Interactive Input/Output

1
Interactive Input/Output
Contents

1. Reading data from the keyboard
2. Extracting separate data items from a String
3. Converting from a String to a primitive numerical
   type
4. An example showing how numerical data is read
   from the keyboard and used to obtain and display
   a result
5. Using dialog boxes to obtain input data and
   display results

2
Keyboard Input
The System class in java provides an InputStream
object System.in And a buffered PrintStream
object System.out
The PrintStream class (System.out) provides
support for outputting primitive data type values.
However, the InputStream class only provides
methods for reading byte values.
To extract data that is at a higher level than
the byte, we must encase the InputStream,
System.in, inside an InputStreamReader object
that converts byte data into 16-bit character
values (returned as an int).
We next wrap a BufferedReader object around the
InputStreamReader to enable us to use the methods
read( ) which returns a char value and readLine(
), which return a String.
3
Keyboard Input
import java.io. //for keyboard input stream
InputStreamReader isr new InputStreamReader(Syst
em.in) BufferedReader br new
BufferedReader(isr) String st br.readLine( )
//reads chars until eol and forms string
4
String Tokenizer
Consider the following program fragment
import java.io. public class TotalNumbers
throws java.io.IOException private String
str private int num public static
void main (String args)
InputStreamReader isr new InputStreamReader(Sys
tem.in) BufferedReader br new
BufferedReader(isr) System.out.print(Ente
r four integers ) str br.readLine( )
str
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String Tokenizer
We need to retrieve the four separate integers
from the string.
str
A token consists of a string of characters
separated by a delimiter.
Delimiters consist of space, tab, newline,
return
A StringTokenizer parses a String and extracts
the individual tokens.
StringTokenizer st new StringTokenizer (str)
//create tokenizer and pass string String
s_temp while (st.hasMoreTokens( ) )
s_temp st.nextToken( ) //now convert
this string to an integer.
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Wrapper Classes
For each of the primitive types there is a
Wrapper class.
Primitive type
object
int num1 6 Integer myInt
Integer(num1) double num2 3.1416 Double pi
Double(num2)
In the statement Integer myInt Integer(num1)
an Integer object named myInt is created and
assigned a value equal to the contents of num1
Wrapper classes begin with an uppercase letter to
distinguish from their primitive type counterpart
(int, long, short, double, float, byte, char,
boolean).
int ? Integer double ? Double float
? Float char ? Character
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Wrapper Classes
Unlike primitive types, objects have operations
called methods that they can be directed to
perform. (These methods have visibility static ?
they can be accessed by using the class name
without instantiating objects of the class.
Wrapper class objects have a method for
converting a string into a primitive type, and a
method for transforming a primitive type into a
string.
Integer parseInt(String st) int
Integer toString(int num) String
Double parseDouble(String st) double
Double toString(double num) String
Float parseFloat(String st) float
Long parseLong(String st) long
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Converting Tokenized String to Primitive Types
Return to the code for extracting tokens from the
input string
int sum 0, num String s while
(st.hasMoreTokens( )) s st.nextToken(
) //convert string to int num
Integer.parseInt(s) sum num
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Review -- Reading stream of integers from
keyboard
Step 1 Prompt user to enter multiple integers
on one line
System.out.print(Enter four integers separated
by spaces )
Step 2 Retrieve keyboard input as a stream of
16-bit chars (retuned as int)
InputStreamReader isr new InputStreamReader(Syst
em.in)
Step 3 Form input stream of characters into a
string (look for eol)
BufferedReader br new BufferedReader(isr) Strin
g str br.readLine( )
Need to throw java.io.IOException in function in
which it is used
Step 4 Create StringTokenizer to extract tokens
from the input string
StringTokenizer st new StringTokenizer(str)
10
Review cont.
Step 5 Parse the input string to extract tokens
String s1 st.nextToken( ) //note can use
while(st.hasMoreTokens( )) to repeatedly extract
each //token in the string
Step 6 Use wrapper class methods to convert
token (string) to primitive type
int num Integer.parseInt(s1)
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Putting it all together
import java.io. //for keyboard input
methods import java.util. //for
StringTokenizer public class TotalNumbers
public static void main (String args) throws
java.io.IOException String str, s
int sum 0, num InputStreamReader isr
new InputStreamReader(System.in)
BufferedReader br new BufferedReader(isr)
System.out.print(Enter four integers
separated by spaces ) //prompt str
br.readLine( ) StringTokenizer st new
StringTokenizer(str) while (st.hasMoreTokens(
)) s st.nextToken( ) num
Integer.parseInt(s) sum num
12
Interactive Input
Using a Dialog box to request input data
Step 1 Import swing package. import
javax.swing.
Step 2 Create an input dialog box (returns a
String)
String str JOptionPane.showInputDialog(the
message)
Input
OK
Cancel
Step 3 Write input value(s) in the text field
//done by user
Step 4 Click on command button (OK,
Cancel) //done by user
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Output to a Dialog Box
Types of Dialog Boxes icon
WARNING_MESSAGE
QUESTION_MESSAGE
INFORMATION_MESSAGE
ERROR_MESSAGE
PLAIN_MESSAGE
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Output Dialog Boxes
To create a dialog box for displaying output use
showMessageDialog
JOptionPane.showMessageDialog(null, message,
title,icon_type)
For example, the statement
JOptionPane.showMessageDialog(null, Hi Mom!,
greetings, JOptionPane.PLAIN_MESSAGE)
produces
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Output Dialog Boxes
Note that the icon-type field in the
showDialogMessage( ) method is a static constant
declared in class JOptionPane, and must be
referenced by a message to this class.
JOptionPane.PLAIN_MESSAGE
The icon_type determines which of the five icons
(the fifth being no icon at all) will appear in
the dialog box.
Note! Without the final statement
System.exit(0) a program that ends with a
message dialog box will still be active after the
dialog box has been closed.
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An Interactive Program that Uses Dialog Boxes
import javax.swing. //for JOptionPane import
java.util. //for StringTokenizer public class
DialogExample public static void
main(String args) String sInput,
str int num, sum 0 sInput
JOptionPane.showInputDialog(enter three
integers ) StringTokenizer st new
StringTokenizer(sInput) while
(st.hasMoreTokens( ) ) str
st.nextToken( ) num Integer.parseInt(str
) sum num //output the result to a
dialog box JOptionPane.showDialogMessage(null,
DialogExample Output, the sum is sum,
JOptionPane.INFORMATION_MESSAGE) System.exit(0)
//needed to close program when using dialog box

17
Review
To perform interactive I/O using dialog boxes
1. import javax.swing.

• Use an input dialog box to prompt the user to
  enter data (returns a String)
• String inString JOptionPane.showInputDial
  og(your message)

• Use a StringTokenizer (if necessary) to extract
  the individual tokens
• from the input String

• Convert (String) tokens into primitive types
  wherever necessary using
• the appropriate wrapper class.

• Use a dialog box to display the output
• JOptionPane.showMessageDialog(null,message,
  title, icon-type)

• When using a Message dialog box, end the program
  with
• System.exit(0)