Lecture 3: Methods for simple classes

Overview

In the last two lectures, we’ve built classes to represent compound data: structures, nested structures, unions, and recursive unions of structures. However our data has been utterly inert, and we have not defined any computations that work with our data. In this lecture, we’ll define methods, which are the object-oriented analogue of the functions we’ve seen before (though they are subtly and importantly different!), work with arithmetic expressions, and build up to methods that consume and produce additional objects.

3.1 Designing methods for simple classes

Suppose we’re modeling books in a bookstore. Recall our definition of the Book class from Lecture 1. (We’ll simplify it for now by representing the author field merely as a String; we’ll reinstate the Author class below.) Let’s remind ourselves how we might manipulate books in Racket, and then we’ll see how to translate these concepts to Java.

;; to represent a book in a bookstore

;; A Book is (make-book String String Number)

(define-struct book (title author price))

;; Examples:

(define htdp (make-book "HtDP" "FFFK" 60))

(define beaches (make-book "Beaches" "PC" 20))

Occasionally, stores have sales: on a sale day, all books are discounted by some fixed percentage. We’ll therefore want to define a function that will produce the book’s sale price. Following the Design Recipe we produce the following function and tests:

;; compute the sale price of a book for the given discount

;; sale-price: Book Num -> Num

(define (sale-price abook discount)

(- (book-price abook) (/ (* (book-price abook) discount) 100)))

(check-expect (sale-price htdp 30) 42)

(check-expect (sale-price beaches 20) 16)

Note that this function is useless for other types of data - per our signature, this function requires that we provide a book.

In Java all computations relevant for one type of data are defined as methods in the class that represents the relevant data. (We say that the methods represent the behavior of the objects that are the members of this class.)

The class diagram and the Java class definitions for the class of books is straightforward:

+---------------+
| Book          |
+---------------+
| String title  |
| String author |
| int price     |
+---------------+

Do Now!

Without looking at Lecture 1, translate this diagram into a Java class definition. Translate the examples of books into Java as well.

According to the design recipe, we need a purpose statement for our method, a signature for it, examples of using it, a template for it, then finally its implementation and tests. However, to write examples or tests for this method...we first need to know how to define and invoke methods! So in this one instance, we’ll skip writing examples first and come back to them afterward. Once you’re familiar with the syntax for defining and invoking methods, you should follow the recipe in order.

Here is the Java code:

3.1.1 Signature and purpose

Every method definition consists of the following parts:

• A purpose statement, much as we wrote in Racket, except we will be very careful to use the pronoun this

• The type of the value returned from the function, known as the return type

• The method name, where the standard naming convention starts with a lowercase letter and uses “camelCase” to distinguish words within the name

• A parenthesized argument list, consisting of the type and name of each argument, separated by commas

• The method body, surrounded by braces; this is the code to execute when the method is invoked

How should we define the method salePrice? What should its signature be? We know it needs a Book and an int sale rate, so should it have two arguments? No! Unlike in Racket, where we have to explicitly pass a Book in as an argument to the sale-price function, in Java every method always has access to this, the object on which we want to invoke the method (in this case, salePrice). This object acts as an implicit argument for the method, and is sometimes called the receiver object. Within the body of the method we can access the fields of the receiver object as this.title or this.price, etc. (These field accesses are selectors, similar to (book-title abook) or (book-price abook) in Racket.) Accordingly, our signature will look like this:

3.1.2 Template

What should the template look like, and where should it be defined? Recall that in Racket, we defined a template once and for all for each data type, rather than once per function that we define. Accordingly, we will define a template just once in each class. For now, the template is very simple, and contains only the fields of the current class. Later we will make this more elaborate:

(A minor note on syntax: /* ... */ denotes a block comment that can span multiple lines, as opposed to // ... which is a single-line comment.)

Just as the template for structs in Racket allowed us to tear the struct apart and access its fields, so too the template here allows us access to the fields of this object.

3.1.3 Method Body

3.1.4 Tests

In order to test our method, we must learn how to invoke a method. We show the completed code for these tests below, which introduces several new features, and then explain the syntax in detail.

The syntax for invoking a method is to first write the receiver object, followed by a period, then the name of the method, then a parenthesized list of arguments to the method: for example, this.htdp.salePrice(30). Here, the receiver object is this.htdp, the method name is salePrice, and the argument is 30.

Do Now!

There are three more method invocations in this code: what are they, what are the receiver objects, and what are the parameters?

Testing our salePrice method actually required writing another method! Specifically, we needed to add a test method to our examples class. After all, running a test means we have to execute some computation, and the only way to execute computations in Java is to define methods. Test methods are slightly special: the tester library looks for methods named test..., with a single Tester parameter, in classes named Examples..., and invokes those methods for us – this is how our tests will get run. The Tester object that is given to these test methods is what implements the check-expect functionality you have seen in Racket. Naturally enough, checkExpect is a method on this object: we are asking the tester library to run some code for us, and give us a report about the passing and failing tests.

Phew! Quite a lot of new content in only four lines of code!

3.2 Aside: Evaluation of arithmetic expressions

The body of the salePrice method contained the following formula:

In Racket, every function expression was wrapped in parentheses and evaluation proceeded predictably left-to-right and inside-out. How does the Java code above evaluate?

Java tries to mimic the conventions used in mathematical formulas by assigning precedences to operators: it defines an order of operations. Specifically, multiplication and division take precedence over addition or subtraction, and equal-precedence operators evaluate from left to right. So the expression above is evaluated as if the parentheses were written:

Savvy programmers add extra parentheses whenever the order of operations is not clear, or to emphasise the meaning of the formula. (Unlike Racket, adding parentheses does not mean “invoke this value as a function”, since Java does not use parenthetical syntax. So adding parentheses for clarity is semantically just fine.)

The moral here, naturally, is to test your arithmetic thoroughly: be sure you understand the consequences of using ints instead of doubles, and check the order of operations carefully.

3.3 Methods for classes with containment: Designing method templates

The example above introduced all the machinery and syntax we’ll need to define methods, but we simplified our data definitions for brevity and clarity. Let’s continue with the original example that included a Book and its Author:

Suppose we would like to determine if the authors of two books are the same.

3.3.1 Signature and purpose

In Racket we might define the signature and purpose of a function to do this as follows:

;; are the two given books by the same author?

;; same-author? : Book Book -> Boolean

(define (same-author? book1 book2)...)

In Java, the first book becomes the implicit argument (this), the instance which invokes the method, and the second book will be the sole explicit argument for the method. Here is the purpose and the header:

3.3.2 Examples

3.3.3 Template

Now we look at the template. The common template for all methods in the class Book now looks like this:

Our template now consists of three sections. The first section, fields, is the same as before. Second, we add all methods defined for the Book class. That includes this.salePrice(int), defined earlier, as well as this.sameAuthor(Book) we are defining now. So, for example, our method this.sameAuthor could invoke this.salePrice (though in this case, knowing a sale price is useless for checking authorship).

The third section is interesting: if any of the fields of this class is an instance of another class, we add to our template all methods defined for that other class, as they can be invoked on that field. Here we would add any methods defined for the Author class, since such methods can be invoked on this.author. We don’t have any such methods...yet.

Using the templates we’ve constructed for each of our classes, we can also construct a template customized for our current method, by adding any information we can extract from the parameters of the method. In this case, such information includes anything that that book can provide: we might access its fields, and we might invoke methods on it.

3.3.4 Method Body

Annoyingly, when we try to finish the design of the method we see that we do not have enough information available. We need to know whether the authors are the same — but only the Author class can define the method that would compare two authors.

Do Now!

What information isn’t available to the Book class, that would be needed to compute whether two authors are the same? Why isn’t that information available?

So, we make a wish list: we need to define the method sameAuthor in the Author class:

We can now add this method to the Author template, and from there add it to the template for sameAuthor in java{Book}:

Now writing the method body is trivial:

Of course, before we test this method, we need to finish designing any remaining methods in our wish list:

with the added tests:

(A note about testing for equality: always test whether two Strings are equal by invoking their equals method; always check whether two ints are equal by using the == operator.)

This pattern of asking another object to “help”, by invoking a method on that object which requires access to information that only it actually has access to, is called delegation, and we’ll see a lot more of this pattern in the upcoming lectures.

3.4 Methods that produce objects

The methods so far produced results of the primitive type. We now look at how to design methods that produce objects.

Suppose the bookstore wants to permanently decrease the price of all books by 20%. We need a method that produces a book with the price reduced as desired.

The purpose and signature will be:

Here are some examples:

We see that the result is a new object and so the method body will contain return new Book(...). The template is as follows:

Conveniently, we can reuse the method defined earlier in the body of our method as follows:

Of course, we finish by running the tests. Notice that the test cases now compare the values of two objects, not just the values of data of the primitive types.