Moving Toward Object-Oriented Programming - Beginning Java Programming: The Object-Oriented Approach - page 78

"Bart Baesens",
"Aimee Backiel",
"Seppe vanden Broucke"
};

book2 = new Book(); // Set second book
book2.title = "Catcher in the Rye";
book2.authors = new String[]{"J. D. Salinger"};

Every instance variable you define defaults to a particular value, depending on the variable type. For
instance, you might wonder what happens if you ask for the title of a book, without specifying a title first:
Book book3 = new Book();
// Oops, forgot to set title...
System.out.println("Title equals: "+book3.title);

The answer is that “Title equals: null”. For variables with a class type, the default value is a special
keyword representing emptiness, null . Since String is a non‐primitive class, its default value is thus
null . Remember, as discussed in Chapter 2, the default values, per data type, are:
➤

For boolean : false

➤

For byte : 0

➤

For short : 0

➤

For int : 0

➤

For long : 0L

➤

For float : 0.0f

➤

For double : 0.0d

➤

For char : \u0000 (the null character)

➤

For String or any object: null

Note that these default values apply only to fields (instance variables). That means if you try to be
clever and add the following to the code snippet:
Book book;
System.out.println("Now, book equals: "+book);
book = new Book();
System.out.println("And now, book equals: "+book);

Eclipse will complain about the book variable not being initialized. This can be fixed like so:
Book book = null;
System.out.println("Now, book equals: "+book);
book = new Book();
System.out.println("And now, book equals: "+book);

The same applies for other variables you define that are not instance variables:
int a = 0;
System.out.println("int a equals"+a);