5.4. Method Comments and Conditions

In industry you often produce code that other people will use and you will use code other people produced. It is important to document your code to facilitate understanding and reuse.

5.4.1. Method Comments

Recall that there are 3 types of comments in Java:

  1. // Single line comment

  2. /* Multiline comment */

  3. /** Documentation comment */

The special characters // are used to mark the rest of the line as a comment in many programming languages. If the comment is going to be multiple lines, we use /* to start the comment and */ to end the comment.

The multi-line comment, /** */ is called the documentation comment. Java has a cool tool called javadoc that comes with the Java JDK that will pull out all of these comments to make documentation of a class as a web page. This tool generates the official Java documentation too, for example for the String class. It’s a good idea to use the documentation comment in front of classes, methods, and instance variables in case you want to use this tool.

The are some special tags that you can use in Java documentation. These are not required but many programmers use them. Here are some common tags:

  • @author Author of the program

  • @since Date released

  • @version Version of program

  • @param Parameter of a method

  • @return Return value for a method

The code below shows example commenting for a class and two enclosed methods.

* The Converter program implements an application that
* converts inches to centimeters and prints
* the equivalent quantities.
* @author  Fred Smith
* @version 1.0
* @since   2020-01-31
public class Converter {

    * This method is used to convert inches to centimeters.
    * @param inches A double representing a quantity of inches.
    * @return double The equivalent quantity of centimeters.
    public static double inchesToCentimeters(double inches)
        return inches * 2.54;

    * The main method demonstrates use of the inchesToCentimeters method.
    * @param args Unused.
    * @return Nothing.
    public static void main(String[] args)
        System.out.println("10 inches = " + inchesToCentimeters(10) + " centimeters");
        System.out.println("15.7 inches = " + inchesToCentimeters(15.7) + " centimeters");


5.4.2. Preconditions and Postconditions

As you write methods in a class, it is a good idea to keep in mind the preconditions and the postconditions for the method and write them in the comments. A precondition is a condition that must be true for your method code to work, for example the assumption that the parameters have values and are not null. The methods could check for these preconditions, but they do not have to. The precondition is what the method expects in order to do its job properly.

A postcondition is a condition that is true after running the method. It is what the method promises to do. Postconditions describe the outcome of running the method, for example what is being returned or the changes to the instance variables. These assumptions are very useful to other programmers who want to use your class and get the correct results.

Let’s consider the substring method in Java. This method has a strong precondition that its arguments refer to indices within the given string.

coding exercise Coding Exercise

The following code breaks the preconditions of the substring method and throws an IndexOutOfBoundsException. Can you fix the code by changing the arguments for the substring method to print out the substring “lo”? What are the preconditions for the substring method?


The method str.substring(beginIndex, endIndex) has the precondition that 0 <= beginIndex <= endIndex <= str.length.

exercise Check your understanding

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