Transcript Reading Input

Reading Input
Overview

Introduction to Wrapper classes.

Introduction to Exceptions (Java run-time
errors).

Console input using the BufferedReader
class.
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WRAPPER CLASSES



Java uses primitive types, such as int and char, for
performance reasons.
However, there are times when a programmer
needs to create an object representation for one of
these primitive types.
Java provides a Wrapper class for each of the
primitive types. All these classes are in the
java.lang package:
Primitive type
Wrapper class
boolean
byte
char
Boolean
Byte
Character
short
int
long
float
Short
Integer
Long
Float
double
Double
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WRAPPER CLASSES (cont.)

Wrapper classes are used to provide constants and
general methods for the primitive data types.

Converting strings into numbers

Each of the Wrapper classes Byte, Short, Integer,
Long, Float, and Double has a method to convert a
string representation of a number of the
corresponding primitive type into its numeric
format:
Wrapper class
Parse method
Byte
parseByte(string)
Short
parseShort(string)
Integer
parseInt(string)
Long
parseLong(string)
Float
parseFloat(string)
Double
parseDouble(string)
Examples:
int numStudents = Integer.parseInt(“500”) ;
String inputLine ;
. . .
double studentGPA = Double.parseDouble( inputLine) ;
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INTRODUCTION TO EXCEPTIONS

A program may have one or more of three types of
errors:
1. Syntax errors or Compile-time errors.
2. Run-time or Execution-time errors.
3. Logic errors.

A Java exception is an object that describes a runtime error condition that has occurred in a piece of
Java code or in the Java run-time System.
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All Java exception classes are subclasses of the
Throwable class.

Most exception classes are defined in the java.io and
java.lang packages. other packages like: java.util,
java.awt, java.net, java.text also define exception
classes.

Catching and throwing an exception
A piece of Java code containing statements to handle
an exception is said to catch the exception; otherwise it
is said to throw that exception.
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An exception that is not caught by any portion of a
Java program will ultimately be caught by the default
exception handler. The default exception handler
displays a string describing the exception.
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INTRODUCTION TO EXCEPTIONS (cont.)

A partial hierarchy of Java exceptions:
Throwable
Exception
Error
...
. . .
IOException
RuntimeException
. . .
. . .
IllegalArgumentException
NullPointerException
NumberFormatException
ArithmeticException
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INTRODUCTION TO EXCEPTIONS (cont.)

Checked and Unchecked exceptions
Java exceptions are classified into two categories: checked
exceptions and unchecked exceptions.
Any exception that derives from the class Error or the class
RuntimeException is an unchecked exception. All other
exceptions are checked exceptions.

The Java rule for thrown exceptions:
A METHOD MUST DECLARE ALL CHECKED
EXCEPTIONS IT MAY THROW, OTHERWISE THE
JAVA COMPILER WILL ISSUE AN ERROR
MESSAGE.
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The throws clause
A method declares that it may throw an exception by a
throws clause in the method header:
access-specifier return-type method-name(parameter-list)
throws exception-list
{
. . .
}
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INTRODUCTION TO EXCEPTIONS (cont.)
Example:
import java.io.* ;
public static void main(String[ ] args) throws
IOException
{
. . .
}
One or more statements that may throw an
IOException that is not handled.
Note: When a method declares that it throws an
exception, then it may throw an exception of that
class or any of its subclasses.
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CONSOLE INPUT USING BufferedReader CLASS
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In Java I/O is handled by streams.
Input stream
An input stream is an object that takes data
from an input source and delivers that data to a
program.
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Output stream
An output stream is an object that delivers data
to an output destination.

In Java, console input is usually accomplished by
reading from the input stream System.in of the
class java.lang.System
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System.in represents the standard input stream
(i.e., the keyboard).
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Unfortunately, System.in has no methods for
reading characters, strings, or numbers. It has a
read method to read a single byte at a time.[Java
uses Unicode in which each character is two
bytes]
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To be able to read characters, strings, or numbers,
System.in must be wrapped in other objects.
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CONSOLE INPUT USING BufferedReader CLASS (cont.)

To turn System.in into a Reader object (i.e., an object
that is capable of reading one character at a time), wrap
System.in in an InputStreamReader object:
InputStreamReader reader =
new InputStreamReader(System.in) ;
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To turn the object referenced by reader into an object
with the ability to read entire lines at a time, wrap the
object in a BufferedReader object:
BufferedReader stdin =
new BufferedReader(reader) ;

The steps of turning System.in into a BufferedReader
object can be combined into a single statement:
BufferedReader stdin = new BufferedReader( new
InputStreamReader(System.in)) ;

Note: Both the BufferedReader class and the
InputStreamReader class are defined in the java.io
package.
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CONSOLE INPUT USING BufferedReader CLASS (cont.)
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The read( ) and readLine( ) methods
The object to which stdin refers to contains a read( )
method that reads one character at a time, and returns its
integer code in the range 0 to 65535:
int read( ) throws IOException
It also contains a readLine( ) method that reads one
input line at a time, and returns it as a string:
String readLine( ) throws IOException

Example: Reading a string
import java.io.*;
public class ReadString
{
public static void main(String[ ] args) throws
IOException
{
BufferedReader stdin = new
BufferedReader(new InputStreamReader(System.in))
;
System.out.println(“Enter a line of text:”) ;
String message = stdin.readLine( ) ;
System.out.println(“You entered: ” + message ) ;
}
}
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CONSOLE INPUT USING BufferedReader CLASS (cont.)
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Example: Reading a character
char ch = (char) stdin.read( ) ;

Numeric input
The Java library contains no classes to read
numbers directly. One way of processing numeric
input is to read it as a string using the readLine( )
method then convert it to its corresponding numeric
value, using the parse method of an appropriate
Wrapper class.

Example:
String inputLine = stdin.readLine( ) ;
int numStudents = Integer.parseInt(inputLine) ;
double speed = Double.parseDouble(stdin.readLine( )) ;
float height = Float.parseFloat(stdin.readLine( ).trim( ));
Note: Each parse method can throw an unchecked
NumberFormatException
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