Computer Science II

Lab 4.0 - Classes & Constructors

School of Computing
College of Engineering
University of Nebraska-Lincoln
University of Nebraska-Omaha

An introduction to using classes and constructors in the Java programming language.

Overview

Resources

Prior to lab you should read/review the following resources.

• Java constructor tutorial:
  https://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html

• Object Creation tutorial:
  http://download.oracle.com/javase/tutorial/java/javaOO/objectcreation.html

Lab Objectives & Topics

Following the lab, you should be able to:

• Use Classes and objects to write Java programs

• Understand and use constructors

• Understand the visibility of a class's methods and how to use them

Peer Programming Pair-Up

To encourage collaboration and a team environment, labs will be structured in a pair programming setup. At the start of each lab, you may be randomly paired up with another student by a lab instructor. One of you will be designated the driver and the other the navigator.

The navigator will be responsible for reading the instructions and telling the driver what is to be done. The driver will be in charge of the keyboard and workstation. Both driver and navigator are responsible for suggesting fixes and solutions together. Neither the navigator nor the driver is "in charge."
Beyond your immediate pairing, you are encouraged to help and interact and with other pairs in the lab.

Each week you should try to alternate: if you were a driver last week, be a navigator next, etc. Resolve any issues (you were both drivers last week) within your pair. Ask the lab instructor to resolve issues only when you cannot come to a consensus.

Because of the peer programming setup of labs, it is absolutely essential that you complete any pre-lab activities and familiarize yourself with the handouts prior to coming to lab. Failure to do so will negatively impact your ability to collaborate and work with others which may mean that you will not be able to complete the lab.

1. Getting Started

Clone this project code for this lab from GitHub in Eclipse using the URL: https://github.com/cbourke/CSCE156-Lab04. Refer to Lab 1.0 for instructions on how to clone a project from GitHub.

2. Classes & Constructors

Java is a class-based Object Oriented Programming Language meaning that it realizes the concept of objects by allowing you to define classes which have member methods and variables. Instances of classes are instantiated through a constructor; a method with the same name as a class and called using the keyword new. This lab will familiarize you with how classes and their constructors are defined and used. In addition, you will be introduced to some ways that Java supports other Object Oriented Principles: Encapsulation and Abstraction.

• Encapsulation is a mechanism by which objects group data and the methods/functions that act on that data. It is also the means by which a class's data is protected from the outside world (outside of the object). Java achieves this by allowing you to define member methods and variables and to specify the visibility of these fields using the keywords private, protected, and public.

• Abstraction refers to the means by which an object exposes a public interface to the outside world while hiding the inner workings (the internal representation or the implementation details). Java's main mechanism for supporting this is the same as with encapsulation though it does provide other means (interfaces, abstract classes, etc.).

• Class Signaling refers to invoking methods on an instance of a class. Java uses the dot (or period) operator to signal a class.

3. Activities

We have provided a Java project that simulates a library collections system. It has several classes already defined to model authors, books, a library (a collection of books) and a text-based interface which allows you to search the collection, add books to the collection, and list the collection.

3.1 Creating Constructors

1. Run the library program (LibraryDemo) to familiarize yourself with its functionality. Note that printing the collection is not fully operational

2. Complete each of the accessor (getter) methods in the class. Best Practice Tip: always use the this keyword to disambiguate the scope of variables and prevent potential problems when subclassing.

3. Observe that the Book class does not have a constructor defined. Examine the addBook() code and determine how it is possible to create instances of the Book class without a constructor.

4. Modify the Book class by adding and implementing the following constructor.

   public Book(String title, Author author, String isbn, String publishDate) {
     //TODO: your code here
   }

5. Adding this constructor will cause syntax errors in other parts of the program. Think about why and then fix these problems by modifying the code appropriately.

3.2 Enforcing Good Encapsulation

The Book class is well-designed: it logically groups data and methods together that semantically define what a book is and how you can use it. The Author class however, is not well-designed. Its data members are publicly exposed and it has no methods at all.

1. Redesign the Author class and make its member variables private.

2. Create and use getter methods to make the members accessible to the outside world. Use these methods where appropriate.

3. Create setter methods (also called mutator methods) to enable code outside of the Author class to change the member variables. Add some data validation: for example, do not allow "invalid" values for member variables.

4. Add and make use of an appropriate constructor to this class.

5. Add a method to return a String that is the author's last name/first name separated by a comma and then utilize it where appropriate (modify the printBooks() method to use this new method instead of formatting the last name/first name directly).

3.3 Adding and Using Methods

The printBooks() method prints out the title, author, and ISBN in a formatted manner one per line. In this activity, you will modify it to output additional information: the year of its publication and its age (the number of years since its publication date).

1. Modify the printBooks() method in the LibraryDemo class to output the publication year of each book in another column. Note that the Book class has a getPublishDate() method already implemented for you.

2. To add the "age" column, you will need to add code (somewhere) to compute the number of years between the publish date and today. Well-designed encapsulation means that code that operates on the data of a class belongs inside the class. Bad encapsulation would mean you locate this code outside the class.

   For example: you could write code in the printBooks() method that extracted the publish date from the class and calculated the age. This would be an example of bad encapsulation as it locates code acting on a class's data (publishDate) outside the class.

   Instead, do this correctly using good encapsulation by adding a new method to the Book class that returns the number of years between the publish date and today. Name your method getAge(). You may find the following code snippet useful (it utilizes the date/time library provided by Java 8):

   int years = Period.between(this.publishDate, LocalDate.now()).getYears();

4. Testing, Submitting & Grading

• Test your programs using the provided JUnit test suite(s). Fix any errors and completely debug your programs.
• Submit the following files:
  • Author.java
  • Book.java
• Verify that all of the tests pass.

Advanced Activity (Optional)

1. The printBooks() method prints books in the order in which they appear in the list. This isn't as useful as if they were sorted in some manner. Read Oracle's tutorial on Object ordering, http://docs.oracle.com/javase/tutorial/collections/interfaces/order.html. Write code to use the Collections.sort() method along with an anonymous Comparator<Book> instance to sort the collection according to the book's title.

2. Composition is when a class owns instance(s) of other classes (the Book class owns an instance of the Author class). If different instances of a class, say A and B are given references to the same object $C$, and changes are made to $C$, the changes will be apparent to both A and B. Sometimes this behavior is desired. Sometimes it is not. One common technique to prevent this is to define a copy constructor which takes an instance of the class and performs a deep copy of an object by copying each of its fields to the new instance. For example, the copy constructor for the Author class may look something like this:

   public Author(Author author) {
     this.firstName = new String(author.firstName);
     this.lastName = new String(author.lastName);
   }

   Design and implement a copy constructor for the Book class and the Library class. How do different fields need to be copied?