Java visibility of class members

Classes and packages are both means of encapsulating and containing the name space and scope of variables and methods. Packages act as containers for classes and other subordinate packages. Class act as container for data and method code. The class is Java’s smallest unit of abstraction. Because of the interplay between classes and packages, Java addresses five categories of visibility for class members:

1. Visibility within the class

2. Visibility in sub-classes in the same package

3. Visibility in non-sub classes in the same package

4. Visibility in sub-classes in different package

5. Visibility in classes that are neither in the same package nor are sub classes.

A top-level class or interface has only two possible access levels: default and public. When a class is declared public, it is accessible by any other code. If a class has default access, then it can only be accessed by other code within the same package.

The visibility of class members is summarized in the following table:

Note: The member visibility has meaning only if the class is visible. If visibility modifier of the class is default then even public members of the class will be visible only within the package.

Example:

Protection.java

package p1;

public class Protection

{

int n = 1;

private int n_pri = 2;

protected int n_pro = 3;

public int n_pub = 4;

public Protection()

{

System.out.println("Base Constructor");

System.out.println(n);

System.out.println(n_pri);

System.out.println(n_pro);

System.out.println(n_pub);

}

}

To compile: javac –d . Protection.java

Derived.java

package p1;

class Derived extends Protection

{

public Derived()

{

 System.out.println(n);

System.out.println("Derived Constructor");

//System.out.println(n_pri);

System.out.println(n_pro);

System.out.println(n_pub);

  }

}

To compile: javac –d . Derived.java

SamePackage.java

package p1;

class SamePackage

{

public SamePackage()

  {

Protection p = new Protection();

System.out.println("Same Package constructor");

System.out.println(p.n);

//System.out.println(p.n_pri);

System.out.println(p.n_pro);

System.out.println(p.n_pub);

}

}

To compile: javac –d . SamePackage.java

Protection2.java

package p2;

class Protection2 extends p1.Protection

{

Protection2()

{

System.out.println("Protection2 Constructor");

//System.out.println(n);

//System.out.println(n_pri);

System.out.println(n_pro);

System.out.println(n_pub);

}

}

To compile: javac –d . Protection2.java

OtherPackage.java

package p2;

class OtherPackage

{

OtherPackage()

{

p1.Protection p = new p1.Protection();

System.out.println("OtherPackage Constructor");

//System.out.println(p.n);

//System.out.println(p.n_pri);

//System.out.println(p.n_pro);

System.out.println(p.n_pub);

}

}

To Compile: javac –d . OtherPackage.java

Test1.java

package p1;

class Demo

{

public static void main(String args[])

{

Protection ob1 = new Protection();

Derived ob2 = new  Derived();

SamePackage ob3 = new SamePackage();

}

}

To Compile: javac –d . Test1.java

Output:

Base Constructor1234
Base Constructor1234
Derived Constructor134
Base Constructor1234
Same Package Constructor134

Test2.java

package p2;

class Demo

{

public static void main(String args[])

{

Protection2 ob1 = new Protection2();

OtherPackage ob2 = new OtherPackage();

}

}

To Compile: javac –d . Test2.java

Output:

Base Constructor1234
Protection2 Constructor
34
Base Constructor1234
OtherPackage Constructor
4