لطفا منتظر باشید ...
Chapter 2. Objects, Classes, and Interfaces
This chapter contains examples that illustrate the object-oriented
nature of Java and show you how to define and use classes and
interfaces. It is designed to be read in conjunction with
Chapter 3
of Java in a Nutshell,
which offers a complete introduction to the object-oriented concepts
and syntax you must understand to program in Java. As a refresher,
the following paragraphs summarize Java's
object-oriented terminology.An object is a
collection of data values, or fields, plus
methods that operate on that data. The data type
of an object is called a class; an object is
often referred to as an instance of its class.
The class defines the type of each field in an object, and it
provides the methods that operate on data contained in an instance of
the class. An object is created using the new
operator, which invokes a constructor of the
class to initialize the new object. The fields and methods of an
object are accessed and invoked using the .
operator.Methods that operate on the fields of an
object are known as instance methods. They are different from the
static, or class, methods that we saw in Chapter 1. Class methods are declared
static; they operate on the class itself, rather
than on an individual instance of the class. Fields of a class may
also be declared static, which makes them class
fields instead of instance fields. While each object has its own copy
of each instance field, there is only one copy of a class field and
it is shared by all instances of the class.The fields and methods of a class may
have different visibility levels, including
public, private, and
protected. These different levels of visibility
allow fields and methods to be used in different contexts. Every
class has a superclass, from which it
inherits fields and methods. When a class
inherits from another class, it is called a
subclass of that class. Classes in Java form a
class hierarchy. The
java.lang.Object class is root of this hierarchy;
Object is the ultimate superclass of all other
classes in Java.An
interface is a Java construct that defines
methods, like a class, but doesn't provide any
implementations for those methods. A class can
implement an interface by defining an
appropriate implementation for each method in the interface.
