Introduction to Objects - Advanced Topics in Java: Core Concepts in Data Structures - page 32

2575

author
title
price
pages
binding
inStock

2575

2599

b

Figure 2-1. An instance of a Book object

As a shortcut, we can declare b and create a book object in one statement, like this:

Book b = new Book();

It is a common error to think that the Book variable b can hold a Book object. It cannot; it can hold only a reference
to a Book object. (In a similar manner, we should be familiar with the idea that a String variable does not hold a
string but, rather, the address of where the string is stored.) However, where the distinction (between an object and a
reference to the object) does not matter, we will speak as if b holds a Book object.
Once an object b is created, we can refer to its instance fields like this:

b.author b.title b.price
b.pages b.binding b.inStock

However, we can do so from outside the class only if the fields are declared public . We will see later how to access
the fields indirectly when they are declared private .
When an object is created, unless we say otherwise, its instance fields are initialized as follows:

0 . • Numeric fields are set to

'\0' (Unicode '\u0000' , to be precise). • Character fields are set to

false .

• Boolean fields are set to

null . (A variable with the value null means that it does not reference or • Object fields are set to

point to anything.)

In our example, the following happens:

b.author (of type String ) is set to null ; remember that String is an object type. •

b.title (of type String ) is set to null . •

b.price (of type double ) is set to 0.0 . •

b.pages (of type int ) is set to 0 . •

b.binding (of type char ) is set to '\0' .

•

b.inStock (of type boolean ) is set to false . •

We could specify an initial value when we declare an instance variable. Consider this code:

public class Book {
private static double Discount = 0.25;
private static int MinBooks = 5;