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Day 5
Chapter
Agenda Day 5
• Questions from last Class??
• Problem set 1 Posted
Introduction on developing java programs
10 programs from Chapter 1 & 2
Due in 4 days (September 19)
• Problem set 2 will be posted by next class
• Quiz 1 will be Oct 3
Chapter 1-4
25 M/C open book, open notes, 40 Min
• Today we will
Finish up on Chapter 2
• Applets
Start on Chap 3
• Classes and Objects
© 2007 Pearson Addison-Wesley. All rights reserved
2-2
Outline
Character Strings
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes
© 2007 Pearson Addison-Wesley. All rights reserved
2-3
Applets
• A Java application is a stand-alone program with a
main method (like the ones we've seen so far)
• A Java applet is a program that is intended to
transported over the Web and executed using a
web browser
• An applet also can be executed using the
appletviewer tool of the Java Software
Development Kit
• An applet doesn't have a main method
• Instead, there are several special methods that
serve specific purposes
© 2007 Pearson Addison-Wesley. All rights reserved
2-4
Applets
• The paint method, for instance, is executed
automatically and is used to draw the applet’s
contents
• The paint method accepts a parameter that is an
object of the Graphics class
• A Graphics object defines a graphics context on
which we can draw shapes and text
• The Graphics class has several methods for
drawing shapes
© 2007 Pearson Addison-Wesley. All rights reserved
2-5
Applets
• The class that defines an applet extends the
Applet class
• This makes use of inheritance, which is explored
in more detail in Chapter 8
• See Einstein.java (page 95)
• An applet is embedded into an HTML file using a
tag that references the bytecode file of the applet
• The bytecode version of the program is
transported across the web and executed by a
Java interpreter that is part of the browser
© 2007 Pearson Addison-Wesley. All rights reserved
2-6
The HTML applet Tag
<html>
<head>
<title>The Einstein Applet</title>
</head>
<body>
<applet code="Einstein.class" width=350 height=175>
</applet>
</body>
</html>
© 2007 Pearson Addison-Wesley. All rights reserved
2-7
Outline
Character Strings
Variables and Assignment
Primitive Data Types
Expressions
Data Conversion
Interactive Programs
Graphics
Applets
Drawing Shapes
© 2007 Pearson Addison-Wesley. All rights reserved
2-8
Drawing Shapes
• Let's explore some of the methods of the
Graphics class that draw shapes in more detail
• A shape can be filled or unfilled, depending on
which method is invoked
• The method parameters specify coordinates and
sizes
• Shapes with curves, like an oval, are usually drawn
by specifying the shape’s bounding rectangle
• An arc can be thought of as a section of an oval
© 2007 Pearson Addison-Wesley. All rights reserved
2-9
Drawing a Line
10
150
X
20
45
Y
page.drawLine (10, 20, 150, 45);
or
page.drawLine (150, 45, 10, 20);
© 2007 Pearson Addison-Wesley. All rights reserved
2-10
Drawing a Rectangle
50
X
20
40
100
Y
page.drawRect (50, 20, 100, 40);
© 2007 Pearson Addison-Wesley. All rights reserved
2-11
Drawing an Oval
175
X
20
80
bounding
rectangle
50
Y
page.drawOval (175, 20, 50, 80);
© 2007 Pearson Addison-Wesley. All rights reserved
2-12
Drawing Shapes
• Every drawing surface has a background color
• Every graphics context has a current foreground
color
• Both can be set explicitly
• See Snowman.java (page100)
© 2007 Pearson Addison-Wesley. All rights reserved
2-13
In class Lab
• Drawing a face from page 30 of lab manual
• Applettemplate.java
© 2007 Pearson Addison-Wesley. All rights reserved
2-14
Summary
• Chapter 2 focused on:
character strings
primitive data
the declaration and use of variables
expressions and operator precedence
data conversions
accepting input from the user
Java applets
introduction to graphics
© 2007 Pearson Addison-Wesley. All rights reserved
2-15
Using Classes
and Objects
Chapter
3
5TH EDITION
Lewis & Loftus
java
Software Solutions
Foundations of Program Design
© 2007 Pearson Addison-Wesley. All rights reserved
Using Classes and Objects
• We can create more interesting programs using
predefined classes and related objects
• Chapter 3 focuses on:
object creation and object references
the String class and its methods
the Java standard class library
the Random and Math classes
formatting output
enumerated types
wrapper classes
graphical components and containers
labels and images
© 2007 Pearson Addison-Wesley. All rights reserved
2-17
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
© 2007 Pearson Addison-Wesley. All rights reserved
2-18
Creating Objects
• A variable holds either a primitive type or a
reference to an object
• A class name can be used as a type to declare an
object reference variable
String title;
• No object is created with this declaration
• An object reference variable holds the address of
an object
• The object itself must be created separately
© 2007 Pearson Addison-Wesley. All rights reserved
2-19
Creating Objects
• Generally, we use the new operator to create an
object
title = new String ("Java Software Solutions");
This calls the String constructor, which is
a special method that sets up the object
• Creating an object is called instantiation
• An object is an instance of a particular class
© 2007 Pearson Addison-Wesley. All rights reserved
2-20
Invoking Methods
• We've seen that once an object has been
instantiated, we can use the dot operator to invoke
its methods
count = title.length()
• A method may return a value, which can be used
in an assignment or expression
• A method invocation can be thought of as asking
an object to perform a service
© 2007 Pearson Addison-Wesley. All rights reserved
2-21
References
• Note that a primitive variable contains the value
itself, but an object variable contains the address
of the object
• An object reference can be thought of as a pointer
to the location of the object
• Rather than dealing with arbitrary addresses, we
often depict a reference graphically
num1
38
name1
© 2007 Pearson Addison-Wesley. All rights reserved
"Steve Jobs"
2-22
Assignment Revisited
• The act of assignment takes a copy of a value and
stores it in a variable
• For primitive types:
Before:
num1
38
num2
96
num2 = num1;
After:
© 2007 Pearson Addison-Wesley. All rights reserved
num1
38
num2
38
2-23
Reference Assignment
• For object references, assignment copies the
address:
Before:
name1
"Steve Jobs"
name2
"Steve Wozniak"
name2 = name1;
name1
After:
"Steve Jobs"
name2
© 2007 Pearson Addison-Wesley. All rights reserved
2-24
Aliases
• Two or more references that refer to the same
object are called aliases of each other
• That creates an interesting situation: one object
can be accessed using multiple reference
variables
• Aliases can be useful, but should be managed
carefully
• Changing an object through one reference
changes it for all of its aliases, because there is
really only one object
© 2007 Pearson Addison-Wesley. All rights reserved
2-25
Garbage Collection
• When an object no longer has any valid references
to it, it can no longer be accessed by the program
• The object is useless, and therefore is called
garbage
• Java performs automatic garbage collection
periodically, returning an object's memory to the
system for future use
• In other languages, the programmer is responsible
for performing garbage collection
© 2007 Pearson Addison-Wesley. All rights reserved
2-26
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
© 2007 Pearson Addison-Wesley. All rights reserved
2-27
The String Class
• Because strings are so common, we don't have to
use the new operator to create a String object
title = "Java Software Solutions";
• This is special syntax that works only for strings
• Each string literal (enclosed in double quotes)
represents a String object
© 2007 Pearson Addison-Wesley. All rights reserved
2-28
String Methods
• Once a String object has been created, neither its value nor
its length can be changed
• Thus we say that an object of the String class is immutable
• However, several methods of the String class return new
String objects that are modified versions of the original
• See the list of String methods on page 119 and in Appendix
M
• Better Yet use Java.sun.com
http://java.sun.com/javase/reference/api.jsp
© 2007 Pearson Addison-Wesley. All rights reserved
2-29
String Indexes
• It is occasionally helpful to refer to a particular
character within a string
• This can be done by specifying the character's
numeric index
• The indexes begin at zero in each string
• In the string "Hello", the character 'H' is at index
0 and the 'o' is at index 4
• See StringMutation.java (page 120)
© 2007 Pearson Addison-Wesley. All rights reserved
2-30
Outline
Creating Objects
The String Class
Packages
Formatting Output
Enumerated Types
Wrapper Classes
Components and Containers
Images
© 2007 Pearson Addison-Wesley. All rights reserved
2-31
