University of Washington, CSE 142

Lab 6: File Processing

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Basic lab instructions

• Mouse over highlighted words if you're not sure what they mean!
• Talk to your classmates for help.
• You may want to bring your textbook to future labs to look up syntax and examples.
• Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
• Complete as much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
• Feel free to complete problems in any order.
• Make sure you've signed in on the sign-in sheet before you leave!

Today's lab

Goals for today:

• use Scanner and File objects to read input data from files
• understand the way a Scanner breaks input into tokens
• explore token-based and line-based input
• mouse over boxed words for a definition if you're not sure what a word means

Files

So far, we've used Scanners to read user input. Now, we're going to look at using Scanners to read over files!

A File object allows you to interact with actual files on your computer. Make sure to put any file you want to read with a Scanner in the same folder as the program that wants to use it!

    // necessary to use Files
    import java.io.*;

    // makes a File object, holding the same info as the file named "actualNameOfFile"
    File fileVariableName = new File("actualNameOfFile");

    // a Scanner that reads over the File!
    Scanner fileScanner = new Scanner(fileVariableName);

Scanner methods

Recap: Scanner method examples

    4.2 abc 4

New methods: Scanner method examples

    4.2 abc 4

Exercise : .hasNext() caution

.hasNext() methods only check the type of the next token. .next() methods consume tokens, changing what token the Scanner is looking at next! For each of the following, enter the output if the loop terminates, or write infinite if the loop loops forever!

Consider the following as part of input.txt

Jello world :)

Scanner input = new Scanner(new File("input.txt"));
while (input.hasNext()) {
   String nextWord = input.next();
   System.out.print(nextWord + " ");
}

Jello world :) 

Scanner input = new Scanner(new File("input.txt"));
while (input.hasNext()) {
   System.out.println("hi");
}

infinite

File Exceptions

When you work with Files, the Java compiler gets concerned that you might be trying to access a File that doesn't exist. So any method that uses Files must declare that it might throw a FileNotFoundException: this basically tells Java that if the desired "actualNameOfFile" cannot be found, it's okay to crash.

Every method that directly or indirectly calls a method that works with Files also needs to declare that it might throw a FileNotFoundException.

• Directly: method A() calls methodB(), and methodB() uses a File. Then A() and B() must declare that they might throw a FileNotFoundException.
• Indirectly: method A() calls method B(), and method B() calls method C(). Method C() uses a File. Then A() and B() and C() must all declare that they might throw a FileNotFoundException.

  public static void main(String[] args) throws FileNotFoundException {
                           ...
  }

.next() vs .nextLine()

• .next() reads and returns the next token as a String. Tokens are separated by white space.
• .nextLine() reads and returns the next line as a String. A line is all the characters up to the next \n (line break).
• In this class, a single Scanner should only ever read its input using either .next() (mixing with .nextDouble() and .nextInt() and .nextBoolean() is fine), or using .nextLine().

String Scanners

Consider the following problem:

We already know how to read full lines or individual words from a Scanner. Now we need a new strategy: we need to read lines from the input, and then we need to somehow read word-by-word through each line.

We can do this by creating a second Scanner! So far, we've used Scanners to read user input and to read Files. Scanners can also read over Strings!

    Scanner fileScannerName = new Scanner(new File("fileName"))
    while (fileScannerName.hasNextLine()) {
       String line = fileScannerName.nextLine(); // reads the next line from the input file
       Scanner lineScanner = new Scanner(line);
       while (lineScanner.hasNext()) {
          String word = lineScanner.next(); // reads the next word from the input line
          ...
       }
    }

Checkpoint: Congratulations!

Nice job making it this far--labs are tough! Feel free to work with the person next to you for the remaining slides. Labs are a unique opportunity (unlike homework) to collaborate directly on ideas, and practice peer programming.

These next problems get a little more challenging as we explore earlier concepts further.

We put a lot of problems in here so that you have plenty to refer back to later when working on homework. Don't feel bad if you don't finish all of them--Brett can't finish them all in a 50 minute lab, either! :)

Forest the cat says good job!

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!