Java Modifiers - Java Software Solutions: Foundations of Program Design - page 684

A Visibility Example
Consider the highly contrived 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 be invoked only 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 instanti-
ated 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();
C1 y = new C1();
C2 z = new C2();

public a()
protected b()
c()
private d()

class Another1

package Two

class C1

class C2

class Another2

FIGURE E.2 A situation demonstrating Java visibility modifiers