Java 天地·Core Java·Lecture Notes·Unitt 3 - Core Java Programming Concepts

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Comments

A comment is a sequence of characters ignored by the Java compiler and interpreter. In Java, comments have one of these three forms:

// The rest of this line is a comment
/* Everything in this
section is a comment */
/** Everything in this section
is a documentation comment */

The computer never gets to process anything marked as a comment, so comments can say anything you like. Different authorities have different ideas about the amount of comments to use, but you should use some ?rather than none. Comments are what will make it possible for you to understand your program if you look at it again after you thought you had finished it. They will also make it easier for another person (e.g., one of your co-workers) to understand your program.

The use of extensive comments in computer programs is becoming less common in modern practice, although some authorities still recommend it. The number of comments in Hello1.java is excessive, and one would not normally comment a program to this extent except for educational purposes. The reason for this is that while comments can give human readers a good idea of what different parts of a program will do when executed, what is hard to understand is the overall structure of the program. This is where techniques like object-oriented design can help, as they document the large-scale structure of a program.

The difference between a standard comment (//, /*) and a `documentation comment' (/**) is that the latter is considered to be part of the overall explanation of how a program works. Software exists that will extract the documentation comments and make them into a report. The distinction is not that important in the context of this course, but anyone who is interested in programming professionally should consider finding out about the standard documentation generation tooljavadoc (which is part of the JDK package).

Consider the following program:

import java.io.*;
class commentme
{
public static void main( String args[] )
    {
    // display some text on screen
    System.out.println("line 1");
    System.out.println("line 2");
    System.out.println("line 3");
    System.out.println("line 4");
    }
}

At present there is a single comment in this class:

// display some text on screen

the 4 lines following this comment will display 4 lines of text.

line 1
line 2
line 3
line 4

Press any key to continue ...

One common technique when debugging a program is to comment out lines, to reduce the number of statements that are compiled and executed. Thus one can isolate parts of a program to help locate errors. By the phrase 'comment out' it is meant that program statements are edited with comments so that they are ignored by the compiler and interpreter. For example, we could comment out the first output statement as follows:

import java.io.*;
class commentme
{
public static void main( String args[] )
    {
    // display some text on screen
    // System.out.println("line 1");
    System.out.println("line 2");
    System.out.println("line 3");
    System.out.println("line 4");
    }
}

The output on screen is now:

line 2
line 3
line 4

Press any key to continue ...

since the first output line has been ignored by the compiler (and so there is no compiled 'line 1' output statement to be interpreted).

Likewise we can comment out larger sections of code using the two-part `slash-asterisk' comment:

/* any characters between the slash-asterisk and asterisk-slash are ignored, even on different lines */

For example we can comment out the 3^rd and 4^th output lines of our program as follows:

class commentme
{
public static void main( String args[] )
    {
    // display some text on screen
    System.out.println("line 1");
    System.out.println("line 2");

    /* System.out.println("line 3");
    System.out.println("line 4");
    */

    }
}

The output on screen is now:

line 1
line 2

Press any key to continue ...

since the third and fourth output lines have been ignored by the compiler (and so there are no compiled 'line 3' or 'line 4' output statements to be interpreted).

However, perhaps the most useful feature of comments is that they facilitate the recording of ideas and design summaries of those writing programs. Such comments are an important source of information about a program, since they can be written and updated at the same time the program is written (as opposed to attempting to keep written records up to date with a changing program). See the listing for Hello1.java for an example of a program with an awful lot of description comments in it.

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