1. 18 File handling1June 15 18 File handling CE00858-1: Fundamental Programming Techniques

2. 18 File handling Objectives In this lecture, we will: open files for input and output close files after processing read data from file write results to file June 152

3. 18 File handling Why use files? the data in programs written so far has either been: hard coded into the program read from the keyboard using a Scanner object more useful to: read data from file store results in a file int width = 4; Scanner kybd = new Scanner(System.in); width = kybd.nextInt(); June 153

4. 18 File handling Using input files must import java.io.* create a Scanner object to read from a file need the name of the file to read in place of System.in file must be in same folder as Java program or checked exception will occur checked exception must be handled can have any number of Scanner objects one to read from each file one to read from keyboard when processing done all files should be closed June 154

5. 18 File handling Reading from file Scanner object methods are used to read from file in same way as they read from keyboard: nextInt() nextDouble() next() nextLine() hasNext() method: returns true if there is more data to read returns false when end of file is reached can use hasNext() method in a while loop to process all data in a file June 155

6. Using input files example - AverageAge program to read in names and ages from a data file called in.txt calculate and output the average age to the screen assume that all data is valid 18 File handling6June 15

7. AverageAge analysis what data is used? sum: double calculated in program to total ages count: integer calculated in program to count number of people age: double input from file name: string input from file what operations are needed? iteration to process contents of file what operations are performed once before the loop? create Scanner and link it to in.txt initialise count and sum to 0 how many times is the loop performed? loop performed until end of file reached 18 File handling7June 15

8. AverageAge analysis continued what operations are repeated inside the loop? read name and age add age to sum add 1 to count what operations are performed once after the loop close in.txt calculate and output average what exceptions may occur file not found error reading from file division by 0 if no data input 18 File handling8June 15

9. 18 File handling //input ages from file and calculate average import java.util.*; import java.io.*; public class AverageAge { public static void main (String [] args) { double sum = 0; int count = 0; try { //create Scanner linked to input file Scanner in = new Scanner(new File("in.txt")); String name; double age; June 159 continued on next slide AverageAge.java

10. 18 File handling //read until end of file reached while (in.hasNext()) { name = in.next(); age = in.nextDouble(); sum = sum + age; count++; } //close file in.close(); double average = sum / count; System.out.println("The average age is: " + average); } //handle any errors catch(ArithmeticException e) { System.out.println("Cannot calculate average - no data"); } catch(IOException e) { System.out.println("Error reading from file"); } June 1510

11. 18 File handling Using output files must import java.io.* create a PrintWriter object to write to a file need the name of the file to write to file will be in same folder as Java program if file already exists contents will be overwritten if file doesn't exist, new one is created checked exception must be handled can have any number of PrintWriter objects June 1511

12. 18 File handling Writing to file PrintWriter object methods are used to write data to file: print() println() printf() June 1512

13. Using output files example - OddsEvens program to read in positive integers entered at the keyboard until -1 is entered output all even numbers to a file called even.txt output all odd numbers to a file called odd.txt assume that all data is valid 18 File handling13June 15

14. OddsEvens analysis what data is used? num: integer input by user what operations are performed? iteration to process all numbers input what operations are performed once before the loop? create PrintWriter and link it to even.txt create PrintWriter and link it to odd.txt read num how many times is the loop performed? loop performed until -1 entered 18 File handling14June 15

15. OddsEvens analysis continued what operations are repeated inside the loop? output num to even.txt if even output num to odd.txt if odd read num what operations are performed once after the loop close even.txt and odd.txt what exceptions may occur error writing to file 18 File handling15June 15

16. 18 File handling //input numbers and output odd ones to odd.txt, even ones to even.txt import java.util.*; import java.io.*; public class OddsEvens { public static void main (String [] args) { try { //create PrintWriters linked to output files Scanner kybd = new Scanner(System.in); PrintWriter even = new PrintWriter("even.txt"); PrintWriter odd = new PrintWriter("odd.txt"); int num = kybd.nextInt(); //read until -1 entered while (num != -1) { June 1516 continued on next slide OddsEvens.java

17. 18 File handling if (num % 2 != 0) { odd.println(num); } else { even.println(num); } num = kybd.nextInt(); } //close files odd.close(); even.close(); } catch(IOException e) { System.out.println("Error writing to file"); } June 1517

18. 18 File handling Handling exceptions opening, reading, writing and closing files can generate a checked exception if these are not handled, the compiler generates an error June 1518

19. 18 File handling19June 15 Summary In this session we have: opened and closed files read data from a file written results to a file