Download Reading Input From A File

Reading Input From A File
In addition to reading in values from the keyboard, the Scanner class also allows us
to read in numeric values from a file.
1. Create and save a text file (.txt or .dat extension) in the same folder with
the .java file.
2. At the very top of your program add the line
import java.io.*; //for File and IOException
import java.util.Scanner; //for Scanner
3. Add throws IOException after the header of the main method. When
working with files, a program may encounter errors that it cannot handle.
Examples are the file could be missing when the program tries to open it, or the
file’s data may be corrupted. At a minimum, a program must acknowledge that an
I/O exception might be thrown.
4. In the main method, create a Scanner object (this one is named reader) to read
from a file (this one is named myfile.txt).
Scanner reader = new Scanner(new
File(“myfile.txt”));
5. In the main method, create a sentinel-controlled loop. The idea of a sentinel
controlled loop is that there is a special value (the sentinel) that is used to say when
the loop is done. In this case, the loop will continue to read from the file until the
hasNext() method returns false (when the end of the file has been reached).
while(reader.hasNext())
{
//statement
//statement
}
6. Close the scanner object when your program is done using it.
reader.close();
reader.hasNext()
reader.hasNextLine()
reader.hasNextInt()
reader.hasNextDouble()
Useful Methods
Returns true if the scanner has another token in its input
Returns true if the scanner has another line
Returns true if the next token in the scanner can be
interpreted as an int
Returns true if the next token in the scanner can be
interpreted as a double
User input is not part of the AP Java subset. There are many possible
ways for supplying user input. If reading input is necessary, it will be
indicated in a way similar to the following:
double x = call to a method that reads a floating-point number;
or
double x = IO.readDouble(); // read user input
A Sample Program
This program reads in numerical data from a file, token by token. Remember, the
Scanner object assumes that whitespace characters (spaces, tabs, and new
lines) are used to separate the input elements (tokens) from each other.
import java.io*;
import java.util.Scanner;
public class ComputeAverage
{
//computes the average of the numbers in a file
public static void main(String[] args) throws IOException
{
Scanner reader = new Scanner(new File(“numbers.txt”));
double number, sum=0;
int count = 0;
//computes the average of the numbers in a file
while(reader.hasNext())
{
number = reader.nextDouble();
sum += number;
count++;
}
if(count == 0)
System.out.println(“The file has no numbers.”);
else
System.out.println(“The average of “ + count +
“ numbers is “ + sum / count);
reader.close(); //close the Scanner
}
}
Another Sample Program
This program reads in data line by line (the first loop) then reads each line token by
token (the nested loop). One scanner is used to read in the lines; a second scanner
is used to
import java.io*;
import java.util.Scanner;
public class ComputeAverage
{
//does something…
public static void main(String[] args) throws IOException
{
Scanner readFile = new Scanner(new File(“numbers.txt”));
Scanner readLine = new Scanner
double number, sum=0;
int count = 0;
//computes the average of the numbers in a file
while(reader.hasNext())
{
number = reader.nextDouble();
sum += number;
count++;
}
if(count == 0)
System.out.println(“The file has no numbers.”);
else
System.out.println(“The average of “ + count +
“ numbers is “ + sum / count);
reader.close(); //close the Scanner
}
}
Errors You May Encounter
InputMismatchException: This exception can be thrown if you try to get the
next token using a next method that does not match the type of the token
FileNotFound Exception: Thrown when the file is missing when the program
tries to open it
Defining Your Own Delimiter
By default, the Scanner will read data as tokens separated by whitespace. If
you want to read in data separated by something else (comma, colon, etc.) you
can create your own delimiter.
scan.useDelimiter(",\\s*"); //comma followed by spaces
scan.useDelimiter("\\s*dog\\s*"); //space then the word dog then space
scan.useDelimiter("/"); //slash
A Comma Delimited Program
import java.io*;
import java.util.Scanner;
public class UseCommas
{
//reads a file of numbers separated by commas
public static void main(String[] args) throws IOException
{
Scanner scan3 = new Scanner(new File(“commadata.txt”));
String line = null;
while(scan3.hasNext())
{
line = scan3.nextLine();
Scanner scan4 = new Scanner(line);
scan4.useDelimiter(", "); //a comma
while(scan4.hasNextInt())
{
System.out.println(scan4.nextInt());
}
}
scan3.close();
}
}