Object Lab 1

Learning Goals

• Add fields (instance variables) to a class
• Declare a variable with a class data type
• Create an object in Java using the new operator
• Assign a variable as an object reference
• Use IntelliJ's Java Visualizer to view object state
• Assign an instance variable an initial value at variable declaration
• Use IntelliJ to auto-generate a toString() method for printing object state
• Use dot notation to access and update object state

Instructions

The starter project contains a class named Book with a main method that contains comments outlining the lab tasks.

You will use IntelliJ's debugger and the Java Visualizer to view object state. Recall that you can stop the debugger session at any point by pressing the red square icon:

You can also press the green arrow icon to continue execution to the next breakpoint or end of program:

Task #1 - Add fields and create 3 Book objects

1. Edit the Book class to add 5 fields named title, isbn, edition, pages, and language. The title, isbn, and language are strings. The edition and pages are integers.

2. Edit the main() method to create 3 instances of the Book class. Declare local variables named book1, book2, and book3 and assign each variable to reference a new Book instance.

3. Use IntelliJ's Java Visualizer to confirm the state of each object is initialized to default values based on data type.

   • Set a breakpoint at the first executable line of code in the main() method (code hidden in screen print below, your line number may be different).

     

   • Click the debug icon.

     

   • Use the Java Visualizer to step through the code.

     

   • Confirm each object has the 5 fields using the IntelliJ Java Visualizer view:

   

If you don't like the crossed arrows due to the horizontal layout of the objects, you can also use the visualizer at https://pythontutor.com/java.html, which lays out the objects vertically as shown below. NOTE: When an object is created by calling new Book(), you will see a frame for a method named <init> show up on the call stack, which represents the constructor method being called. Keep pressing the Next > button until the Book object appears on the heap.

Task #2 - Edit field (instance variable) declarations to assign initial values

Do not modify the main() method for this task.

Let's assume most books are first edition and written in English.

1. Edit the edition instance variable declaration to assign an initial value of 1.
2. Edit the language instance variable declaration to assign an initial value of "English".
3. Press the debug icon and use the Java Visualizer to confirm the edition and language are initialized for the 3 books.

Task #3 - Printing Object State

Edit the main method to add the following code. Place this after creating the 3 Book objects.

// print initial object state
System.out.println("Object state after instantiation:");
System.out.println(book1);
System.out.println(book2);
System.out.println(book3);

If you run the main method, you'll see the memory location of each object printed:

Object state after instantiation:
Book@4617c264
Book@36baf30c
Book@7a81197d

Use IntelliJ to generate a toString() method to print the values of the fields. Place the cursor before the main method, then right-click, select generate, select toString(), select all 5 fields.

Run the main() method and confirm the output. The order is based on the order the fields were declared in the class. You might need to reorder your fields and regenerate the toString() method to match the output.

Object state after instantiation:
Book{title='null', isbn='null', edition=1, pages=0, language='English'}
Book{title='null', isbn='null', edition=1, pages=0, language='English'}
Book{title='null', isbn='null', edition=1, pages=0, language='English'}

Task #4 - Updating Object State

Edit the main() method to update the state for the objects by assigning new values to the fields.

Place the new assignment statements after the 3 print statements that print the initial default state.

Don't add unnecessary assignment statements, i.e. when the book's language is English or the edition is 1.

First try adding the code to update the values for book1 ("Effective Java", "978-0134685991", 3rd edition, 416 pages).

Then add the code to update book2 ("Spring Boot: Up & Running", "978-1492076988"; 328 pages).

Finally, add the code to update book3 ( "HTML para novatos", "978-1493714148", 105 pages, "Spanish").

Edit the main method to add additional print statements after the assignment statements. The print statements should output the state for each object.

// print final object state
System.out.println("Object state after assigning values to instance variables:");
System.out.println(book1);
System.out.println(book2);
System.out.println(book3);

Run the main method and confirm your output is as shown below:

Object state after instantiation:
Book{title='null', isbn='null', edition=1, pages=0, language='English'}
Book{title='null', isbn='null', edition=1, pages=0, language='English'}
Book{title='null', isbn='null', edition=1, pages=0, language='English'}
Object state after assigning values to instance variables:
Book{title='Effective Java', isbn='978-0134685991', edition=3, pages=416, language='English'}
Book{title='Spring Boot: Up & Running', isbn='978-1492076988', edition=1, pages=328, language='English'}
Book{title='HTML para novatos', isbn='978-1493714148', edition=1, pages=105, language='Spanish'}

Task #5 - Run Junit Tests

Within the test folder is a Junit test class named BookTest that tests the output produced from executing the main() method defined in the Book class.

Run BookTest and confirm the test passes.