Java Reference
In-Depth Information
Class
A
class
is a collection of data fields that hold values and methods that operate
on those values. A class defines a new reference type, such as the
Point
type
defined in
Chapter 2
.
The
Point
class defines a type that is the set of all possible two-dimensional points.
Object
An
object
is an
instance
of a class.
A
Point
object is a value of that type: it represents a single two-dimensional point.
Objects are often created by
instantiating
a class with the
new
keyword and a con‐
structor invocation, as shown here:
Point
p
=
new
Point
(
1.0
,
2.0
);
Constructors are covered later in this chapter in
“Creating and Initializing Objects”
on page 106
.
A class definition consists of a
signature
and a
body
. The class signature defines the
name of the class and may also specify other important information. The body of a
class is a set of
members
enclosed in curly braces. The members of a class usually
include fields and methods, and may also include constructors, initializers, and nes‐
ted types.
Members can be
static
or nonstatic. A static member belongs to the class itself while
a nonstatic member is associated with the instances of a class (see
“Fields and Meth‐
ods” on page 100
).
There are four very common kinds of members—class fields,
class methods, instance fields, and instance methods. The
majority of work done with Java involves interacting with
these kinds of members.
The signature of a class may declare that the class
extends
another class. The exten‐
ded class is known as the
superclass
and the extension is known as the
subclass
. A
subclass
inherits
the members of its superclass and may declare new members or
override
inherited methods with new implementations.
The members of a class may have
access modiiers
public
,
protected
, or
private
.
1
These modifiers specify their visibility and accessibility to clients and to subclasses.
This allows classes to control access to members that are not part of their public
API. This ability to hide members enables an object-oriented design technique
known as
data encapsulation
, which we discuss in
“Data Hiding and Encapsulation”
on page 121
.
1
There is also the default, aka package, visibility that we will meet later.
