It's actually not that easy of a problem, but there is hope!  You can
probably find a Java source parser out there somewhere by using a little
website I call Google.  Check it out at Google.com.

Run the code through a C preprocessor.

/gordon

--

Why?

Have your class call javac.

You would do it with a little finite state machine, or a parser.

See http://mindprod.com/jgloss/finitestate.html
http://mindprod.com/jgloss/parser.html

You can see an example of such a parser as part of
http://mindprod.com/products1.html#JDISPLAY
see com.mindprod.jprep.JavaTokenizer

You can strip out all the code except that which deals with comments,
by collapsing other states (each implemented with an enum constant)
into one.

You could simply search for all // with indexOf and rip out till nl or
all /* and rip out till */

However make sure you handle // embedded in /* ... */
and /* embedded in //.

You have to scan for next /* or // whichever comes first, then process
that.

THen there is the simplest solution of all.  Google for "strip Java
comments" and see what comes up.

All you need is a lexer (lex) to pick up tokens--no parsing (yacc or bison) required.  I have a version in C and
lex for MS-DOS which was slapped together in 1990. You can find it at  
http://sourceforge.net/projects/cshroud .  Comments are naturally disposed of since that is half the job of
shrouding.

This is harder than you think.  Use a real parser.

Here are some more test cases for you:
 public class Comment {
   public static void main(String[] args) {
       String note = "// 1 "; // this is a comment
       System.out.println(note);

       /* // comment */ note = "2";
       System.out.println(note);

       char ch = '"'; // code = "3 if broken"
       System.out.println(note);

       note=\u0022 // 4";
       System.out.println(note);
   }
 }

The output should be
// 1
2
2
// 4
--
Mark Rafn    dagon@dagon.net    <http://www.dagon.net/>

Assuming the use of correct Java sources as an input, the code below
should do the trick.  (Warning: not tested intensively!)

Note that it tries to preserve as much of the original code as possible.
 That is, the line numbers, positions, and escape sequences of the code
in output should be the same as in input (that may help in debugging).

piotr

import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.io.Reader;
import java.util.ArrayDeque;
import java.util.Deque;

public class CommentStripper {

  public static void main(String[] args) throws Exception {
    InputStream in = new BufferedInputStream(
        new FileInputStream("CommentStripper.java"));
    Reader source = new InputStreamReader(in);
    PrintWriter out = new PrintWriter(System.out, true);
    stripComments(source, out);
  }

  public static void stripComments(
      Reader source, PrintWriter out) throws IOException {
    SourceReader reader = new SourceReader(source);

    StringBuilder outbf = new StringBuilder();
    boolean inComment = false;
    for(Char next; (next = reader.next()) != Char.EOF;) {

      int commentCharsInLine = 0;
      for(Char sc; !(sc = next).isEOL();) {
        next = reader.next();

        if (inComment) {
          if (sc.codePoint == '*' && next.codePoint == '/') {
            // end of comment

            // read next
            next = reader.next();

            if (!next.isEOL()) {
              // write out spaces
              int ix = outbf.length();
              outbf.setLength(ix + commentCharsInLine + 2);
              for(final int len = outbf.length(); ix < len; ++ix) {
                outbf.setCharAt(ix, ' ');
              }
            }

            commentCharsInLine = 0;
            inComment = false;
          } else {
            commentCharsInLine++;
          }

        } else if (sc.codePoint == '/' && next.codePoint == '*') {
          // start of multiline comment
          inComment = true;
          commentCharsInLine = 2;

          // read next
          next = reader.next();

        } else if (sc.codePoint == '/' && next.codePoint == '/') {
          // single line comment

          // skip to the end of line
          while(!next.isEOL()) {
            next = reader.next();
          }

        } else if (sc.codePoint == '"' || sc.codePoint == '\'' ) {
          // text literal...

          sc.appendSource(outbf);

          // lookup end of literal (should be in the same line)
          boolean literalEndFound = false;
          for(; !next.isEOL(); next = reader.next()) {
            next.appendSource(outbf);
            if (next.codePoint == '\\') {
              // read & write next
              next = reader.next();
              if (!next.isEOL()) {
                next.appendSource(outbf);
              }
              continue;
            }
            if (literalEndFound = next.codePoint == sc.codePoint) {
              // read next
              next = reader.next();
              break;
            }
          }
          if (!literalEndFound) {
            // syntax error in input...
            throw new IOException("End of text literal not found");
          }

        } else {
          // write out source "as is"
          sc.appendSource(outbf);
        }
      }

      // flush buffered line
      String outLine = outbf.toString();
      if (outLine.trim().length() == 0) {
        out.println();
      } else {
        out.println(outLine);
      }

      outbf.setLength(0);
    }
  }

  private static abstract class Char {
    final int codePoint;

    Char(int codePoint) {
      this.codePoint = codePoint;
    }

    boolean isEOL() {
      return codePoint == '\n';
    }

    abstract void appendSource(StringBuilder sb);

    static final Char EOF = new Char(-1) {

      @Override
      public void appendSource(StringBuilder sb) {
        // write nothing
      }

      @Override
      boolean isEOL() {
        return true;
      }
    };

    static Char newInstance(final InputChar c) {
      return new Char(c.value) {

        @Override
        void appendSource(StringBuilder sb) {
          c.appendSource(sb);
        }
      };
    }

    static Char newInstance(int codePoint, final InputChar c) {
      return new Char(codePoint) {

        @Override
        void appendSource(StringBuilder sb) {
          c.appendSource(sb);
        }
      };
    }

    static Char newInstance(int codePoint, final InputChar... chars) {
      return new Char(codePoint) {

        @Override
        void appendSource(StringBuilder sb) {
          for(InputChar c : chars) {
            c.appendSource(sb);
          }
        }
      };
    }

    @Override
    public String toString() {
      StringBuilder sb = new StringBuilder();
      appendSource(sb);
      return "[" + codePoint + "]=" + sb.toString();
    }

  }

  private static abstract class InputChar {
    final int value;

    static final InputChar EOF = new InputChar(-1) {

      @Override
      void appendSource(StringBuilder sb) {
        // write nothing
      };
    };

    InputChar(int value) {
      this.value = value;
    }

    abstract void appendSource(StringBuilder sb);

    static InputChar newCharInstance(int value) {
      return new InputChar(value) {

        @Override
        void appendSource(StringBuilder sb) {
          sb.append((char)value);
        }
      };
    }

    static InputChar newEscapeSequenceInstance(int value, final
CharSequence seq) {
      return new InputChar(value) {

        @Override
        void appendSource(StringBuilder sb) {
          sb.append(seq);
        }
      };
    }

  }

  private static class SourceReader {
    private Reader in;

    SourceReader(Reader in) {
      this.in = in;
    }

    private Deque<InputChar> inputChars = new ArrayDeque<InputChar>();

    Char next() throws IOException {
      InputChar nc = nextInputChar();
      if (nc == InputChar.EOF) {
        return Char.EOF;
      }

      InputChar fc = nextInputChar();

      if (nc.value == '\r' && fc.value == '\n') {
        return Char.newInstance('\n', nc, fc);
      }
      if (nc.value == '\r' || nc.value == '\n') {
        unread(fc);
        return Char.newInstance('\n', nc);
      }

      if (Character.isSurrogatePair((char)nc.value, (char)fc.value)) {
        return Char.newInstance(
            Character.toCodePoint((char)nc.value, (char)fc.value), nc, fc);
      }

      unread(fc);
      return Char.newInstance(nc);
    }

    private void unread(InputChar c) {
      if (inputChars == null) {
        if (c != InputChar.EOF) {
          inputChars = new ArrayDeque<InputChar>();
        } else {
          return;
        }
      }
      inputChars.addFirst(c);
    }

    private InputChar nextInputChar() throws IOException {
      if (inputChars == null) {
        return InputChar.EOF;
      }
      if (!inputChars.isEmpty()) {
        return inputChars.removeFirst();
      }

      int r0 = in.read();
      if (r0 == -1) {
        inputChars = null;
        return InputChar.EOF;
      }
      if (r0 == '\\') {
        int r1 = in.read();
        if (r1 == '\\') {
          // double backslash, read each separately
          inputChars.add(InputChar.newCharInstance(r0));
          return inputChars.peek();
        }
        if (r1 == 'u') {
          // escape sequence
          StringBuilder seqbf = new StringBuilder();
          // collect all 'u's
          seqbf.append((char)r0);
          do {
            seqbf.append((char)r1);
            r1 = in.read();
          } while(r1 == 'u');
          // parse escape sequence value
          parseSeq: if (r1 != -1) {
            seqbf.append((char)r1);
            for(int i = 3; i > 0; --i) {
              r1 = in.read();
              if (r1 == -1) break parseSeq;
              seqbf.append((char)r1);
            }
            if (r1 != -1) {
              int val = Integer.parseInt(
                  seqbf.substring(seqbf.length() - 4), 16);
              return InputChar.newEscapeSequenceInstance(val, seqbf);
            }
          }
          // incorrect escape sequence...
          throw new IOException("Incorrect escape sequence: '" + seqbf
+ "'");
        }
        // unknown...
        inputChars.add(InputChar.newCharInstance(r1));
      }
      return InputChar.newCharInstance(r0);
    }

    void close() throws IOException {
      if (in != null) {
        in.close();
      }
      in = null;
      inputChars = null;
    }
  }

}