CS100J       Lab 08. Reading files    Spring 2008 

Name ___________________       Section time _____________        Section instructor _________________

This lab discusses input —reading a file. After the lab, study section 5.9 of the text —better yet, listen to the lectures on lesson page 5-7 of the ProgramLive CD. The lectures are much clearer than the paper version. Start this lab by downloading files Lab08.java into a directory, opening them in DrJava, and compiling. You will also need this text file: test.txt.

Streams

A "stream" is a sequence of data values that is processed --either read or written-- from beginning to end. When the data is being read, or input, the stream is called an "input stream"; when it is being written, or output, the stream is called an "output stream". Input/output of streams is done in Java using classes in package java.io.*;

The basic way to create an input stream for a file is by creating an instance of class FileReader:

FileReader fr= new FileReader(an arg that describes which file to read);

However, the standard way to read using FileReader fr is to read one character at a time, using function

fr.read()

This is too low-level for us. We would like to be able to read not one character but one line at a time. Java includes also a class BufferedReader to help us here. Instead of the above, use this:

FileReader fr= new FileReader(an arg that describes which file to read);
BufferedReader br= new BufferedReader(fr);

Now, execution of

String lin= br.readLine();

reads the next line of the file and stores it in variable lin ---if there are no more lines to read, null is stored in lin. We will see how to use this later.

Using a JFileChooser dialog box

In order to read a file, you have to indicate which file should be read. The easiest way to do this is to use a dialog window to navigate to the appropriate directory and select the file, using an instance of class JFileChooser, in package javax.swing. Execute the following in the interactions pane:

br= Lab09.getReader(null);

A dialog box opens. Its title is "Choose input file". And it allows you to navigate anywhere you want and then select a file. Do a bit of navigating and select a file. Then take a look at function getReader(p). Here's what it does:

  1. Declare local variable jd, of class JFileChooser.
  2. Store in jd a new instance of class JFileChooser. In the new expression new JFileChooser(), you have no control over the directory that appears in dialog window initially. In the new expression new JFileChooser(p), String p is supposed to be a path on your computer of the directory to open in the dialog window. This choice allows you to dispense with a lot of navigating.
  3. Set the title of jd to "Choose input file".
  4. Execute jd.showOpenDialog(null);. This causes the dialog window to open on your monitor, and the program pauses until you have closed it. Nothing happens until you have chosen a file (or canceled the interaction).
  5. Create a new FileReader, with a file that you selected (its name is jd.getSelectedFile()) as the argument, and store its name in fr.
  6. Create and return a BufferedReader that is attached to fr.

The function for obtaining the next line from a BufferedReader br is:

br.readLine()      // = the next line of BufferedReader br ---null if there are no more lines

In the interactions pane, you can continue to evaluate br.readLine(). Each time you do, the next line of the file you selected is printed. Try it.

Processing the lines of a file

Function lines in class Lab08 illustrates the basic way of processing the lines of a file given by a BufferedReader. In the interactions pane, put a call on this method and let it read some file --you will see how many lines the file has in it.

Study this method. Any loop that you write that processes a file should be similar to this one --the "processing" of each line will change, but the basic structure of the loop that does the processing will not. Note that this method contains a while-loop, which will be introduced on Tuesday, 11 March. Here is how a while-loop

    while ( <condition> ) <repetend>

is executed (where <condition> is a boolean expression and <repetend> is any statement:

1. If the <condition> evaluates to false, stop execution of the while-loop.
2. Execute the <repetend> and begin again at step 1.

Here are important points:

  1. The first line is read before the while loop (and stored in variable lin). lin will be null if the file is empty.
  2. The loop stops when lin is null, indicating that there are no more lines in the file.
  3. The repetend first processes the line given by variable lin and then reads the next line into lin.

The header of the method contains a new construct: throws IOException. It is needed because function br.readLine() might create some sort of I/O (input/output) error, and this is how we handle it. We will explain this later in the course.

Write your own method

Write a procedure that prompts the caller for an input file and then reads the file, printing every line that contains a '*'. Use the statement

System.out.println(lin);

to print line lin. Test the function on the text file test.txt, which you obtained at the beginning of the lab. When you are finished, show your procedure to your TA.

Writing files

Writing files is not much different from reading them. Look at Sec 5.10 on page 207 of the text --and the accompanying material in ProgramLive-- for information.