CS231 lecture notes, Fall 1999
Week 7, Friday

Development plan for Checker.java (complete with errors and how
I found them)

Step One
--------

  public static void parseChar (char c) {
    switch (c) {
    case '{': case '(': case '[':
      System.out.print (c);
      break;
    default:
    }
  }

  public static void parseLine (String line) {
    for (int i = 0; i<line.length(); i++) {
      parseChar (line.charAt(i));
    }
  }

Step Two
--------

  public static void parseChar (char c) {
    switch (c) {
    case '{': case '(': case '[':
      System.out.print (c);
      break;
    case '}': case ')': case ']':
      System.out.print (c);
      break;
    default:
    }
  }

Step Three
----------

  public static Stack stack = new Stack ();

  public static void parseChar (char c) {
    switch (c) {
    case '{': case '(': case '[':
      stack.push ((Object)(new Character (c)));
      break;
    case '}': case ')': case ']':
      System.out.print (c);
      break;
    default:
    }
  }

Oops.  Forgot to import java.util.Stack

Step Four
---------

  public static void checkCloser (char c) {
    if (stack.empty()) {
      System.out.println ("Unbalanced");
      return;
    }
    char d = ((Character) stack.pop()).charValue();
    System.out.print (d);
  }

  public static void parseChar (char c) {
    switch (c) {
    case '{': case '(': case '[':
      stack.push ((Object)(new Character (c)));
      break;
    case '}': case ')': case ']':
      checkCloser (c);
      break;
    default:
    }
  }

Step Five
---------

  public static void match (char c, char d) {
    switch (c) {
    case '{': return (d == '}');
    case '(': return (d == ')');
    case '[': return (d == ']');
    default:
      System.out.println ("Something insane has happened.");
    }
  }    

Oops.  Should be returning boolean, not void.

Oops.  Need a return at the end.

I got the following...

{Something insane has happened.
(Something insane has happened.
(Something insane has happened.
...

Oops.  Should have the closers in the switch and the openers in
the comparison!

    Lesson: should have used better names for the variables!

Step Six
--------

  public static boolean match (char opener, char closer) {
    switch (closer) {
    case '}': return (opener == '{');
    case ')': return (opener == '(');
    case ']': return (opener == '[');
    default:
      System.out.println ("Something insane has happened.");
    }
    return true;
  }    

  public static void checkCloser (char closer) {
    if (stack.empty()) {
      System.out.println ("Unbalanced");
      return;
    }
    char opener = ((Character) stack.pop()).charValue();
    if (!match (opener, closer)) {
      System.out.println ("Unbalanced");
    }
  }

Step Seven
----------

At the end of checkFile, I added

    if (stack.empty()) {
      System.out.println ("All is well.");
    } else {
      System.out.println ("Unbalanced.");
    }

Step Eight
----------

So far I have only been testing this on correct programs.
Now I need to make sure it catches errors correctly.  I
created a new file called Test.java, and tested three kinds
of errors:

1) a closer without an opener

Unbalanced
All is well.

Oops!  Once we see an error, we should exit.

      System.exit (1);

A non-zero exit value usually indicates an error.

2) a leftover opener on the stack at the end of the program
3) a closer that does not match an opener


My test sequence looked like

)
()
([])
([])(
([])()
([])([)]

Each one followed by the correct program.  Each test succeeded.


Step Nine
---------

I tested the program by giving it itself, and got the following:

  public boolean match (char opener, char closer) {
    switch (closer) {
    case '}':
      return (opener == '{');
Unbalanced (wrong closer)

Oops!  Apparently it doesn't realize that that open squiggly
is in quotes!

We need to teach this program Java's rules for when things
are in quotes or not.  Ugh!