Lab: Instance methods

Objective

In this lab, we will enhance our understanding of object oriented programming in C# by refactoring local functions or static methods into instance methods. We will work with a simple library system, focusing on the behaviors of books, to illustrate the power of instance methods and the principle of encapsulation.

Provided Code

Carefully review the provided code. Notice the static method describe which prints details of a Book object. This demonstrates a procedural approach that we will transform into a more object oriented pattern by enabling each Book to handle its own description.

class Book
{
    public string Title;
    public string Author;
    public int PageCount;
}

// Static method
static void describe(Book book)
{
    Console.WriteLine($"'{book.Title}' by {book.Author} ({book.PageCount} pages).");
}

// Example usage
Book book = new Book();
book.Title = "A Random Walk down Wall Street";
book.Author = "Burton G. Malkiel";
book.PageCount = 432;

// Passing the book to the static method.
describe(book);

'A Random Walk down Wall Street' by Burton G. Malkiel (432 pages).

Here, we have a Book class with fields that represent the state of a book in a library. Our objective is to modify this procedural code into object oriented code by using instance methods.

Instructions

We will now turn the describe method into an instance method within the class Book. This will allow any Book object to describe itself by accessing its own state.

Step 1: Convert describe to an Instance Method

Redefine the describe method to be an instance method. It will not take any parameters, as it should now refer to the internal state of the instance.

class Book
{
    public string Title;
    public string Author;
    public int PageCount;

    // Instance method to describe book details
    public void Describe()
    {
        // Add the implementation here...
    }
}

Step 2: Update Method Invocation

Change the way you call the Describe method so that it is called on an instance of Book rather than as a static method.

Book book = new Book();
book.Title = "How to Win Friends and Influence People";
book.Author = "Dale Carnegie";
book.PageCount = 320;

book.Describe();

'How to Win Friends and Influence People' by Dale Carnegie (320 pages).

🤔 Reflection

In the transition from a static method to an instance method, consider how the context of Describe has changed. Why does the instance method Describe not need any parameters? How does this relate to encapsulation?

Challenge

Add an instance method called ReadPage to the Book class that simulates reading a page by incrementing a field called CurrentPage and printing a message that a page has been read.

The ReadPage method should:

1. Increase CurrentPage by one each time it is called.

2. Not allow CurrentPage to exceed the PageCount.

3. Print a message each time a page is read, and a different message if there are no more pages to read.

After adding the ReadPage method to the Book class, create two instances of Book in the Main method and try reading a few pages of them instance.

Book b1 = new Book();
Book b2 = new Book();

// Set attributes of b1.
b1.Title = "Built to sell";
b1.Author = "John Warrillow";
b1.PageCount = 3;

// Set attributes of b2.
// ...

// Read pages of b1.
b1.ReadPage();
b1.ReadPage();
b1.ReadPage();
b1.ReadPage();

// Read pages of b2.
// ...

Reading page 1 of 'Built to sell'. 2 remaining.

Reading page 2 of 'Built to sell'. 1 remaining.

Reading page 3 of 'Built to sell'. 0 remaining.

You have finished reading 'Built to sell'!

🤔 Reflection

Does reading pages in b1 affect anything in b2?

🤔 Reflection

Reflect on the implementation of the ReadPage method. How does managing state within an instance affect the way we think about the data and behavior of objects in object oriented programming?

Conclusion

🤔 Reflection

Since all fields in Book are public we can access these fields on any Book object. For example, this means that we can update the CurrentPage of a book instance by saying book.CurrentPage = 12 (if the variable containing a Book object is called book). Why is this problematic?

Excellent work! 🌟