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Building Java Programs

Chapter 6

Line-Based File Input

reading: 6.3 - 6.

Hours answer (flawed) // This solution does not work! import java.io.; // for File import java.util.; // for Scanner public class HoursWorked { public static void main(String[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("hours.txt")); while (input.hasNext()) { // process one person int id = input.nextInt(); String name = input.next(); double totalHours = 0.0; int days = 0; while ( input.hasNextDouble() ) { totalHours += input.nextDouble() ; days++; } System.out.println(name + " (ID#" + id + ") worked " + totalHours + " hours (" + (totalHours / days) + " hours/day)"); } } }

Flawed output Ross (ID#123) worked 488.3 hours ( 97.66 hours/day) Exception in thread "main" java.util.InputMismatchException at java.util.Scanner.throwFor(Scanner.java:840) at java.util.Scanner.next(Scanner.java:1461) at java.util.Scanner.nextInt(Scanner.java:2091) at HoursWorked.main(HoursBad.java:9) — The inner while loop is grabbing the next person's ID. — We want to process the tokens, but we also care about the line breaks (they mark the end of a person's data).

— A better solution is a hybrid approach:

— First, break the overall input into lines. — Then break each line into tokens.

Consuming lines of input 23 3.14 John Smith "Hello" world 45.2 19

— The Scanner reads the lines as follows:

23 \t3.14 John Smith\t"Hello" world\n\t\t45.2 19 \n ^ — String line = input.nextLine(); 23 \t3.14 John Smith\t"Hello" world \n\t\t45.2 19 \n ^ — String line2 = input.nextLine(); 23 \t3.14 John Smith\t"Hello" world\n \t\t45.2 19 \n ^ — Each \n character is consumed but not returned.

Scanners on Strings

— A Scanner can tokenize the contents of a String:

Scanner __ = new Scanner( __ ); — Example: String text = "15 3.2 hello 9 27.5"; Scanner scan = new Scanner(text) ; int num = scan.nextInt(); System.out.println(num); // 15 double num2 = scan.nextDouble(); System.out.println(num2); // 3. String word = scan.next(); System.out.println(word); // "hello"

Hours question

— Given a file hours.txt with the following contents:

123 Alex 12.5 8.2 7.6 4. 456 Alina 4.2 11.6 6.3 2.5 12. 789 Ryan 16.0 12.0 8.0 20.0 7. — Consider the task of computing hours worked by each person: Alex (ID#123) worked 32.3 hours (8.075 hours/day) Alina (ID#456) worked 36.6 hours (7.32 hours/day Ryan (ID#789) worked 63.5 hours (12.7 hours/day)

Hours answer, corrected // Processes an employee input file and outputs each employee's hours. import java.io.; // for File import java.util.; // for Scanner public class Hours { public static void main(String[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("hours.txt")); while (input.hasNextLine()) { String line = input.nextLine(); processEmployee(line); } } public static void processEmployee(String line) { Scanner lineScan = new Scanner(line); int id = lineScan.nextInt(); // e.g. 456 String name = lineScan.next(); // e.g. ”Erika" double sum = 0.0; int count = 0; while (lineScan.hasNextDouble()) { sum = sum + lineScan.nextDouble(); count++; } double average = sum / count; System.out.println(name + " (ID#" + id + ") worked " + sum + " hours (" + average + " hours/day)"); } }

Output to files

— PrintStream : An object in the java.io package that lets

you print output to a destination such as a file.

— Any methods you have used on System.out (such as print, println) will work on a PrintStream. — Syntax: PrintStream __ = new PrintStream(new File(" __ ")); Example: PrintStream output = new PrintStream(new File("out.txt")); output.println("Hello, file!"); output.println("This is a second line of output.");

Details about PrintStream PrintStream __ = new PrintStream(new File(" __ ")); — If the given file does not exist, it is created. — If the given file already exists, it is overwritten. — The output you print appears in a file, not on the console. You will have to open the file with an editor to see it. — Do not open the same file for both reading (Scanner) and writing (PrintStream) at the same time. — You will overwrite your input file with an empty file (0 bytes).

PrintStream question

— Modify our previous Hours program to use a PrintStream

to send its output to the file hours_out.txt.

— The program will produce no console output. — But the file hours_out.txt will be created with the text: Ross (ID#123) worked 32.3 hours (8.075 hours/day) Erika (ID#456) worked 36.6 hours (7.32 hours/day Alex (ID#789) worked 63.5 hours (12.7 hours/day)

PrintStream answer // Processes an employee input file and outputs each employee's hours. import java.io.; // for File import java.util.; // for Scanner public class Hours2 { public static void main(String[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("hours.txt")); PrintStream out = new PrintStream(new File("hours_out.txt")); while (input.hasNextLine()) { String line = input.nextLine(); processEmployee(out, line); } } public static void processEmployee( PrintStream out, String line) { Scanner lineScan = new Scanner(line); int id = lineScan.nextInt(); // e.g. 456 String name = lineScan.next(); // e.g. ”Erika" double sum = 0.0; int count = 0; while (lineScan.hasNextDouble()) { sum = sum + lineScan.nextDouble(); count++; } double average = sum / count; out.println (name + " (ID#" + id + ") worked " + sum + " hours (" + average + " hours/day)"); } }

File Scanner Question

— Write a program called Spammer.java that asks the user

for an email domain and searches a file called

address_book.txt. If an email with the domain name is

found, the user is prompted whether the contact should

be added to the "spam list" (User input in bold)

Email domain to spam? @gmail.com Would you like to spam [email protected]? yes

— The program should output the contacts that the user

selected to a file named spam_list.txt

Schmerlock Schmolmes