Transcript Java Software Solutions Foundations of Program Design

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:
–
–
–
–
–
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
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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
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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
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The Coordinate System
<0, 0>
X
x
y
<x, y>
<width-1, height-1>
Y
Chapter 7
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Inheritance -- Introduction
• Another fundamental object-oriented technique is called
inheritance, which, when used correctly, supports reuse
and enhances software designs
• Chapter 8 focuses on:
– the concept of inheritance
– inheritance in Java
– the protected modifier
– adding and modifying methods through inheritance
– creating class hierarchies
Chapter 8
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Inheritance
• Inheritance allows a software developer to derive a new
class from an existing one
• The existing class is called the parent class, or
superclass, or base class
• The derived class is called the child class or subclass.
• As the name implies, the child inherits characteristics of
the parent
• In programming, the child class inherits the methods and
data defined for the parent class
Chapter 8
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Inheritance
• Inheritance relationships are often shown graphically,
with the arrow pointing to the parent class:
Vehicle
Car
• Inheritance should create an is-a relationship, meaning
the child is-a more specific version of the parent
Chapter 8
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Deriving Subclasses
• In Java, the reserved word extends is used to establish
an inheritance relationship
class Car extends Vehicle {
// class contents
}
• See Words.java
Chapter 8
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The protected Modifier
• The visibility modifiers determine which class members
get inherited and which do not
• Variables and methods declared with public visibility
are inherited, and those with private visibility are not
• But public variables violate our goal of encapsulation
• The protected visibility modifier allows a member to
be inherited, but provides more protection than public
does
• The details of each modifier are given in Appendix F
Chapter 8
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The super Reference
• Constructors are not inherited, even though they have
public visibility
• Yet we often want to use the parent's constructor to set
up the "parent's part" of the object
• The super reference can be used to refer to the parent
class, and is often used to invoke the parent's constructor
• See Words2.java
Chapter 8
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Defined vs. Inherited
• A subtle feature of inheritance is the fact that even if a
method or variable is not inherited by a child, it is still
defined for that child
• An inherited member can be referenced directly in the
child class, as if it were declared in the child class
• But even members that are not inherited exist for the
child, and can be referenced indirectly through parent
methods
• See Eating.java and School.java
Chapter 8
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Overriding Methods
• A child class can override the definition of an inherited
method in favor of its own
• That is, a child can redefine a method it inherits from its
parent
• The new method must have the same signature as the
parent's method, but can have different code in the body
• The object type determines which method is invoked
• See Messages.java
Chapter 8
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Overloading vs. Overriding
• Don't confuse the concepts of overloading and overriding
• Overloading deals with multiple methods in the same
class with the same name but different signatures
• Overriding deals with two methods, one in a parent class
and one in a child class, that have the same signature
• Overloading lets you define a similar operation in
different ways for different data
• Overriding lets you define a similar operation in different
ways for different object types
Chapter 8
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The super Reference Revisited
• The super reference can be used to invoke any method
from the parent class
• This ability is often helpful when using overridden
methods
• The syntax is:
super.method(parameters)
• See Firm.java and Accounts.java
Chapter 8
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Class Hierarchies
• A child class of one parent can be the parent of another
child, forming class hierarchies:
Business
Retail_Business
Macy's
Chapter 8
Service_Business
K-Mart
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Kinko's
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Class Hierarchies
• Two children of the same parent are called siblings
• Good class design puts all common features as high in
the hierarchy as is reasonable
• Class hierarchies often have to be extended and modified
to keep up with changing needs
• There is no single class hierarchy that is appropriate for
all situations
• See Accounts2.java
Chapter 8
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The Object Class
• All objects are derived from the Object class
• If a class is not explicitly defined to be the child of an
existing class, it is assumed to be the child of the
Object class
• The Object class is therefore the ultimate root of all
class hierarchies
• The Object class contains a few useful methods, such
as toString(), which are inherited by all classes
• See Test_toString.java
Chapter 8
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References and Inheritance
• An object reference can refer to an object of its class, or
to an object of any class related to it by inheritance
• For example, if the Holiday class is used to derive a
child class called Christmas, then a Holiday
reference could actually be used to point to a
Christmas object:
Holiday day;
day = new Christmas();
Chapter 8
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References and Inheritance
• Assigning a predecessor object to an ancestor reference
is considered to be a widening conversion, and can be
performed by simple assignment
• Assigning an ancestor object to a predecessor reference
can also be done, but it is considered to be a narrowing
conversion and must be done with a cast
• The widening conversion is the most useful
Chapter 8
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Polymorphism
• A polymorphic reference is one which can refer to one of
several possible methods
• Suppose the Holiday class has a method called
celebrate, and the Christmas class overrode it
• Now consider the following invocation:
day.celebrate();
• If day refers to a Holiday object, it invokes
Holiday's version of celebrate; if it refers to a
Christmas object, it invokes that version
Chapter 8
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Polymorphism
• In general, it is the type of the object being referenced,
not the reference type, that determines which method is
invoked
• See Messages2.java
• Note that, if an invocation is in a loop, the exact same
line of code could execute different methods at different
times
• Polymorphic references are therefore resolved at runtime, not during compilation
Chapter 8
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Polymorphism
• Note that, because all classes inherit from the Object
class, an Object reference can refer to any type of
object
• A Vector is designed to store Object references
• The instanceOf operator can be used to determine the
class from which an object was created
• See Variety.java
Chapter 8
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Polymorphism
• See Firm2.java
Staff_Member
Employee
Hourly
Chapter 8
Volunteer
Executive
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Enhanced Class Design -- Introduction
• We now examine several features of class design and
organization that can improve reusability and system
elegance
• Chapter 9 focuses on:
– abstract classes
– formal Java interfaces
– packages
Chapter 9
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Abstract Classes
• An abstract class cannot be instantiated
• It is used in a class hierarchy to organize common
features at appropriate levels
• An abstract method has no implementation, just a name
and signature
• An abstract class often contains abstract methods
• Any class that contains an abstract method is by
definition abstract
Chapter 9
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Abstract Classes
• The modifier abstract is used to define abstract
classes and methods
• The children of the abstract class are expected to define
implementations for the abstract methods in ways
appropriate for them
• If a child class does not define all abstract methods of the
parent, then the child is also abstract
• An abstract class is often too generic to be of use by
itself
Chapter 9
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Abstract Classes
• See Dinner.java
Food
Beans
Chapter 9
Franks
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Pepperoni
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Abstract Classes
• See Printer.java
File
Binary_File
Text_File
Image_File
Chapter 9
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Abstract Classes
• An abstract method cannot be declared as final,
because it must be overridden in a child class
• An abstract method cannot be declared as static,
because it cannot be invoked without an implementation
• Abstract classes are placeholders that help organize
information and provide a base for polymorphic
references
Chapter 9
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Interfaces
• We've used the term interface to mean the set of service
methods provided by an object
• That is, the set of methods that can be invoked through
an object define the way the rest of the system interacts,
or interfaces, with that object
• The Java language has an interface construct that
formalizes this concept
• A Java interface is a collection of constants and abstract
methods
Chapter 9
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Interfaces
• A class that implements an interface must provide
implementations for all of the methods defined in the
interface
• This relationship is specified in the header of the class:
class class-name implements interface-name {
}
• See Soap_Box.java
Chapter 9
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Interfaces
• An interface can be implemented by multiple classes
• Each implementing class can provide their own unique
version of the method definitions
• An interface is not a class, and cannot be used to
instantiate an object
• An interface is not part of the class hierarchy
• A class can be derived from a base class and implement
one or more interfaces
Chapter 9
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Interfaces
• Unlike interface methods, interface constants require
nothing special of the implementing class
• Constants in an interface can be used in the
implementing class as if they were declared locally
• This feature provides a convenient technique for
distributing common constant values among multiple
classes
• See File_Protection.java
Chapter 9
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Interfaces
• An interface can be derived from another interface, using
the extends reserved word
• The child interface inherits the constants and abstract
methods of the parent
• Note that the interface hierarchy and the class hierarchy
are distinct
• A class that implements the child interface must define
all methods in both the parent and child
Chapter 9
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Interfaces
• An interface name can be used as a generic reference
type name
• A reference to any object of any class that implements
that interface is compatible with that type
• For example, if Philosopher is the name of an
interface, it can be used as the type of a parameter to a
method
• An object of any class that implements Philosopher
can be passed to that method
Chapter 9
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Interfaces
• Note the similarities between interfaces and abstract
classes
• Both define abstract methods that are given definitions
by a particular class
• Both can be used as generic type names for references
• However, a class can implement multiple interfaces, but
can only be derived from one class
• See Printer2.java
Chapter 9
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Interfaces
• A class that implements multiple interfaces specifies all
of them in its header, separated by commas
• The ability to implement multiple interfaces provides
many of the features of multiple inheritance, the ability
to derive one class from two or more parents
• Java does not support multiple inheritance
• See Readable_Files.java
Chapter 9
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Packages
• A Java package is a collection of classes
• The classes in a package may or may not be related by
inheritance
• A package is used to group similar and interdependent
classes together
• The Java API is composed of multiple packages
• The import statement is used to assert that a particular
program will use classes from a particular package
Chapter 9
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Packages
• A programmer can define a package and add classes to it
• The package statement is used to specify that all classes
defined in a file belong to a particular package
• The syntax of the package statement is:
package package-name;
• It must be located at the top of a file, and there can be
only one package statement per file
Chapter 9
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Packages
• The classes must be organized in the directory structure
such that they can be found when referenced by an
import statement
• There is a CLASSPATH environment variable on each
computer system that determines where to look for
classes when referenced
• See Simple_IO_Test.java
Chapter 9
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Packages
• The import statement specifies particular classes, or an
entire package of classes, that can be used in that
program
• Import statements are not necessary; a class can always
be referenced by its fully qualified name in-line
• See Simple_IO_Test2.java
• If two classes from two packages have the same name
and are used in the same program, they must be
referenced by their fully qualified name
Chapter 9
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Packages
• As a rule of thumb, if you will use only one class from a
package, import that class specifically
• See Simple_IO_Test3.java
• If two or more classes will be used, use the * wildcard
character in the import statement to provide access to all
classes in the package
Chapter 9
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Graphical User Interfaces -- Introduction
• Users have become accustomed to using a graphical user
interface (GUI) through which they interact with a
program
• Java provides strong support for building GUIs through
the java.awt package
• Chapter 10 focuses on:
–
–
–
–
Chapter 10
GUI components
event-driven programming
containers and component hierarchies
layout managers
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GUI Elements
• The key elements of a Java graphical user interface are:
– GUI components
– layout managers
– event processing
• GUI components, such as text fields and buttons, are the
screen elements that a user manipulates with the mouse
and keyboard
• Layout managers govern how the components appear on
the screen
• Events signal important user actions, like a mouse click
Chapter 10
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Event-Driven Programming
• Programs with GUIs must respond to events, generated
by GUI components, that indicate that specific actions
have occurred
• A special category of classes, called listeners, wait for
events to occur
• Therefore, a GUI program is composed of:
– the code that presents the GUI to the user
– the listeners that wait for events to occur
– the specific code that is executed when events occur
Chapter 10
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Event-Driven Programming
• There is a listener interface defined for each event type
• Each listener interface contains the abstract methods
required to respond to specific events
• A listener class implements a particular listener interface
• Listeners are "added" to a particular GUI component
• When a component generates an event, the method
corresponding to that event is executed in the listener
• See Mimic.java
Chapter 10
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The GUI Program Model
Listeners
Handle events
Add listeners
GUI
Event effects
Chapter 10
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Programspecific
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Event Interfaces
• Multiple listeners can be added to a component
• Multiple components can be processed by the same
listener
• Furthermore, one listener class can implement multiple
listener interfaces
• Therefore one class can listen for many types of events
• See Events.java
Chapter 10
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Containers
• A container is a special category of GUI components that
group other components
• All containers are components, but not all components
are containers
• An applet is a container
• Therefore, buttons, text fields, and other components can
be added to an applet to be displayed
• Each container has an associated layout manager to
control the way components in it are displayed
Chapter 10
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Containers
• Some containers must be attached to another graphical
surface:
– panel
– applet
• An applet is attached to a browser or appletviewer
window
• Other containers can be moved independently:
– window
– frame
– dialog
Chapter 10
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Containers
Component
Container
Window
Panel
Applet
Chapter 10
Frame
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Dialog
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Component Hierarchies
• A GUI is created when containers and other components
are put together
• The relationships between these components form a
component hierarchy
• For example, an applet can contain panels which contain
other panels which contain buttons, etc.
• See Rings_Display.java
• Careful design of the component hierarchy is important
for visually pleasing and consistent GUIs
Chapter 10
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GUI Components
• There are several GUI components that permit specific
kinds of user interaction:
–
–
–
–
–
–
Chapter 10
labels
text fields
text areas
lists
buttons
scrollbars
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Labels
• A label defines a line of text displayed on a GUI
• Labels are static in the sense that they cannot be selected
or modified by the human user once added to a container
• A label is instantiated from the Label class
• The Label class contains several constructors and
methods for setting up and modifying a label's content
and alignment
Chapter 10
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Text Fields and Text Areas
• A text field displays a single line of text in a GUI
• It can be made editable, and provide a means to get input
from the user
• A text area is similar, but displays multiple lines of text
• They are defined by the TextField and TextArea
classes
• A text area automatically has scrollbars on its bottom and
right sides
• See Fahrenheit.java
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Lists
• A list, in the Java GUI sense, is used to display a list
selectable strings
• A list component can contain any number of strings and
can be instantiated to allow multiple selections within it
• The size of the list is specified by the number of visible
rows or strings within it
• A scrollbar will automatically appear on the right side of
a list if the number of items exceed the visible area
• A list is defined by the List class
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Buttons
• The java.awt package supports four distinct types of
buttons:
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–
–
Push buttons
Choice Buttons
Checkbox buttons
Radio buttons
• Each button type serves a particular purpose
Chapter 10
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Push Button
• A push button is a single button which can be created
with or without a label
• A system is usually designed such that when a push
button is pressed, a particular action occurs
• It is defined by the Button class
Chapter 10
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Choice button
• A choice button is a single button which displays a list of
choices when pushed
• The user can then scroll through and choose the
appropriate option
• The current choice is displayed next to the choice button
• It is defined by the Choice class
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Checkbox button
• A checkbox button can be toggled on or off
• A set of checkbox buttons are often used to define a set
of options as a group, though one can be used by itself
• If used in a group, more than one option can be chosen at
any one time
• Defined by the Checkbox class
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Radio buttons
• A radio button, like a checkbox button, is toggled on or
off
• Radio buttons must be grouped into a set, and only one
button can be selected at any one time
• When one button of a group is selected, the currently
selected button in that group is automatically reset
• They are used to select among a set of mutually
exclusive options
• Radio button sets are defined by the Checkbox and
CheckboxGroup classes
Chapter 10
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Scrollbars
• A scrollbar is a slider that indicates a relative position or
quantity
• They are automatic on text areas and list components, but
can be used independently
• The position of the slider in the range corresponds to a
particular numeric value in a range associated with the
scrollbar
• A scrollbar is defined by the Scrollbar class
• See Zoom.java
Chapter 10
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Layout Managers
• There are five predefined layout managers in the
java.awt package:
–
–
–
–
–
flow layout
border layout
card layout
grid layout
grid bag layout
• Each container has a particular layout manager
associated with it by default
• A programmer can also create custom layout managers
Chapter 10
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Flow Layout
• Components are placed in a row from left to right in the
order in which they are added
• A new row is started when no more components can fit
in the current row
• The components are centered in each row by default
• The programmer can specify the size of both the vertical
and horizontal gaps between the components
• Flow layout is the default layout for panels and applets
• See Flow.java
Chapter 10
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Grid Layout
• Components are placed in a grid with a user-specified
number of columns and rows
• Each component occupies exactly one grid cell
• Grid cells are filled left to right and top to bottom
• All cells in the grid are the same size
• See Grid.java
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Border Layout
• Defines five locations each of which a component or
components can be added
– North, South, East, West, and Center
• The programmer specifies the area in which a component
should appear
• The relative dimensions of the areas are governed by the
size of the components added to them
• See Border.java
Chapter 10
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Border Layout
North
West
Center
East
South
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Card Layout
• Components governed by a card layout are "stacked"
such that only one component is displayed on the screen
at any one time
• Components are ordered according to the order in which
they were added to the container
• Methods control which component is currently visible in
the container
• See Card.java
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Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Grid Bag Layout
• Designed as a two-dimensional grid of columns and rows
• However, not all cells in the grid are the same size
• Components may span multiple columns and rows
• Each component in a grid bag layout is associated with a
set of constraints, defined by the
GridBagConstraints class
• A grid bag layout is the most versatile, and most
complex, of the predefined layout managers
• See Grid_Bag.java
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Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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GUI Design
• Careful design of a graphical user interface is key to a
viable software system
• To the user, the user interface is the system
• For each situation, consider which components are best
suited and how they should best be arranged
• See Quotes.java
Chapter 10
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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