Objects, Classes and Applets
Computer Programming II
Metcalfe County High School
Justin Smith- Instructor
Introduction to Objects
 An object represents something with which we can
interact in a program
 An object provides a collection of services that we
can tell it to perform for us
 A class represents a concept, and an object
represents the embodiment of a class
2
Objects and Classes
A class
(the concept)
Bank Account
An object
(the realization)
John’s Bank Account
Balance: $5,257
Bill’s Bank Account
Balance: $1,245,069
Multiple objects
from the same class
Mary’s Bank Account
Balance: $16,833
3
The print Method
 The System.out object provides another service as
well
 The print method is similar to the println method,
except that it does not advance to the next line
 Therefore anything printed after a print statement
will appear on the same line
 The use of print and println is showcased in the
coutdown program on page 61
 See Countdown.java (page 61)
4
Character Strings
 The string concatenation operator (+) is used to
append one string to the end of another
 Code, compile and run the Facts program on
page 64 to see the use and implementation of the
string concatenation operator.
5
String Concatenation
 The plus operator (+) is also used for arithmetic
addition
 The function that the + operator performs depends
on the type of the information on which it operates
 If both operands are strings, or if one is a string and
one is a number, it performs string concatenation
 If both operands are numeric, it adds them
 Code, compile and run the Addition program on page
65 to see the 2 different ways the + operator can be
used.
6
Escape Sequences
 What if we wanted to print a double quote character?
 The following line would confuse the compiler
because it would interpret the second quote as the
end of the string
System.out.println ("I said "Hello" to you.");
 An escape sequence is a series of characters that
represents a special character
 An escape sequence begins with a backslash
character (\), which indicates that the character(s)
that follow should be treated in a special way
System.out.println ("I said \"Hello\" to you.");
7
Escape Sequences
 Some Java escape sequences:
Escape Sequence
Meaning
\b
\t
\n
\r
\"
\'
\\
backspace
tab
newline
carriage return
double quote
single quote
backslash
 Code, compile and run Roses.java (page 67) to see
numerous escape sequences and how they can be
implemented into a program.
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Assignment
Design a program that utilizes only one
println statement and provides a brief
introduction about yourself. The
program must include 3 of the 7
available escape sequences that have
been discussed.
The finished program is worth 50 points
and should contain appropriate
comments
9
Assignment 2
 Using only the tab escape sequence \t and the println
statement design a program that will print out the
following column headings first name, last name and
hometown and under each column heading include
the first and last name of at least 5 people, and their
hometown. Please show your finished program when
finished(50 point program). An Example is below.
First Name
Justin
John
Jane
Last Name
Smith
Doe
Doe
Hometown
Center
Edmonton
Summer Shade
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Assignment 3
 A Table of Student Grades
 Write a Java program that prints a table with a list of at least 5
students together with their grades earned (lab points, bonus
points, and the total) in the format below.








///////////////////\\\\\\\\\\\\\\\\\\\
== Student Points ==
\\\\\\\\\\\\\\\\\\\///////////////////
Name
---Joe
William
Mary Sue
Lab
--43
50
39
Bonus Total
--------7
50
8
58
10
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 1. Use tab characters to get your columns aligned and you must
use the + operator both for addition and string concatenation.
11
Variables
 A variable is a name for a location in memory
 A variable must be declared by specifying the
variable's name and the type of information that it will
hold
data type
variable name
int total;
int count, temp, result;
Multiple variables can be created in one declaration
12
Variables
 A variable can be given an initial value in the
declaration
int sum = 0;
int base = 32, max = 149;
 When a variable is referenced in a program, its
current value is used
13
Assignment
 An assignment statement changes the value of a
variable
 The assignment operator is the = sign
total = 55;
 The expression on the right is evaluated and the
result is stored in the variable on the left
 The value that was in total is overwritten
 You can assign only a value to a variable that is
consistent with the variable's declared type
14
Assignment
 Code, Compile and Run PianoKeys.java (page 68) to
see how variables can be implemented.
 Code, Compile and Run Geometry.java (page 70).
 Design a program that will print out the following
using variables and assignment statements. Please
refer to the program on page 70(Geometry Program)
for additional help.
• Number of days in a week EX: The number of days in a week
is 7
• Number of months in a year
• Number of sides in a pentagon
15
Constants
 A constant is an identifier that is similar to a variable
except that it holds one value while the program is
active
 In Java, we use the final modifier to declare a
constant
final int MIN_HEIGHT = 69;
16
Primitive Data
 There basic data types used in Java
 To represent integers:
•
int EX: int total = 100;
 To represent floating point numbers(decimals):
•
double EX: double pi=3.14
 To represent characters:
• Char EX: char fail = ‘F’;
 To represent boolean values(True/False):
• Boolean EX boolean game = false;
17
Arithmetic Expressions
 An expression is a combination of one or more
operands and their operators
 Arithmetic expressions compute numeric results and
make use of the arithmetic operators:
Addition
Subtraction
Multiplication
Division
Remainder
+
*
/
%
 If either or both operands associated with an
arithmetic operator are floating point, the result is a
floating point
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Division and Remainder
 If both operands to the division operator (/) are
integers, the result is an integer (the fractional part is
discarded)
14 / 3
equals?
4
8 / 12
equals?
0
 The remainder operator (%) returns the remainder
after dividing the second operand into the first
14 % 3
equals?
2
8 % 12
equals?
8
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Operator Precedence
 Operators can be combined into complex
expressions
result
=
total + count / max - offset;
 PEMDAS
 Multiplication, division, and remainder are evaluated
prior to addition, subtraction, and string
concatenation
 Arithmetic operators with the same precedence are
evaluated from left to right
 Parentheses can be used to force the evaluation
order
20
Operator Precedence
 What is the order of evaluation in the following
expressions?
a + b + c + d + e
1
2
3
4
a + b * c - d / e
3
1
4
2
a / (b + c) - d % e
2
1
4
3
a / (b * (c + (d - e)))
4
3
2
1
21
Assignment
 Code and Run the program listed on page 77(The
temp converter program) when finished please
submit for a daily work grade.
22
Test
 We are now finished with the first part of chapter 2.
 A test will be given over the covered material. Please
use your Ch2 on the go sheet to help prepare for the
exam.
23
Chapter 2------ Part 2
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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
25
Creating Objects
 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
 Once an object has been instantiated, we can use the
dot operator to invoke its methods
title.length()
 Refer to page 81 to see the string class and the many
methods that the class contains.
26
Class Libraries
 A class library is a collection of classes that we can
use when developing programs
 The Java standard class library is part of any Java
development environment
 The System class and the String class are part of
the Java standard class library
 Other class libraries can be obtained through third
party vendors, or you can create them yourself
27
Packages
 The classes of the Java standard class library are
organized into packages
 Some of the packages in the standard class library
are:
Package
java.applet
java.awt
javax.swing
Purpose
Creating applets for the web
Graphics and graphical user interfaces
Additional graphics capabilities and components
28
The import Declaration
 When you want to use a class from a package, you
could use its fully qualified name
java.util.Random
 Or you can import the class, and then use just the
class name
import java.util.Random;
 To import all classes in a particular package, you can
use the * wildcard character
import java.util.*;
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The import Declaration
 The Random class is part of the java.util package
 It provides methods that generate pseudorandom
numbers
 *********************Assignment*********************
 ******Create the String Mutation program on page 83
to help showcase the multiple string methods that
can be used within the Java language.******
 On page 90 create the random numbers program that
requires the import code to be utilized and random
numbers be created.
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The Keyboard Class
 The Keyboard class is NOT part of the Java standard
class library
 The Keyboard class is part of a package called cs1
 It contains several static methods for reading
particular types of data
 This class can be used to capture user input such as
a name or ask for a number such as a password.
 The complete list of Keyboard class functions can be
found on page 93 with examples on pages 94 and 95
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Formatting Output
 The NumberFormat class has static methods that
return a formatter object to allow data to be either in
a percentage or currency format.
getCurrencyInstance()
getPercentInstance()
 Each formatter object has a method called format
that returns a string with the specified information in
the appropriate format
32
Assignment
 Code, compile and run the 2 programs found on page
94 and 95 within the java textbook to showcase the
power of the Keyboard class(ECHO & QUADRATIC).
 Code, Compile and Run the Price.java program on
(page 97)
 Complete the paint program that will be distributed
on Smith’s website. You will be required to calculate
the area of a wall and determine the amount of pain
needed to paint a room.
 The overall point value will be 100 points.
 Utilize the Keyboard class and other features that
have been demonstrated and practiced during last
few sections of this unit.
33
Programming Projects
 Before we explore applets complete the following
programming projects from page 117 – 118 to test
your skills. Groups of 2 will be used an only one
program per group is required.
 Complete the following
•
•
•
•
•
2.2
2.4
2.6
2.11
2.13
34
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 or by embedding the code within a web page
 An applet doesn't have a main method
35
Applets
 The paint method, for instance, is executed
automatically and is used to draw the applet’s
contents
 public void paint(Graphics page)
 The Graphics class has several methods for drawing
shapes
36
Assignment
Code the Einstein program on (page 101)
This will be your first Applet design.
Notice the difference in some of the coding that is used
within the Applet compared to the Applications we have
completed in class so far.
In order for the code to work correctly in JCreator you may
need to use Build File and then Run Project to make the
applet show up correctly.
Leave your Einstein program open to use in a discussion.
37
How Did The Information Show?
 Think about your Einstein Applet code and answer
the following question.
• How did the program know where to place my items within
the program?
 Looking at page 104 and your program code lets
generate some answers and examples.
 Java uses a grid system of X and Y coordinates
within the Java Applet screen.
 Using the supplied grid sheet lets diagram out the
program to see how it worked.
 Shapes with curves, like an oval, are usually drawn
by specifying the shape’s bounding rectangle
38
Drawing a Line
10
150
X
20
45
Y
page.drawLine (10, 20, 150, 45);
or
page.drawLine (150, 45, 10, 20);
39
Drawing a Rectangle
50
X
20
40
100
Y
page.drawRect (50, 20, 100, 40);
40
Drawing an Oval
175
X
20
80
bounding
rectangle
Y
50
page.drawOval (175, 20, 50, 80);
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Applet Task – 50 point task
 Grid out an Applet that will showcase the following
shapes within your program.
• Circle, Square, Rectangle, Line and the challenging Arc(use
page 104 and 105 to assist) the dimensions are up to each
programmer and how they wish to layout the applet.
• Use Text to Identify each item from above. The identification
can be below, above or to the side of the item being created.
• Example
Square
• After the grid design is complete, now code your applet into
Jcreator to ensure that we can take project from the grid
paper into program design. Refer to the Einstein program
and page 104 for code assistance.
42
The Color Class
 A color is defined in a Java program using an object
created from the Color class
 The Color class also contains several static
predefined colors, including:
Object
RGB Value
Color.black
Color.blue
Color.cyan
Color.orange
Color.white
Color.yellow
0, 0, 0
0, 0, 255
0, 255, 255
255, 200, 0
255, 255, 255
255, 255, 0
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Let’s Set Some Color
 To set the page background code similar to:
• setBackground (Color.orange);
 To set the color of a shape or multiple shapes insert
the following before the shape is created.
• Page.setColor (Color.black);
• Or you can use the new color code.
 Java has a set of colors but what if you need a darker
or lighter color.
 Never fear the new color code is here.
 page.setColor (new Color(255,255,255));
 Lets look at an RGB color chart using google.com
44
Assignment 1
 Code the snowman applet found on page 107
45
Assignment Applet 2
 Using your snowman applet add the following items
into the picture.
•
•
•
•
Add 3 black buttons into the center of the snowman
Change the color of the ground to black
Add yellow rays coming from the sun 3-4 diagonal lines
Add white circular snowflakes in the background (15-20
snowflakes spread out over the screen)
• If needed use the Java grid system to code in the snowman
program.
• 100 point enhancement
46
Assignment Applet 3
Design an applet that draws a smiling
face. Give the face a nose, ears, a
mouth, and eyes with pupils.
50 point program
47
Assignment Applet 4
 Design a program that draws a side view of a car,
truck or van. The vehicle will be a combination of
various Java ovals, rectangles, lines, arc’s and will
utilize various colors to accomplish the task of
creating the side view of a vehicle.
 100 Points
48
Assignment Applet 5















Design a program that draws the front view of a house.
The house must contain the following elements of design.
v A Blue Background (sky)
v The sun within the background
v A 2 story house with attached garage (garage can be 1
story)
v An arched roof (no flat roof homes allowed)
v At least 4 windows with shutters (2 per window)
v A front door with door handle
v A chimney
v Driveway that run to the attached garage
v Green Grass in front of the house (this will be a rectangle)
v A sidewalk that goes to the front door
Appropriate colors used throughout the house applet
100 Point Program
49