Creating new data types: class

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data
types: class

If
everything is an object, what determines how a particular class of object looks
and behaves? Put another way, what establishes the
type
of an object? You might expect there to be a keyword called “type”
and that certainly would have made sense. Historically, however, most
object-oriented languages have used the keyword
class
to mean “I’m about to tell you what a new type of object looks
like.” The
class
keyword (which is so common that it will not be emboldened throughout the book)
is followed by the name of the new type. For example:

class
ATypeName { /* class body goes here */ }

This
introduces a new type, so you can now create an object of this type using
new:

ATypeName
a = new ATypeName();

In
ATypeName,
the class body consists only of a comment (the stars and slashes and what is
inside, which will be discussed later in this chapter) so there is not too much
that you can do with it. In fact, you cannot tell it to do much of anything
(that is, you cannot send it any interesting messages) until you define some
methods for it.

Fields
and methods

When
you define a class (and all you do in Java is define classes, make objects of
those classes and send messages to those objects), you can put two types of
elements in your class: data members (sometimes called
fields)
and member functions (typically called
methods).
A data member is an object (that you communicate with via its handle) of any
type. It can also be one of the primitive types (which isn’t a handle).
If it is a handle to an object, you must initialize that handle to connect it
to an actual object (using
new,
as seen earlier) in a special function called a
constructor
(described fully in Chapter 4). If it is a primitive type you can initialize it
directly at the point of definition in the class. (As you’ll see later,
handles can also be initialized at the point of definition.)

Each
object keeps its own storage for its data members; the data members are not
shared among objects. Here is an example of a class with some data members:

class DataOnly {
  int i;
  float f;
  boolean b;
}

This
class doesn’t
do
anything, but you can create an object:

DataOnly
d = new DataOnly();

You
can assign values to the data members, but you must first know how to refer to
a member of an object. This is accomplished by stating the name of the object
handle, followed by a period (dot), followed by the name of the member inside
the object (
objectHandle.member).
For example:

d.i = 47;
d.f = 1.1f;
d.b = false;

It
is also possible that your object might contain other objects that contain data
you’d like to modify. For this, you just keep “connecting the
dots.” For example:

myPlane.leftTank.capacity
= 100;

The
DataOnly
class
cannot do much of anything except hold data, because it has no member functions
(methods). To understand how those work, you must first understand
arguments
and
return
values
,
which will be described shortly.


Default
values for primitive members

When
a primitive data type is a member of a class, it is guaranteed to get a default
value if you do not initialize it:

Primitive
type

Default

Boolean

false

Char

‘u0000’
(
null)

byte

(byte)0

short

(short)0

int

0

long

0L

float

0.0f

double

0.0d

Note
carefully that the default values are what Java guarantees when the variable is
used
as
a member of a class
.
This ensures that member variables of primitive types will always be
initialized (something C++ doesn’t do), reducing a source of bugs.

However,
this guarantee doesn’t apply to “local” variables –
those that are not fields of a class. Thus, if within a function definition you
have:

int
x;

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