Chapter 2: Objects and Primitive Data

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Chapter 2: Objects and Primitive Data

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Title: Chapter 2: Objects and Primitive Data

1
Chapter 2 Objects and Primitive Data
2
Objects and Primitive Data

We can now explore some more fundamental
programming concepts
Chapter 2 focuses on
predefined objects
primitive data
the declaration and use of variables
expressions and operator precedence
class libraries
Java applets
drawing shapes

3
Software Engineering

Goal
to make software reliable and maintainable
As the complexity of a program increases, its
cost to develop and revise grows exponentially

cost
complexity
4
Software Components

Programs are easier to construct and modify if
they are made up of separate components
A software component can be thought of
any program element that transforms input into
output

Input
Component
Output
15 38 16
Compute average
22
5
Software Components

Components can be combined to make larger
components

6
Object-Oriented Programming

Java is object-oriented language
Programs are made from software components called
objects
Initially, we can think of an object as
data for it and a collection of services
Object
contains data and methods to provide the service

7
Object-Oriented Programming

Class
A class represents a concept
An object is defined by a class
Multiple objects can be created from the same
class

8
Object-Oriented Programming

A class represents a concept and
an object represents the realization of that
concept

Objects
Class
Car
9
Introduction to Objects

Recall the Lincoln program,
We invoked the println method of the System.out
object

System.out.println ("Whatever you are, be a good
one.")
10
Input and Output

Java I/O is based on input streams and output
streams
There are three predefined standard streams
The print and println methods write to standard
output

Stream System.in System.out System.err
Purpose reading input writing output writing
errors
Default Device keyboard monitor monitor
11
The println and print Methods

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
See Countdown.java (page 53)

12
Countdown.java

class Countdown
public static void main (String args)
System.out.print ("Three... ")
System.out.print ("Two... ")
System.out.print ("One... ")
System.out.print ("Zero... ")
System.out.println ("Liftoff!")
System.out.println ("Houston, we have a
problem.")
// method main
// class Countdown

13
The String Class

Every character string is an object in Java,
defined by the String class
Every string literal represents a String object
A string literal cannot be broken across two
lines in a program

14
String Concatenation and Addition

The operator serves two purposes
When applied to two strings,
they are combined into one (string
concatenation)
When applied to a string and a number,
that value is converted to a string and
concatenated
When applied to two numeric types,
they are added together arithmetically
See Antarctica.java and Sum.java

15
Antarctica.java

class Antarctica
public static void main (String args)
System.out.print ("The international "
"dialing code ")
System.out.println ("for Antarctica is "
672)
// method main
// class Antarctica

16
Sum.java

class Sum
public static void main (String args)
System.out.println ("The sum of 16 and 9 is
" (169))
System.out.println (16 and 9 concatenated"
16 9)
// method main
// class Sum

17
Variables

A variable is a name for a location in memory
A variable must be declared before use
its name and the type of information that will be
held in it

int total
int count, temp, result
Multiple variables can be created in one
declaration
18
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

See PianoKeys.java (page 60)

19
Piano_Keys.java

class Piano_Keys
public static void main (String args)
int keys 88
System.out.println ("The number of piano
keys " keys)
// method main
// class Piano_Keys

20
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
Assigned values must be consistent with the
variable's declared type

21
Assignment Statements

class United_States
public static void main (String args)
int states 13
System.out.println ("States in 1776 "
states)
states 50
System.out.println ("States in 1959 "
states)
// method main
// class United_States

22
Primitive Data Types

Data type
A set of values and the operators you can perform
on them
Each value stored in memory is associated with a
particular data type
Primitive data types
The Java language has 8 predefined data types
Reserved words for primitive types
byte, short, int, long, float, double, boolean,
char

23
Primitive Data

Four of them represent integers
byte, short, int, long
Two of them represent floating point numbers
float, double
One of them represents characters
char
And one of them represents boolean values
boolean

24
Integers

There are four separate integer primitive data
types
They differ by the amount of memory used to store
them

Type byte short int long
Storage 8 bits 16 bits 32 bits 64 bits
Min Value -128 -32,768 -2,147,483,648
Max Value 127 32,767 2,147,483,647 9 x 1018
25
Floating Point

There are two floating point types
The float type stores 7 significant digits
The double type stores 15 significant digits

Approximate Min Value -3.4 x 1038 -1.7 x 10308
Approximate Max Value 3.4 x 1038 1.7 x 10308
Type float double
Storage 32 bits 64 bits
26
Characters

A char value stores a single character from the
Unicode character set
A character set is an ordered list of characters
The Unicode character set uses sixteen bits per
character, allowing for 65,536 unique characters
Character literals are delimited by single
quotes
'a' 'X' '7' '' ',' '\n'

27
Characters

The ASCII character set is still the basis for
many other programming languages
ASCII is a subset of Unicode, including

uppercase letters lowercase letters punctuation di
gits special symbols control characters
A, B, C, a, b, c, period, semi-colon, 0, 1,
2, , , \, carriage return, tab, ...
28
Boolean

A boolean value represents a true or false
condition
The reserved words
true and false are the only valid values for a
boolean type
boolean done false

29
Wrappers

Wrapper class(?? ???) for each primitive type
Wrapper classes are useful in situations where
you need an object instead of a primitive type
They also contain some useful methods

Primitive Type int double char boolean
Wrapper Class Integer Double Character Boolean
30
Constants

A constant is similar to a variable except that
they keep the same value throughout their
existence
They are specified using the reserved word final
For example
final double PI 3.14159
final int STUDENTS 25

31
Constants

When appropriate, constants are better than
variables
they prevent inadvertent errors because their
value cannot change
They are better than literal values because
they make code more readable by giving meaning to
a value
they facilitate change because the value is only
specified in one place

32
Expressions

An expression is a combination of operators and
operands
every expression has its result value.
The arithmetic operators
, -, , /, , ...
Operands
literal values, variables, or other sources of
data
The programmer usually
store or print the result of an expression.

33
Division

Division A/B
If A and B are both integers, the result is an
integer (the fractional part is truncated)
If one or more operands are floating point
values, the result is a floating point value
The remainder operator AB
returns the integer remainder after dividing
The operands to the operator must be integers
The remainder result takes the sign of the
numerator
See Division.java

34
Division.java

class Division
public static void main (String args)
int oper1 9, oper2 4
double oper3 4.0
System.out.println ("Integer division "
oper1/oper2)
System.out.println ("Floating division "
oper1/oper3)
System.out.println ("Modulus division "
oper1oper2)
// method main
// class Division

35
Division
Expression 17 / 5 17.0 / 5 17 / 5.0 9 / 12 9.0
/ 12.0 6 2 14 5 -14 5
Result 3 3.4 3.4 0 0.75 0 4 -4
36
Operator Precedence

Operator precedence
The order in which operands are evaluated in an
expression
Associativity
Operators at the same level of precedence are
evaluated
(left to right ) or (right to left)
Parentheses can be used to force precedence
See Appendix D

37
Operator Precedence

Multiplication, division, and remainder have a
higher precedence than addition and subtraction
Both groups associate left to right

5 12 / 5 - 10 3 6
Expression Order of evaluation Result
4
3
2
1
38
Operator Precedence

What is the order of evaluation in the following
expressions?

a b c d e
a b c - d / e
1
4
3
2
3
2
4
1
a / (b c) - d e
2
3
4
1
a / (b (c (d - e)))
4
1
2
3
39
Operator Precedence
Expression 2 3 4 / 2 3 13 2 (3 13)
2 3 (13 2) 4 (11 - 6) (-8 10) (5 (4
- 1)) / 2
Result 8 41 41 45 40 7
40
Assignment Revisited

The assignment operator has a lower precedence
than the arithmetic operators

First the expression on the right hand side of
the operator is evaluated
answer sum / 4 MAX lowest
1
4
3
2
Then the result is stored in the variable on the
left hand side
41
Assignment Revisited

The right and left hand sides of an assignment
statement can contain the same variable

First, one is added to the original value of count
count count 1
Then the result is stored back into
count (overwriting the original value)
42
Data Conversions

Sometimes it is convenient to convert data from
one type to another
For example, we may want to treat an integer as a
floating point value during a computation
Conversions must be handled carefully to avoid
losing information

43
Data Conversions

Widening conversions are safest
they tend to go from a small data type to a
larger one (such as a short to an int)
Narrowing conversions can lose information
they tend to go from a large data type to a
smaller one (such as an int to a short)

44
Data Conversions

In Java, data conversions can occur in three
ways
assignment conversion(ONLY WIDENING)
arithmetic promotion(ONLY WIDENING)
casting
Assignment conversion occurs
when a value of one type is assigned to a
variable of another
Arithmetic promotion happens automatically
when operators in expressions convert their
operands

45
Data Conversions

Casting is the most powerful, and dangerous,
technique for conversion
Both widening and narrowing conversions
To cast, the type is put in parentheses in front
of the value being converted.
result (float) total / count

46
Creating Objects

A variable either holds a primitive type, or it
holds a reference to an object
A class name can be used as a type to declare an
object reference variable
String title
No object has been created with this declaration
An object reference variable holds the address of
an object
The object itself must be created separately

47
Creating Objects

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

48
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 only works for
strings
Once an object has been instantiated, we can use
the dot operator to invoke its methods
title.length()

49
String Methods

The String class has several methods that are
useful for manipulating strings
Many of the methods return a value, such as an
integer or a new String object
See the list of String methods on page 75 and in
Appendix M
See StringMutation.java (page 77)

50
StringMutation.java

public class StringMutation
public static void main (String args)
String phrase new String ("Change is
inevitable")
String mutation1, mutation2, mutation3,
mutation4
System.out.println ("Original string \""
phrase "\"")
System.out.println ("Length of string "
phrase.length())
mutation1 phrase.concat (", except from
vending machines.")
mutation2 mutation1.toUpperCase()
mutation3 mutation2.replace ('E', 'X')
mutation4 mutation3.substring (3, 30)

51
StringMutation.java

// Print each mutated string
System.out.println ("Mutation 1 "
mutation1)
System.out.println ("Mutation 2 "
mutation2)
System.out.println ("Mutation 3 "
mutation3)
System.out.println ("Mutation 4 "
mutation4)
System.out.println ("Mutated length "
mutation4.length())

52
Class Libraries

The Java API is a class library,
a group of classes that support program
development
The classes in the Java API is separated into
packages
The System class, for example, is in package
java.lang
Each package contains a set of classes that
relate in some way

53
Packages

The classes of the Java standard class library
are organized into packages

54
The Java API Packages

Other packages in the Java API

java.applet java.awt java.beans java.io java.lang
java.math
java.net java.rmi java.security java.sql java.text
java.util
55
Importing Packages

(1) Using a class from Java API by a fully
qualified name
java.lang.System.out.println ()
(2) Package can be imported using an import
statement
import java.applet.
import java.util.Random
to import all classes in a particular package

56
The import Declaration

All classes of the java.lang package are
automatically imported into all programs
System or String classes
The Random class is part of the java.util package
It provides methods for pseudo-random numbers
scale and shift a number into an appropriate
range
See RandomNumbers.java (page 82)

57
RandomNumbers.java

import java.util.Random
public class RandomNumbers
public static void main (String args)
Random generator new Random()
int num1
float num2
num1 generator.nextInt()
System.out.println ("A random integer "
num1)
num1 Math.abs (generator.nextInt()) 10
System.out.println ("0 to 9 " num1)

58
RandomNumbers.java

num1 Math.abs (generator.nextInt()) 20
10
System.out.println ("10 to 29 " num1)
num2 generator.nextFloat()
System.out.println ("A random float
between 0-1 " num2)
num2 generator.nextFloat() 6 // 0 to
5
num1 (int) num2 1
System.out.println ("1 to 6 " num1)

59
Class Methods

class methods or static methods
can be invoked through the class name, instead of
through an object of the class
The Math class contains many static methods,
absolute value, trigonometry functions, square
root, etc.
temp Math.cos(90) Math.sqrt(delta)

60
The Keyboard Class

The Keyboard class is NOT part of the Java API
by the authors to make reading input from the
keyboard easy
contains several static methods for reading
particular types of data
The Keyboard class is part of a package called
cs1
For now we will simply make use of it
Details of the Keyboard class are explored in
Chapter 8
See Echo.java (pp 86) and Quadratic.java (pp 87)

61
Echo.java

import cs1.Keyboard
public class Echo
public static void main (String args)
String message
System.out.println ("Enter a line of
text")
message Keyboard.readString()
System.out.println ("You entered \""
message "\"")

62
Quadratic.java

import cs1.Keyboard
public class Quadratic
public static void main (String args)
int a, b, c // ax2 bx c
System.out.print ("Enter the coefficient of
x squared ")
a Keyboard.readInt()
System.out.print ("Enter the coefficient of
x ")
b Keyboard.readInt()
System.out.print ("Enter the constant ")
c Keyboard.readInt()
double discriminant Math.pow(b, 2) - (4
a c)
double root1 ((-1 b)
Math.sqrt(discriminant)) / (2 a)
double root2 ((-1 b) -
Math.sqrt(discriminant)) / (2 a)
System.out.println ("Root 1 " root1)

63
Formatting Output

The NumberFormat class has static methods that
return a formatter object
getCurrencyInstance()
getPercentInstance()
Each formatter object has a method called format
that returns a string with the specified
information in the appropriate format
See Price.java (page 89)

64
Price.java

import cs1.Keyboard
import java.text.NumberFormat
public class Price
public static void main (String args)
final double TAX_RATE 0.06 // 6 sales
tax
int quantity
double subtotal, tax, totalCost, unitPrice
System.out.print ("Enter the quantity ")
quantity Keyboard.readInt()
System.out.print ("Enter the unit price
")
unitPrice Keyboard.readDouble()

65
Price.java

subtotal quantity unitPrice
tax subtotal TAX_RATE
totalCost subtotal tax
// Print output with appropriate formatting
NumberFormat money
NumberFormat.getCurrencyInstance()
NumberFormat percent NumberFormat.getPerce
ntInstance()
System.out.println ("Subtotal "
money.format(subtotal))
System.out.println ("Tax "
money.format(tax) " at "
percent.format(TAX_RAT
E))
System.out.println ("Total "
money.format(totalCost))

66
Formatting Output

The DecimalFormat class can be used to format a
floating point value in generic ways
For example, you can specify that the number be
printed to three decimal places
The constructor of the DecimalFormat class takes
a string that represents a pattern for the
formatted number
See CircleStats.java (page 91)

67
CircleStats.java

import cs1.Keyboard
import java.text.DecimalFormat
public class CircleStats
public static void main (String args)
int radius
double area, circumference
System.out.print ("Enter the circle's
radius ")
radius Keyboard.readInt()
area Math.PI Math.pow(radius, 2)
circumference 2 Math.PI radius

68
CircleStats.java

// Round the output to three decimal places
DecimalFormat fmt new DecimalFormat
("0.")
System.out.println ("The circle's area "
fmt.format(area))
System.out.println ("The circle's
circumference "
fmt.format(circumferen
ce))

69
Input and Output

The Java API allows you to create many kinds of
streams to perform various kinds of I/O
To read character strings, we will convert the
System.in stream to another kind of stream using
BufferedReader stdin new BufferedReader
(new InputStreamReader (System.in))
This declaration creates a new stream called
stdin
We will discuss object creation in more detail
later

70
Numeric Input

Converting a string that holds an integer into
the integer value using Integer wrapper class
value Integer.parseInt (my_string)
A value can be read and converted in one line
num Integer.parseInt (stdin.readLine())
See Addition.java and Addition2.java

71
Escape Sequences

See Echo.java
An escape sequence is a special sequence of
characters preceded by a backslash (\)
They indicate some special purpose, such as

Escape Sequence \t \n \" \' \\
Meaning tab new line double quote single
quote backslash
72
Echo.java

import java.io.
class Echo
public static void main (String args) throws
IOException
BufferedReader stdin new BufferedReader
(new InputStreamReader(System.in))
String message
System.out.println ("Enter a line of text")
message stdin.readLine()
System.out.println ("You entered \""
message "\"")
// method main
// class Echo

73
Java Applets

A Java applet
a Java program that is intended to be sent across
a network and executed using a Web browser
A Java application is a stand alone program
Applications have a main method, but applets do
not
Special methods of applets
The paint method, for instance, is automatically
executed and is used to draw the applets contents
Applets are derived from the java.applet.Applet

74
Java Applets

Links to applets can be embedded in HTML
documents
actually the bytecode version of the program that
is transported across the web
The applet is executed by a Java interpreter that
is part of the browser(Netscape or Explorer)
Appletviewer in JDK
appletviewer xxx.html

75
Execution of Java Applets
local computer
Java compiler
Java source code
Java bytecode
Web browser
remote computer
Java interpreter
76
Applets

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
See Einstein.java (page 93)

77
Einstein.java

import java.applet.Applet
import java.awt.
public class Einstein extends Applet
// Draws a quotation by Albert Einstein among
some shapes.
public void paint (Graphics page)
page.drawRect (50, 50, 40, 40) //
square
page.drawRect (60, 80, 225, 30) //
rectangle
page.drawOval (75, 65, 20, 20) //
circle
page.drawLine (35, 60, 100, 120) // line
page.drawString ("Out of clutter, find
simplicity.", 110, 70)
page.drawString ("-- Albert Einstein", 130,
100)

78
Drawing Shapes

A shape can be filled or unfilled, depending on
which method is invoked
fillArc( ), fillOval( ), fillRect( )
The method parameters specify coordinates and
sizes
Java coordinate system has the origin in the
upper left corner
Many shapes with curves, like an oval, are drawn
by specifying its bounding rectangle
An arc can be thought of as a section of an oval

79
Drawing a Line
10
150
20
45
80
Drawing a Rectangle
50
20
page.drawRect (50, 20, 100, 40)
81
Drawing an Oval
175
20
bounding rectangle
page.drawOval (175, 20, 50, 80)
82
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
Every graphics context has a current foreground
color
Every drawing surface has a background color
See Snowman.java (page 99-100)

83
Snowman.java