Exercise 03: Book Exercise

Not yet reviewed and published for WS 2019/20 Term! Books. Abhi Sharma

This week’s lab work is intended to practice creating classes with data fields, constructors, methods, assignments and conditional statements.

Pre-lab

What to Bring to Lab

Please finish these exercises before coming to lab.

P1. Why does the following version of refundBalance not give the same results as what we saw in class?

public int refundBalance() {
  balance = 0;
  return balance;
}

P2.

What happens if you try to compile the TicketMachine class with the following version of refundBalance?

public int refundBalance() {
  return balance;
  balance = 0;
}

P3. We have seen a few arithmetic operators in Java. Find a table with the Java operators and their priorities. Note down where you found this. What does the ternary operator in x ? a : b do?

P4. What do you expect the the following expressions to evaluate to? Do not use a compiler or the codepad, only your head!

What to hand in

0. A file Book.java with your source code for the Book Class
1. Your completed assignment, submitted in Moodle as a pdf, should include:

• the strategy you chose for working in a team & how it worked out
• a summary of what you did & learned in the lab
• details on which challenges you encountered, how you tried to solve them and their solution

Lab assignments are due the night before your next lab at 22:00. They may, of course, be turned in earlier. You hand them in by preparing the report in PDF and submitting it to Moodle.

Assignment

CodePad

1. Open BlueJ and find the CodePad. Use it to test your predictions for P4 and record where you were correct and where you made mistakes.

Making a Book

1. Open the BookExercise project. Add two accessor methods to the class - getAuthor and getTitle - that return the author and title fields as their respective results. Test your class by creating some instances and calling the methods.

2. Add two methods, printAuthor and printTitle, to the Book class. These should print the author and title fields, respectively, to the terminal window.

3. Add a further field, pages, to the Book class to store the number of pages. This should be of type int, and its initial value should be passed to the single constructor, along with the author and title strings as the third parameter. Include an appropriate getPages accessor method for this field.

4. Add a method, printDetails, to the Book class. This should print details of the author, title, and pages to the terminal window. It is your choice how the details are formatted. You might want to include some explanatory text.

5. Add a further field, refNumber, to the Book class. This field can store a reference number for a library, for example. It should be of type String and initialized to the zero-length string in the constructor. Add the corresponding accessor(getter) and the setter with the signatures below. The body of this method should assign the value of the parameter to the refNumber field.

    ```java
   
    public String getRefNumber()
   
    public void setRefNumber (String ref)
    ```
   

6. Modify your printDetails method to include printing the reference number. However, the method should print the reference number only if it has been set, and the exact string “no reference number” otherwise. Hint: use a conditional! Note that Strings have a length method.

7. Modify your setRefNumber mutator so that it sets the refNumber field only if the parameter is a string of at least three characters. If it is less than three, then print an error message and leave the field unchanged.

Exercise Adapted from https://people.f4.htw-berlin.de/~weberwu/info1/Labs/Lab2.shtml , which was itself adapted from Objects First with Java, A Practical Introduction Using BlueJ. David Barnes & Michael Kölling, 2009