Encapsulation: Private, Public, Friendly, and Protected...

Preventing unauthorized access to data.

Date: December 18, 2004

It is often necessary to hide certain aspects of a class from public view: one may not want to bother a user with unnecessary details, or one would like to prevent unauthorized access to data. To prevent this from happening, a process called 'data encapsulation' is used:

Encapsulation using Private, Public, Friendly, or Protected:

These are always asked in an interview!!!

public: a field, method, or class that is accessible to every class.

protected: a field, method, or class that is accessible to the class itself, subclasses, and all classes in the same package or directory.

friendly: a field, method, or class that is accessible to the class itself and to all classes in the same package or directory. Note that friendly is not a separate keyword. A field or method is declared friendly by virtue of the absence of any other access modifiers.

private: a field or method that is accessible only to the class in which it is defined. Note that a class can not be declared private as a whole.

Question: Write a class containing a private, public, and friendly field. Then try to access these fields from another class?

We need to write two classes: one class with various fields, and a second class to attempt to access these fields.

class AccessTest
{ // declare each of the fields
  private int privateNum = 1;
  public int publicNum = 3;
  int friendlyNum = 4;
}

class Tester
{ // instantiate a methods
  public static void main(String args[])
  {
    AccessTest a = new AccessTest();
    System.out.println(a.privateNum);
    System.out.println(a.friendlyNum);
    System.out.println(a.publicNum);
  }
}

Variable privateNum in class AccessTest not accessible from class Tester. Good Luck!