Class Notes - October 26

Class Notes - October 28

First off, we figured out what was wrong with the FileWriter. When using the FileWriter, you have to close the PrintWriter using pw.close(). We'll discuss this below...

Today we will focus on the StringTokenizer. This is an essential Java tool you will use frequently in your future programming. If you continue on to ICS 211 and 311, you will most likely get some sort of assignment (after our Project 2) that will require you to use this.

Topics for today:

We Fixed It! - FileReader and FileWriter
The String Tokenizer
Project 2

Alternative File Reader / Writer: FileReader and FileWriter

Code Example: NewFile.java (fixed)

Last Thursday's Way: New way using File Reader / File Writer:

Using FileInputStream:

File f = new File("file.txt");
FileInputStream fis = new FileInputStream(f);
InputStreamReader isr = new InputStreamReader(fis);
BufferedReader br = new BufferedReader(isr);

br.readLine will read a line of the file

FileReader:

File f = new File("file.txt");
FileReader fr = new FileReader(f);
BufferedReader br = new BufferedReader(fr);

br.readLine will read a line from the user

Using FileOutputStream:

File fout = new File ("outputFile.txt");
FileOutputStream fos = new FileOutputStream (fout);
PrintStream ps = new PrintStream(fos);

ps.println() will write a line of output to the file

FileWriter:

File fout = new File ("outputFile.txt");
FileWriter fw = new FileWriter(fout);
BufferedWriter bw = new BufferedWriter(fw);
PrintWriter pw = new PrintWriter(bw);

pw.println() will write a line of output to the file

// at the end of your program, do this to write the file
// this is the step that we forgot to do earlier...
pw.close(); // to close the PrintWriter and write the file

The String Tokenizer:

What is it?

It is a way to read a String, and break it up into pieces.

Why would we want to use it?

Say we want to store data for a program. We would probably make a data file. In that file, we can store data for different sets of data. For instance, for project 2, we would probably have a taxpayer per line in our data file. Therefore, each individual will have all of their data on one line of the file. We can separate the data by using a special character, such as a colon (":") that hopefully the user will not type as part of their input.

For example, in project 2, we need to store the taxpayer's name, income, and dependants. Our data file might look like this:

Frank:15000:0
Joe Schmoe:54000:2
Julie:12000:1
Toni Schmoni:36000:4

In this case, data is in the form of (Name : Income : # Dependants). Using the String Tokenizer, we can separate on ":" to pull the pieces apart and get the data back out.
Note: If the user was to type in "Mary Ja:ne" for their name, this would mess up the String Tokenizer if we separate based on a ":"

How it works:

import java.util.StringTokenizer; // (goes at the top of the program)
StringTokenizer st = new StringTokenizer(String sToTokenize, String delimiter); // (goes in method that uses the string tokenizer)
- String sToTokenize is the String that we want to tokenize
- String delimiter is a String containing the character that we want to split up the String sToTokenize
StringTokenizer st = new StringTokenizer(String sToTokenize); // (goes in method that uses the string tokenizer)
- Works the same as above- but the difference is that we don't specify the delimiter. If we don't specify the delimiter, the StringTokenizer will separate the String sToTokenize on spaces (" ")
A typical implementation after this is the following:

while (st.hasMoreTokens()) {
System.out.println(st.nextToken());
System.out.println("Number of Tokens Left: " + st.countTokens());
}
- In the above example, st.nextToken() gets us the next token out of the String
- In the above example, st.countTokens() counts the number of tokens remaining
- In the above example, st.hasMoreTokens() returns a boolean (true or false) depending on whether there are tokens remaining

String Tokenizer Methods:

As shown above, the most common methods used on a String Tokenizer are:

nextToken() - returns us a String, gets us the next token out of the String
countTokens() - returns us an int, counts the number of tokens remaining
hasMoreTokens() - returns a boolean (true or false) depending on whether there are tokens remaining

Code Examples:

StringTok1.java (simple String Tokenizer, separate w/ spaces)

StringTok2.java (simple String Tokenizer, separate w/ colon)

StringTok3.java (using a String Tokenizer to read tokens from lines of a file) using input.txt

Discussion of Project 2:

http://www2.hawaii.edu/~tp_200/ics111/fall2004/project2.html

- You'll use the StringTokenizer and the file readers / writers, plus lots of concepts from earlier in the class