1. File Handling in Java January 19
Part of this lecture will be reserved for working through solutions to selected exercises from last week. Notes relating to this do not appear in the slides.
January 19

2. Objectives In this lecture, we will:
learn how to read data from a file
learn how to write data to a file
discuss the structure of a program which uses files
January 19

3. Why use files?
In all the programs we have written so far, any data we need has been either
hard coded into the program
String name = "Cathy";
or read in from the keyboard using a Scanner object
Scanner kybd = new Scanner(System.in);
System.out.println("What is your name?");
name = kybd.next();
It is tedious to type in all the data to be processed each time the program is run
It is not very useful if the data and results cannot be saved
imagine a payroll program where all information about the employees has to be entered each time the program is run
January 19

4. Opening a file to read from
A Scanner object can be set up to read from a file
so far all our Scanner objects have read from the keyboard
The name of the file to read from is required
First create a File object corresponding to this file
need to import java.io.*
this is a library of java classes for input and output
Then create a Scanner object using the File object as the source of the input
instead of System.in
If the input file is in the same directory as the Java program it makes life easier
if the file is not found, there will be an error (exception) when the program runs
January 19

5. Reading from a file import java.util.*; import java.io.*;
public class Payroll {
public static void main(String args[])
String fName = "payroll.txt";
Scanner inFile = new Scanner(new File(fName)); …… or
Scanner inFile =
new Scanner(new File("payroll.txt"));
January 19

6. Reading from Scanner objects
Once the Scanner object is created, we can use its methods to read from the file
just like when reading from the keyboard
String name = inFile.next();
double hourlyPay = inFile.nextDouble();
Multiple Scanner objects can be created, as long as they are given different names
for example, to read from two different files
or from a file and the keyboard
Scanner kybd = new Scanner(System.in);
Scanner inFile = new Scanner(new File(fName));
January 19

7. Reading from a file
The hasNext() method is useful when you do not know how much data is in the file
returns true if there is more data to read
returns false if you have reached the end of the file
It can be used in a while loop to process all the data in the file
Imagine a text file is available where each line contains information about one employee:
name (as a String)
followed by hourly pay (as a double)
For example the file could contain the data
Nick 4.95
Fred 5.94
Dave 9.45
The code overleaf would process all the data in turn
January 19

8. Reading all the data in a file
while (inFile.hasNext()) {
name = inFile.next();
hourlyPay = inFile.nextDouble();
System.out.println("Hours worked by "+name+"?");
hoursWorked = kybd.nextInt();
double pay = hourlyPay * hoursWorked;
System.out.println("Pay is " + pay); }
inFile.close();
January 19

9. Closing a file
When you have finished reading from a file, you should close it
A file is closed by calling the close() method of the Scanner object that was set up to read the file
inFile.close();
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10. Tips for Input Files Decide on the format of the data
repeating rows of
name (as a String)
followed by hourly pay (as a double)
Make sure the information is
in the correct order
of the correct type
to match the input statements in your program
Any text editor can be used to create and edit the file
or it could be output from a program
which writes to the file using the correct format
January 19

11. Writing to a file The name of the file to write to is required
First create a PrintWriter object for this file
need to import java.io.*
same library of java classes as FileReader
Typically the output file will be in the same directory as the Java program
If a file of this name already exists
it will be opened
all the data currently in the file will be lost
If the file does not already exist, a new one will be created
PrintWriter pw = new PrintWriter("Payroll.txt");
January 19

12. Writing to a file
Once the PrintWriter object is created, we can use its methods to write to the file
can use print(), println(), printf()
just like when writing to the console output window
For example to print an employee’s data on one line
pw.print(name);
pw.printf("%6.2f", hourlyPay);
pw.println();
Close the PrintWriter when you have finished writing to the file
pw.close();
January 19

13. Structure of a program that uses files
A program which reads data from a file may do a lot of processing on it
do calculations (totals, averages)
add to it (input by user)
delete some of it
sort it
search all of it for a particular data value
It is awkward to search for and retrieve only the required data from a sequential file for each process
and to write changes back to the original file
January 19

14. Structure of a program that uses files
It is sometimes better to
open the input file
read all the data in the file into an appropriate data structure
such as an array, or several arrays
close the input file
Do all the processing in memory, then write the final version of the data to a file
either with the same name as the input file
original data is lost
or a new file
Most of the program (data structures, processing) is the same as when all the data is entered via the keyboard
just add the file-reading code at the beginning of the program
and the file-writing code at the end
January 19

15. Files and Exceptions
File handling is one area that is prone to things going wrong
A file may not exist
A file may not be accessible
The format of the data in the file may be incorrect
Whenever dealing with files it is best to make use of try catch blocks to handle any exceptions
Try to anticipate what could go wrong
January 19

16. Summary In this lecture we have: learned how to read data from a file
learned how to write data to a file
discussed the structure of a program which uses files
January 19