Working with java input and 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
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2. Reading information into a program.
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Writing information from a program

3. User input classes
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 To get user input, use the BufferedReader and
InputStreamReader classes.
 The InputStreamReader class - reads the user's
input.
 The BufferedReader class - buffers the user's
input to make it work more efficiently.

4. Keyboard Input
The System class in java provides an
InputStream object: System.in
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.
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5. Keyboard Input
BufferedReader
InputStreamReader
int stringbyte
InputStreamReader isr = new InputStreamReader(System.in);
BufferedReader br = new BufferedReader(isr);
String st = br.readLine( );
import java.io.*; //for keyboard input stream
//reads chars until eol and forms string
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6. 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(System.in);
BufferedReader br = new BufferedReader(isr);
System.out.print(“Enter four integers: “);
str = br.readLine( );
str = 6 2 4 3 2 1
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7. String Tokenizer
We need to retrieve the four separate integers from the string.
str = 6 2 4 3 2 1
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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8. StringTokenizer :Method
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 int countTokens()
Calculates the number of times that this
tokenizer's nextToken method can be called before it
generates an exception.
 boolean hasMoreElements()
Returns the same value as the hasMoreTokens
method.
 boolean hasMoreTokens()
Tests if there are more tokens available from this
tokenizer's string.

9. StringTokenizer :Method
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 Object nextElement()
Returns the same value as the nextToken method,
except that its declared return value is Object rather
than String.
 String nextToken()
Returns the next token from this string
tokenizer.
 String nextToken(String delim)
Returns the next token in this string tokenizer's
string.

10. 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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11. 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 Classes
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.
wrapper method name return type
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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12. 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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13. 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(System.in);
Step 3 – Form input stream of characters into a string (look for eol)
BufferedReader br = new BufferedReader(isr);
String str = br.readLine( );
Step 4 – Create StringTokenizer to extract tokens from the input string
StringTokenizer st = new StringTokenizer(str);
Need to throw java.io.IOException in function in
which it is used
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14. 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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15. 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; }
}
} RAJESHREE KHANDE15