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Java Modifiers
E
This appendix summarizes the modifiers that give particular characteristics to Java
classes, interfaces, methods, and variables. For discussion purposes, the set of all
Java modifiers is divided into two groups: visibility modifiers and all others.
Java Visibility Modifiers
The table in Figure E.1 describes the effect of Java visibility modifiers on various
constructs. Some relationships are not applicable (N/A). For instance, a class cannot be declared with protected visibility. Note that each visibility modifier operates in the same way on classes and interfaces and in the same way on methods
and variables.
Default visibility means that no visibility modifier was explicitly used. Default
visibility is sometimes called package visibility, but you cannot use the reserved
word package as a modifier. Classes and interfaces can have default or public visibility; this visibility determines whether a class or interface can be referenced outside of its package. Only an inner class can have private visibility, in which case
only the enclosing class may access it.
Modifier
default (no modifier)
Classes and interfaces
Methods and variables
Visible in its package.
Visible to any class in the same package as its class.
public
Visible anywhere.
Visible anywhere.
protected
N/A
Visible by any class in the same package as its class.
private
Visible to the enclosing
class only
Not visible by any other class.
FIG URE E .1 Java visibility modifiers
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Java Modifiers
A Visibility Example
Consider the situation depicted in the Figure E.2. Class P is the parent class that
is used to derive child classes C1 and C2. Class C1 is in the same package as P, but
C2 is not. Class P contains four methods, each with different visibility modifiers.
One object has been instantiated from each of these classes.
The public method a() has been inherited by C1 and C2, and any code with
access to object x can invoke x.a(). The private method d() is not visible to C1
or C2, so objects y and z have no such method available to them. Furthermore,
d() is fully encapsulated and can only be invoked from within object x.
The protected method b() is visible in both C1 and C2. A method in y could
invoke x.b(), but a method in z could not. Furthermore, an object of any class
in package One could invoke x.b(), even those that are not related to class P by
inheritance, such as an object created from class Another1.
Method c() has default visibility, since no visibility modifier was used to
declare it. Therefore object y can refer to the method c() as if it were declared
locally, but object z cannot. Object y can invoke x.c(), as can an object instantiated from any class in package One, such as Another1. Object z cannot invoke
x.c().
These rules generalize in the same way for variables. The visibility rules may
appear complicated initially, but they can be mastered with a little effort.
package One
class P
P x = new P();
public a()
protected b()
c()
private d()
C1 y = new C1();
C2 z = new C2();
class Another1
package Two
class C1
class C2
class Another2
FIG URE E .2 A situation demonstrating Java visibility modifiers
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Other Java Modifiers
Figure E.3 summarizes the rest of the Java modifiers, which address a variety of
issues. Furthermore, a modifier has different effects on classes, interfaces, methods, and variables. Some modifiers cannot be used with certain constructs and
therefore are listed as not applicable (N/A).
The transient modifier is used to indicate data that need not be stored in a
persistent (serialized) object. That is, when an object is written to a serialized
stream, the object representation will include all data that is not specified as
transient.
Modifier
Class
abstract The class may contain abstract methods. It cannot be
instantiated.
Interface
Method
Variable
No method body is
defined. The method
requires implementation
when inherited.
N/A
The class cannot be N/A
used to drive new
classes.
The method cannot be
overridden.
The variable is a constant,
whose value cannot be
changed once initially set.
native
N/A
N/A
No method body is necessary since implementation
is in another language.
N/A
static
N/A
N/A
Defines a class method. It
does not require an instantiated object to be invoked.
It cannot reference nonstatic methods or variables.
It is implicitly final.
Defines a class variable. It
does not require an instantiated object to be referenced. It is shared (common memory space) among
all instances of the class.
synchro- N/A
nized
N/A
The execution of the
method is mutually exclusive among all threads.
N/A
transient N/A
N/A
N/A
The variable will not
be serialized.
volatile N/A
N/A
N/A
The variable is changed
asynchronously. The
compiler should not
perform optimizations
on it.
final
All interfaces are
inherently abstract.
The modifier is
optional.
FIG URE E .3 The rest of the Java modifiers
Java Modifiers
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