Ovals.java and Rotating_Disk.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
Lewis and Loftus
Rectangles
• Rectangles can be drawn
– filled or unfilled
– with squared or rounded corners
– with a slight three-dimensional effect or not
• The primary parameters for all rectangle drawing
methods define the upper left corner of the rectangle and
its width and height
• The shape of the rounded corner of a rounded rectangle
are defined by an arc width and height
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Rectangles
• A three dimensional rectangle is shown using a small
highlight on two sides of the rectangle
• The highlight appears on the bottom and right or the top
and left as specified by a boolean parameter to the 3D
drawing methods
• See Rectangles.java,
Rounded_Rectangles.java, and
Three_D_Rectangles.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Arcs
• An arc is defined as a segment of an oval
• The first four parameters to the arc drawing methods
define the bounding rectangle of the oval (top left corner,
width, and height)
• The other two parameters define the start angle and the
arc angle
• The start angle indicates where the arc begins and the arc
angle determines how far the arc sweeps across its
defining oval
• See Arc.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Arcs
• The start angle can be specified using both positive and
negative values:
90
-270
45
-315
0
360
-360
180
-180
270
-90
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Arcs
• An arc angle can also be positive or negative
• A positive arc angle sweeps counterclockwise, and a
negative arc angle sweeps clockwise
• Therefore, the same arc can be specified using four
different combinations of start and arc angles
• Arcs can also be filled or unfilled
• See Arcs.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Polygons
• A polygon is a multisided figure defined by a series of
ordered points
• Line segments connecting the points form the polygon
• The points are defined by corresponding arrays of x and
y coordinate values, and can already be incorporated into
an object of the Polygon class
• Polygons are closed, forming a line segment from the last
point back to the first
• See Polygons.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Polylines
• A polyline is similar to a polygon except that it is not
closed
• That is, there is no line segment from the last point back
to the first unless explicitly specified
• They are convenient for specifying certain kinds of
complex shapes
• Polylines cannot be filled
• See Polylines.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Fonts
• A font defines the look of each character when it is
printed or drawn
• The Font class provides methods for specifying fonts in
a Java program
• Each computer system supports a specific set of fonts
• See Font_Lister.java
• The setFont method defines the current font for a
program
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
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Fonts
• A font is defined using the Font class constructor and a
combination of:
– font name
– font style: plain, bold, italic, or bold+italic
– font size, in points
• Constants are defined in the Font class to specify the
font style
• See Entropy.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
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Animations
• Simple animations can be accomplished by drawing a
shape, then erasing it, then drawing it again in a slightly
altered position
• Erasing can be accomplished through careful use of
XOR mode
• Timing must be controlled so that the animation does not
move too fast
• See Bouncing_Ball.java
Chapter 7
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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