# Casting, Wrapper Classes,

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3 Νοε 2013 (πριν από 4 χρόνια και 6 μήνες)

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Casting, Wrapper Classes,

Static Methods,

JOptionPane Class

Casting

Casting is used to convert one data type
to another.

Implicit casting

mixed arithmetic expressions

assignments

byte
-
> short
-
>
int

-
> long
-
> float
-
> double

explicit casting

use the cast operator

(
type
)
expression

How to code an explicit cast

(
type
)
operand

Example

double average = 93.25;

public class Casting

{

public static void main(String[]
args
)

{

float num1 = 3;

double num2 = 2;

int

num3 = 0;

num3 = (
int
)(num1/num2);

System.out.println
(num3);

}

}

The syntax for declaring a main method

public static void main(String[] args)

{

statements

}

The main method of
an
InvoiceApp class

public class InvoiceApp

{

public static void main(String[] args)

{

System.out.println("Invoice app");

}

}

Type
-
wrapper Classes

All the primitive data types have corresponding
classes that provide some general methods that
are useful when dealing with data of the specified
type.

Byte, Double, Float, Integer, Long

The classes are known as wrapper classes since
they “wrap” the primitive data type in a class.

Look up the specific classes for their methods:

http://
docs.oracle.com/javase/6/docs/api/index.html

Integer Class

The Integer class wraps a value of the primitive
type
int

in an object.

There are two constructors for the Integer class:

Integer
(
int

value)

Constructs a newly allocated Integer object that represents
the specified
int

value

Integer
(String

s)

Constructs a newly allocated Integer object that represents
the
int

value indicated by the String parameter.

The class provides several methods for
converting an
int

to a String and a String to an
int.

Double Class

The Double class wraps a value of the primitive
type double in an object.

There are two constructors for the Integer class:

Double
(double

value)

Constructs a newly allocated Double object that represents
the primitive double argument.

Double
(String

s)

Constructs a newly allocated Double object that represents
the floating
-
point value of type double represented by the
string.

In addition, this class provides several methods
for converting a double to a String and a String
to a double.

Static Methods

Both the Integer class and the Double class
have static methods.

Unlike a regular method which is called from
an object, a static method is called from a
class.

Example:

int quantity = Integer.parseInt(quantityString);

The syntax for using a static method of a class

class
.
method
Name
(
arguments
)

Two static methods of the Integer class

Method

Description

parseInt(
String
)

Attempts to convert the String object that’s
supplied as an argument to an int type.

toString(
int
)

Conve
rts the int value that’s supplied as an
argument to a String object and returns that
String object.

Two static methods of the Double class

Method

Description

parseDouble(
String
)

Attempts to convert the String object
that’s supplied as an argument to a double
type.

toString(
double
)

Converts the double value that’s supplied
as an argument to a String obj
ect and
returns that String object

How to convert a String object to a primitive type

For an int

String quantityString = “25”;

int quantity = Integer.parseInt(quantityString);

For a double

String priceString = “9.95”;

double price = Double.parseDouble(priceString);

How to convert a primi
tive type to a String object

For an int

int counter = 15;

String counterString = Integer.toString(counter);

For a double

double price = 37.95;

String priceString = Double.toString(price);

import javax.swing.JOptionPane;

{

public static void main(String[] args)

{

String firstNumber;

String secondNumber;

int number1;

int number2;

int sum;

firstNumber = JOptionPane.showInputDialog(null, "Enter first integer",

"Input", JOptionPane.QUESTION_MESSAGE);

secondNumber = JOptionPane.showInputDialog(null, "Enter second integer",

"Input", JOptionPane.QUESTION_MESSAGE);

number1 = Integer.parseInt(firstNumber);

number2 = Integer.parseInt(secondNumber);

sum = number1 + number2;

JOptionPane.showMessageDialog(null, "The sum is " + sum, "Results",

JOptionPane.PLAIN_MESSAGE);

System.exit(0);

}

}

The Java Swing Package

Java provides two different technologies for
building a graphical user interface

Abstract Window Toolkit (AWT)

Swing

The Swing classes are stored in the
javax.swing

package

Use the
JOptionPane

class of the
javax.swing

package to display

dialog boxes

message boxes

To use the
JOptionPane

class

import
javax.swing.JOptionPane
;

showInputDialog

Method

Use the static method
showInputDialog

of the
JOptionPane

class to get input from a user.

Example:

String
firstNumber
;

firstNumber

=
JOptionPane.showInputDialog
(null, "Enter first integer",

"Input",
JOptionPane.QUESTION_MESSAGE
);

showMessageDialog

Method

Use the static method
showMessageDialog

of the
JOptionPane

class to display text to a user.

Example:

sum = 6;

JOptionPane.showMessageDialog
(null, "The sum is " + sum, "Message",

JOptionPane.PLAIN_MESSAGE
);

System.exit
(0);

Main loop structure

boolean

finished = false;

while(!finished)

{

do something

if(
exit condition
)

{

finished = true;

}

else

{

do something more

}

}