Using Classes and Objects

Chapter 3
Using Classes and
Objects


Outline
Last Time:
Creating Objects
The GregorianCalendar Class
The String Class
Wrapper Classes

Packages
Math Class
Formatting Output
Enumerated Types
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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

•

Its classes are not part of the Java language per se, but we
rely on them heavily

•

Various classes we've already used (System , Scanner,
String) are part of the Java standard class library

•

Other class libraries can be obtained through third party
vendors, or you can create them yourself

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Packages
•

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

•

Some of the packages in the standard class library are:

Package

Purpose

java.lang
General support
java.applet
Creating applets for the web
java.awt
Graphics and graphical user interfaces
javax.swing
Additional graphics capabilities
java.net
Network communication
java.util
Utilities
javax.xml.parsers
XML document processing

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The import Declaration
•

When you want to use a class from a package, you could use
its fully qualified name

java.util.Scanner
•

Or you can import the class, and then use just the class
name

import java.util.Scanner;
•

To import all classes in a particular package, you can use
the * wildcard character

import java.util.*;

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The import Declaration
•

All classes of the java.lang package are imported
automatically into all programs

•

It's as if all programs contain the following line:
import java.lang.*;

•

That's why we didn't have to import the System or String
classes explicitly in earlier programs

•

The Scanner class, on the other hand, is part of the
java.util package, and therefore must be imported

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Outline

Packages
Math Class
Formatting Output
Enumerated Types

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The Math Class
•

The Math class is part of the java.lang package

•

The Math class contains methods that perform various
mathematical functions

•

These include:

 absolute value
 square root
 exponentiation
 trigonometric functions
 generate random numbers

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The Math Class
•

The methods of the Math class are static methods (also
called class methods)

•

Static methods can be invoked through the class name – no
object of the Math class is needed

value = Math.cos(90) + Math.sqrt(delta);
•

See Quadratic.java (page 129)

discriminant
root1 = ((-1
(2 *
root2 = ((-1
(2 *
•

= Math.pow(b, 2) - (4 * a * c);
* b) + Math.sqrt(discriminant)) /
a);
* b) - Math.sqrt(discriminant)) /
a);

We discuss static methods further in Chapter 6

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Math Methods

Method

Purpose

Argument

abs(x)

Returns the absolute
value of x

any numeric type same as
argument

ceil(x)

Returns smallest
whole number >= x

double

double

exp(x)

Returns ex where e =
2.71828…

double

double

floor(x)

Returns the largest
whole number <= x

double

double

log(x)

Returns the natural
logarithm of x (base e)
for x > 0.0

double

double

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Result Type

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Math Methods
Method

Purpose

Argument

max(x,y)

Returns the larger of x
and y

any numeric type same as
argument

min(x,y)

Returns the smaller of
x and y

any numeric type same as
argument

pow(x,y)

Returns xy. An error
any numeric type double
will occur if x = 0 and y
<=0, or x < 0 and y is
not a whole number

random()

Returns a
pseudorandom
number between 0.0
and 1.0

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double

Result Type

double

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Math Methods
Method

Purpose

Argument

Result Type

rint(x)

Returns the closest
whole number to x

double

double

round(x)

Returns the integer
value closet to x

double or float

long or int

sqrt(x)

Returns the positive
square root of x for
x > 0.0

double

double

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Equations

4 3
V = πr
3

• Volume of a sphere


• Kepler’s Law

a 3 GM
=
2
p
4π 2

2
GMp
a=3
4π 2

 where a = radius of orbit, p = period of orbit, G = gravity

x = 10

• Brightness of stars

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( m2 − m1 )

∆m = 2.5 log10 x

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rLat

Equations
•

radius of a circle of latitude rlat = rE cos Θ

•

distance between two points dist =

•

Simulating the rolling of a die – random number
between 1 and 6

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Θ
r

( x2 − x1 ) 2 + ( y2 − y1 ) 2

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Outline

Packages
Math Class
Formatting Output
Enumerated Types

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Formatting Output
•

It is often necessary to format values in certain ways so
that they can be presented properly

•

The Java standard class library contains classes that
provide formatting capabilities

•

The NumberFormat class allows you to format values as
currency or percentages

•

The DecimalFormat class allows you to format values based on
a pattern

•

Both are part of the java.text package

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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

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See Purchase.java (page 131)
NumberFormat fmt1 =
NumberFormat.getCurrencyInstance();
NumberFormat fmt2 =
NumberFormat.getPercentInstance();
// Print output with appropriate formatting
System.out.println ("Subtotal: " +
fmt1.format(subtotal));
System.out.println ("Tax: " + fmt1.format(tax) +
" at " + fmt2.format(TAX_RATE));
System.out.println ("Total: " +
fmt1.format(totalCost));

Output

Enter the quantity: 3
Enter the unit price: 1.50
Subtotal: $4.50
Tax: $0.27 at 6%
Total: $4.77

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Formatting Output
•

The DecimalFormat class can be used to format a floating
point value in various ways

•

For example, you can specify that the number should be
truncated to three decimal places

•

The constructor of the DecimalFormat class takes a string
that represents a pattern for the formatted number

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See CircleStats.java (page 134)
// 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(circumference));

Output
Enter the circle's radius: 7
The circle's area: 153.938

The circle's circumference: 43.982
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Outline

Packages
Math Class
Formatting Output
Enumerated Types

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Enumerated Types
•

Java allows you to define an enumerated type, which can then
be used to declare variables

•

An enumerated type establishes all possible values for a
variable of that type

•

The values are identifiers of your own choosing

•

The following declaration creates an enumerated type called
Season

enum Season {winter, spring, summer, fall};
•

Any number of values can be listed

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Enumerated Types
•

Once a type is defined, a variable of that type can be
declared
Season time;
and it can be assigned a value
time = Season.fall;

•

The values are specified through the name of the type

•

Enumerated types are type-safe – you cannot assign any value
other than those listed

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Ordinal Values
•

Internally, each value of an enumerated type is stored as an
integer, called its ordinal value

•

The first value in an enumerated type has an ordinal value
of zero, the second one, and so on

•

However, you cannot assign a numeric value to an enumerated
type, even if it corresponds to a valid ordinal value

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Enumerated Types
•

The declaration of an enumerated type is a special type of
class, and each variable of that type is an object

•

The ordinal method returns the ordinal value of the object

•

The name method returns the name of the identifier
corresponding to the object's value

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