CS231 lecture notes, Fall 1998
Week 7, Friday

Quiz on Java Object Handout

Review of wrapper classes, in the context of the bracket checker.

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.