Java Reference
In-Depth Information
C
HAPTER
S
UMMARY
1.
A
class
is a template for
objects
. It defines the
properties
of objects and provides
constructors
for creating objects and methods for manipulating them.
2.
A class is also a data type. You can use it to declare object
reference variables
. An
object reference variable that appears to hold an object actually contains a reference to
that object. Strictly speaking, an object reference variable and an object are different,
but most of the time the distinction can be ignored.
3.
An object is an
instance
of a class. You use the
new
operator to create an object, and the
dot operator
(
.
) to access members of that object through its reference variable.
4.
An
instance variable
or
method
belongs to an instance of a class. Its use is associated
with individual instances. A
static variable
is a variable shared by all instances of the
same class. A
static method
is a method that can be invoked without using instances.
5.
Every instance of a class can access the class's static variables and methods. For clarity,
however, it is better to invoke static variables and methods using
ClassName.variable
and
ClassName.method
.
6.
Visibility modifiers specify how the class, method, and data are accessed. A
public
class, method, or data is accessible to all clients. A
private
method or data is acces-
sible only inside the class.
7.
You can provide a getter (accessor) method or a setter (mutator) method to enable
clients to see or modify the data.
8.
A getter method has the signature
public returnType getPropertyName()
.
If the
returnType
is
boolean
, the
get
method should be defined as
public
boolean isPropertyName()
. A setter method has the signature
public void
setPropertyName(dataType propertyValue)
.
9.
All parameters are passed to methods using pass-by-value. For a parameter of a primi-
tive type, the actual value is passed; for a parameter of a
reference type
, the reference
for the object is passed.
10.
A Java array is an object that can contain primitive type values or object type values.
When an array of objects is created, its elements are assigned the default value of
null
.
11.
Once it is created, an
immutable object
cannot be modified. To prevent users from
modifying an object, you can define
immutable classes
.
12.
The scope of instance and static variables is the entire class, regardless of where the
variables are declared. Instance and static variables can be declared anywhere in the
class. For consistency, they are declared at the beginning of the class in this topic.
13.
The keyword
this
can be used to refer to the calling object. It can also be used inside
a constructor to invoke another constructor of the same class.
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