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050Commenting out Your Code?

Author: Dr. Heinz M. KabutzDate: 2002-06-10Java Version: 1.1Category: Language

Abstract: Source control systems give us insight into how a method has changed over time. It is usually a bad idea to comment out code, especially without adequate comments why. It confuses the person maintaining the code.


Welcome to the 50th edition of The Java(tm) Specialists' Newsletter. Last week I thought that I was presenting my Design Patterns Course at "The Shuttleworth Foundation". I really thought I was. However, when I arrived at TSF, I discovered that I had gone to the wrong place :-( My booking had been with Thawte Consulting, the company that Mark Shuttleworth sold to Verisign, and TSF has nothing to do with Thawte. I had simply assumed, and assumption is one of the biggest failings of a programmer.

When I finally got there, I was amazed that all my students were there already, waiting for me. (The course was supposed to start at 8:30 and I got there at 8:20). The next day, when I arrived at 8:20 again (after missing my turnoff due to daydreaming), the students were *again* all there, waiting for me. WOW, I thought, these guys are *really* keen! The bubble burst when they told me that somehow they got the message that the course started at 8:00 instead of 8:30!

The course went extremely well, partly due to very bright students and partly because I am getting more comfortable presenting this new course. Last Monday at 2:00am I at long last *clicked* with the GoF Factory Method. If you think you understand the Factory Method (like I did for a few years), my bet is that you actually don't ;-) You might understand a different type of pattern commonly known as "Factory Method", but there are very few people who understand the Factory Method according to GoF. Please visit our new self-study course catalog to see how you can upskill your Java knowledge.

Commenting out Your Code?

Something I dislike even more than useless comments is code that is commented out. When I work with code, instead of commenting it out, I delete it. That way I don't have to remember why I commented out broken code, and it makes my code much easier to maintain. It's like telling the truth: you don't have to remember what you said. However, this newsletter is not a rant about commented out code, it is about how even commented out code can cause compiler failures, and how commented out code can end up as part of your class.

I want to thank Clark Updike from for the ideas that sparked this newsletter. He picked it up while studying for the Sun Certified Java Programmer (SCJP) Examination, which I did not author, by the way. I know the guys at Sun can be quite evil with the SCJP, but I don't think they would go this far. The SCJP seems to be getting more difficult to keep pace with the immense number of Java developers arriving on the scene. When I wrote it, you basically had to show up to get 91%.

I promised you last week that I would show you some code that could not compile, even though the problem code was commented out. Let's first look at the original program:

public class A1 {
  Character aChar = new Character('\u000d');

Try compile it, and you will get an error, such as: illegal line end in character literal
  Character aChar = new Character('\u000d');
1 error

Imagine sitting with this code, and not getting it to compile. What would you do? You would probably comment out the offending line to try get it to compile:

public class A2 {
  // Character aChar = new Character('\u000d');

You compile it, and what do you get: 2 errors instead of the 1 that you had without the comment! unclosed character literal
^ <identifier> expected
2 errors

But we can get even more confusing, have a look at class A3:

public class A3 {
  // Character aChar = new Character('\u000d{System.out.println("Hello");}

Amazingly, it compiles, but does it do anything? Let's have a look at

public class A3Test {
  public static void main(String[] args) {
    new A3();

The test compiles, and when we run it, the output is:


Say what? I'm sure that you know what happens. The unicode character '\u000d' gets converted to a newline character as part of the "preparation for compile". At some point, before the class A2 actually gets compiled, it looks like this:

public class A2 {
  // Character aChar = new Character('

Obviously that does not compile! Similarly, at some point, A3 looks like this:

public class A3 {
  // Character aChar = new Character('

I wish I could say: There, you should not comment out code! However, I can only say that you will get strange effects if you use '\u000a' or '\u000d' in your code.

Until the next newsletter ...




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