Reworking some existing code

To solve the administrator’s problem, you need to modify the code in Listing 12-5. The new version gets an email address from a disk file and writes a username to another disk file. The new version is in Listing 14-1.

LISTING 14-1 From One File to Another

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;

class ListOneUsername {

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

Scanner diskScanner = new Scanner(new File("email.txt"));
PrintStream diskWriter = new PrintStream("usernames.txt");
char symbol;

symbol = diskScanner.findWithinHorizon(".",0).charAt(0);

while (symbol != ’@’) {
diskWriter.print(symbol);
symbol = diskScanner.findWithinHorizon(".",0).charAt(0);
}

diskWriter.println();

diskScanner.close();
diskWriter.close();
}
}

Listing 14-1 does almost the same thing as its forerunner in Listing 12-5. The only difference is that the code in Listing 14-1 doesn’t interact with the user. Instead, the code in Listing 14-1 interacts with disk files.

Running your code

Here’s how you run the code in Listing 14-1:

1. Create a file named email.txt in your Eclipse project directory.

   In the email.txt file, put just one email address. Any address will do, as long as the address contains an @ sign.

2. Put the ListOneUsername.java file (the code from Listing 14-1) in your project’s src/(default package) directory.

3. Run the code in Listing 14-1.

   When you run the code, you see nothing interesting in the Console view. What a pity!

4. View the contents of the usernames.txt file.

   If your email.txt file contains [email protected], the usernames.txt file contains John.

For more details on any of these steps, see the discussion accompanying Listings 13-2, 13-3, and 13-4, over in Chapter 13. (The discussion is especially useful if you don’t know how to view the usernames.txt file’s contents.)

Checking for the end of a file

Now all you need is a way to test the loop’s condition. How do you know when you’re at the end of the email.txt file?

The answer comes from Java’s Scanner class. This class’s hasNext method answers true or false to the following question:

Does the email.txt file have anything to read in it (beyond what you’ve already read)?

If the program’s findWithinHorizon calls haven’t gobbled up all the characters in the email.txt file, the value of diskScanner.hasNext() is true. So, to keep looping while you’re not at the end of the email.txt file, you do the following:

while (diskScanner.hasNext()) {
Execute the statements in Listing 14-1
}

The first realization of this strategy is in Listing 14-2.

LISTING 14-2 The Mechanical Combining of Two Loops

/*
* This code does NOT work (but you learn from your mistakes).
*/

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;

class ListAllUsernames {

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

Scanner diskScanner = new Scanner(new File("email.txt"));
PrintStream diskWriter = new PrintStream("usernames.txt");
char symbol;

while (diskScanner.hasNext()) {
symbol = diskScanner.findWithinHorizon(".",0).charAt(0);

while (symbol != ’@’) {
diskWriter.print(symbol);
symbol = diskScanner.findWithinHorizon(".",0).charAt(0);
}

diskWriter.println();
}

diskScanner.close();
diskWriter.close();
}
}

When you run the code in Listing 14-2, you get the disappointing response shown in Figure 14-3.

How it feels to be a computer

What’s wrong with the code in Listing 14-2? To find out, I role-play the computer. “If I were a computer, what would I do when I execute the code in Listing 14-2?”

The first several things that I’d do are pictured in Figure 14-4. I would read the J in John, and then write the J in John, and then read the letter o (also in John).

After a few trips through the inner loop, I’d get the @ sign in [email protected], as shown in Figure 14-5.

Finding this @ sign would jump me out of the inner loop and back to the top of the outer loop, as shown in Figure 14-6.

I’d get the B in BurdBrain and sail back into the inner loop. But then (horror of horrors!) I’d write that B to the usernames.txt file. (See Figure 14-7.)

There’s the error! You don’t want to write host names to the usernames.txt file. When the computer found the @ sign, it should have skipped past the rest of John’s email address.

At this point, you have a choice. You can jump straight to the corrected code in Listing 14-3 (a couple of sections from here), or you can read on to find out about the error message in Figure 14-3.

Why the computer accidentally pushes past the end of the file

Ah! You’re wondering why Figure 14-3 has that nasty error message.

I role-play the computer to help me figure out what’s going wrong. Imagine that I’ve already role-played the steps in Figure 14-7. I shouldn’t process the first letter B (let alone the entire BurdBrain.com host name) with the inner loop. But unfortunately, I do.

I keep running and processing more email addresses. When I get to the end of the last email address, I grab the m in BurdBrain.com and go back to test for an @ sign, as shown in Figure 14-8.

Now I’m in trouble. This last m certainly isn’t an @ sign. So I jump into the inner loop and try to get yet another character. (See Figure 14-9.) The email.txt file has no more characters, so Java sends an error message to the computer screen. (Refer to the NullPointerException error message in Figure 14-3.)

Here’s why I get a NullPointerException: The email.txt file has no more characters, so the call to findWithinHorizon(".",0) comes up empty. (There’s nothing to find.) In Java, a more precise way of describing that emptiness is with the word null. The call findWithinHorizon(".",0) is null, so pointing to a character that was found (charAt(0)) is a fruitless endeavor. Thus, Java displays a NullPointerException message.

Solving the problem

Listing 14-3 has the solution to the problem described with Figures 14-1 and 14-2. The code in this listing is almost identical to the code in Listing 14-2. The only difference is the added call to nextLine. When the computer reaches an @ sign, this nextLine call swallows the rest of the input line without actually tasting it. (The nextLine call gets the rest of the email address, but doesn’t output that part of the email address. The idea works because each email address is on its own separate line.) After gulping down @BurdBrain.com, the computer moves gracefully to the next line of input.

LISTING 14-3 That’s Much Better!

/*
* This code is correct!!
*/

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;

class ListAllUsernames {

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

Scanner diskScanner = new Scanner(new File("email.txt"));
PrintStream diskWriter = new PrintStream("usernames.txt");
char symbol;

while (diskScanner.hasNext()) {
symbol = diskScanner.findWithinHorizon(".",0).charAt(0);

while (symbol != ’@’) {
diskWriter.print(symbol);
symbol = diskScanner.findWithinHorizon(".",0).charAt(0);
}

diskScanner.nextLine();
diskWriter.println();
}

diskScanner.close();
diskWriter.close();
}
}

To run the code in Listing 14-3, you need an email.txt file — a file like the one shown earlier, in Figure 14-1. In the email.txt file, type several email addresses. Any addresses will do, as long as each address contains an @ sign and each address is on its own separate line. Save the email.txt file in your project directory along with the ListAllUsernames.java file (the code from Listing 14-3). For more details, see the discussion accompanying Listings 13-2, 13-3, and 13-4 in Chapter 13.

With Listing 14-3, you’ve reached an important milestone. You’ve analyzed a delicate programming problem and found a complete, working solution. The tools you used included thinking about strategies and role-playing the computer. As time goes on, you can use these tools to solve bigger and better problems.

Nothing is more challenging for novice programmers than creating nested loops. That’s why I cover looping techniques in this chapter, and in the next two chapters. You might say that I loop through my coverage of programming loops.

Try writing the code that I suggest in the next few paragraphs. Don’t be afraid to make lots of mistakes. If you get stuck, slow down, take a step back, and think about what the computer will do when it follows instructions to the letter.

The solutions are on my web page at www.allmycode.com/BeginProg. But don’t jump to the solutions until you’ve experimented with lots of different ideas. Follow this tried-and-true formula:

Write some code;
Run your code;
while (your program doesn’t work correctly) {
Step through your code, one statement after another, keeping track
of the values of the variables and the computer’s output as
Java follows your instructions exactly as they’re written;
In the step by step execution of statements, notice the place where
Java does something that you don’t want it to do;
Ask yourself how you’d change the statements so that Java would do
what you want it to do;
Change the statements in your code;
Run your code again;
}