5.3. Comments and Conditions


Adding comments to your code helps to make it more readable and maintainable. In the commercial world, software development is usually a team effort where many programmers will use your code and maintain it for years. Commenting is essential in this kind of environment and a good habit to develop. Comments will also help you to remember what you were doing when you look back to your code a month or a year from now.

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.

There is also a special version of the multi-line comment, /** */, 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. Although you do not have to use this in the AP exam, it’s a good idea to use the documentation comment in front of classes, methods, and instance variables in case you want ot use this tool.

exercise Check your understanding

    5-3-1: Drag the definition from the left and drop it on the correct symbols on the right. Click the "Check Me" button to see if you are correct. Review the section above.
  • single-line comment
  • //
  • multi-line comment
  • /* */
  • Java documentation comment
  • /** */

The compiler will skip over comments, and they don’t affect how your program runs. They are for you, your teacher, and other programmers working with you. Here are some examples of good commenting:

* MyClass.java
* @author My Name
* @since Date
* This class keeps track of the max score.
public class MyClass()
   private int max = 10; // this keeps track of the max score
   /* The print() method prints out the max */
   public print() {  System.out.println(max); }

Note that most IDEs will tend to show comments formatted in italics – to make them easier to spot.

Notice that there 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

5.3.2. 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.

Here is an example of preconditions, postconditions, and @param in the Turtle code that we have used in the past for our drawing turtles.

  * Constructor that takes the x and y position for the
  * turtle
  * Preconditions: parameters x and y are coordinates from 0 to
  *    the width and height of the world.
  * Postconditions: the turtle is placed in (x,y) coordinates
  * @param x the x position to place the turtle
  * @param y the y position to place the turtle
 public Turtle(int x, int y)
   xPos = x;
   yPos = y;

coding exercise Coding Exercise

Try to break the preconditions of the Turtle constructor below. Does the Turtle constructor behave properly if you break the preconditions that x and y are between 0 and 300. Try giving the Turtle constructor x and y values out of these ranges. What happens? Does the method give good results? Does it give any warnings? What about the t.forward() method? Does it have any preconditions that you can break?

(If the code below does not work for you, you can copy the code into this repl.it link (refresh page after forking and if it gets stuck) or download the files here to use in your own IDE.)

The Turtle constructor’s precondition is that x and y should be between 0 and the width and height of the world. If it receives values out of this range, it sets x and y to the closest legal values that it can so that the turtle appears just at the edge of the world. Similarly, the forward() method will not allow the turtle to leave the world.

exercise Check your understanding

    5-3-2: Consider the following class definition.

       public class TestScore
           private String studentName;
           private double score;
           private double extraCredit;
           public TestScore (String n, double s, double ec)
               studentName = n;
               score = s;
               extraCredit = ec;
           /* Other methods not shown */
    Which of the following preconditions are reasonable for the TestScore constructor?
  • /* Precondition: s <= 0 */
  • It is not reasonable the s which sets the score should be negative.
  • /* Precondition: score >= 0 */
  • The precondition should be about the parameters of the constructor. score is not the parameter variable.
  • /* Precondition: s and ec >= 0 */
  • Correct. It is reasonable that the score and extraCredit should be set to positive values using the parameters s and ec.
  • /* Precondition: n is not the empty String */
  • Correct. It is reasonable that the parameter n which sets the name should be not empty.
  • /* Precondition: studentName is not the empty String */
  • The precondition should be about the parameters of the constructor. score is not the parameter variable.

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

    5-3-3: The following method is intended to return the substring starting at index i until the end of the string. For example, getiToEnd(“012”,1) should return “12”. Which of the following is the most appropriate precondition for the method so that it does not throw an exception?

    /* missing precondition */
    public String getiToEnd(String str, int i)
        return str.substring(i, str.length());
  • /* Precondition: i >= 0 */
  • This is true but it could still throw an exception if i is a large value.
  • /* Precondition: i <= str.length() */
  • This is true but it could still throw an exception if i is a negative value.
  • /* Precondition: 0 < i < str.length() */
  • This is true but a little too restrictive.
  • /* Precondition: 0 <= i < str.length() */
  • Correct. i can refer to character 0 up to str.length().

5.3.3. Software Validity and Use-Case Diagrams

Preconditions and postconditions are covered on the AP CS A exam. Software validity, testing, and use-case diagrams which are discussed in this subsection are not covered on the AP CS A exam, but they are described here because they use preconditions and postconditions and are used by professional programmers.

Determining the preconditions and postconditions help us to test our code and determine the validity of our software. Software validity tests whether the software does what it is supposed to do before it is released. This is sometimes very important. For example, if the code is part of a satellite going to outerspace or is going to be used in an emergency condition, we want to test it thoroughly and make sure it works and is valid before it is put into use.

Good software testers actually try to break the code! They try all kinds of input to see what the software will do because you never know what users will try or what conditions there will be. So, always think what the preconditions of your code are and see what happens when you break them, and then see if you can protect or warn against that.

Preconditions and postconditions can also help us to design better software systems. Software designers often first draw a high-level Use-Case Diagram of a system that shows the different ways that a user might interact with a system before they build it. Here is a simple Use-Case Diagram of a restaurant system. It shows 2 actors in the system: the customer and the staff at the restaurant, and 3 use-cases in circles. A Use-case is a particular user interaction or situation in the system or software, and they often become methods in the program.

Use Case Diagram

Figure 1: Use-Case Diagram of a Restaurant System

After drawing a Use-Case Diagram, designers write down the preconditions and the postconditions for each Use-Case. Often the successful post-condition for one use-case becomes the preconditions for the next use-case. For example, for the “Order Food” and “Eat Food” Use Cases:

  • Preconditions for “Order Food”: Customer enters restaurant. Staff is ready to take the order.

  • Postconditions for “Order Food”: Customer orders the food. Staff takes the order.

  • Preconditions for “Eat Food”: Customer has already ordered food. Staff has delivered food.

  • Postcondition for “Eat Food”: Customer eats the food.

exercise Check your understanding

5-3-4: What are the preconditions and postconditions of the use-case “Pay for food”? Remember that these are often related to the other use-case conditions “order food” and “eat food”.

5.3.4. Agile Software Development

There are many different models for software development. The waterfall model, developed in the 1970s, is a step by step model where each phase is finished before the next phase begins. This model has recently been criticized because it is not very adaptable. The more recent Agile development model involves iterative, incremental development where teams works in short 2-3 week sprints to completely develop, test, and release a component of the project to the customer for feedback. It is very adaptable as project requirements change because of early testing, immediate customer feedback and collaboration.


Figure 2: Waterfall vs Agile Models

One very popular type of agile development is called Scrum. The following short video describes software development with Scrum.

Video: (TRcReyRYIMg)

groupwork Group Exercise

Try the Wake Up In the Morning Game in groups to practice the iterative and incremental agile development process.

5.3.5. groupwork Programming Challenge : Comments and Conditions

Working in pairs or groups, come up with 4 steps that a user must do to purchase a product, for example a book on Java, in an online store, and list the preconditions and postconditions for each step. You could pretend to buy something online to come up with the steps. (You could use an online drawing tool like Creately.com (choose Use-Case Diagrams) to draw a Use-Case diagram for the Online Store System, but it is not required). Don’t forget to list the preconditions and postconditions for each step. You can type in your answer below.

5-3-5: Write down 4 steps that a user must do to purchase a product, for example a book on Java, in an online store, and list the preconditions and postconditions for each step.

Here is a simple class called User that could be used in an online store. Add good commenting to this code before the class, the instance variables, and the methods.

5.3.6. Summary

  • Comments are ignored by the compiler and are not executed when the program is run.

  • Three types of comments in Java include /* */, which generates a block of comments, //, which generates a comment on one line, and /** */, which are Javadoc comments and are used to create API documentation.

  • A precondition is a condition that must be true just prior to the execution of a section of program code in order for the method to behave as expected. There is no expectation that the method will check to ensure preconditions are satisfied.

  • A postcondition is a condition that must always be true after the execution of a section of program code. Postconditions describe the outcome of the execution in terms of what is being returned or the state of an object.

  • Programmers write method code to satisfy the postconditions when preconditions are met.

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