Access Modifiers in Java

Access Modifiers


              Same Class	Same Package	Subclass	Other packages
public                 	Y	Y	Y	Y
protected	        Y	Y	Y	N
default         	Y	Y	N	N
private	                Y	N	N	N

Source of the above table: http://javapapers.com/core-java/access-modifiers-in-java-explain/

Access level of class:

Java class has mainly two type of access level:

default: class objects are accessible only inside the package.

public: class objects are accessible in code in any package.

private: can only be accessed within that class. Not anywhere outside it

protected: Can be accessed within the class and all the classes which inherit this class and only
within that package.



Source: http://staff.science.uva.nl/~heck/JAVAcourse/ch4/ss2_2.html

        http://javapapers.com/core-java/access-modifiers-in-java-explain/
        
Access Specifiers


One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of 
data in a class and making this class available only through methods. In this way the chance of 
making accidental mistakes in changing values is minimized. Java allows you to control access to 
classes, methods, and fields via so-called access specifiers.
Java offers four access specifiers, listed below in decreasing accessibility:

public
protected
default (no specifier)
private
We look at these access specifiers in more detail.
public

public classes, methods, and fields can be accessed from everywhere.
The only constraint is that a file with Java source code can only contain one 
public class whose name must also match with the filename. If it exists, this
public class represents the application or the applet, in which case the public
keyword is necessary to enable your Web browser or appletviewer to show the applet.
You use public classes, methods, or fields only if you explicitly want to offer access
to these entities and if this access cannot do any harm. An example of a square determined 
by the position of its upper-left corner and its size:
public class Square {  // public class
  public x, y, size;   // public instance variables
}
protected

protected methods and fields can only be accessed within the same class to which the methods and
fields belong, within its subclasses, and within classes of the same package, but not from anywhere 
else. You use the protected access level when it is appropriate for a class's subclasses to have access 
to the method or field, but not for unrelated classes.
default (no specifier)

If you do not set access to specific level, then such a class, method, or field will be accessible
from inside the same package to which the class, method, or field belongs, but not from outside this 
package. This access-level is convenient if you are creating packages. For example, a geometry package 
that contains Square and Tiling classes, may be easier and cleaner to implement if the coordinates of
the upper-left corner of a Square are directly available to the Tiling class but not outside the 
geometry package.
private

private methods and fields can only be accessed within the same class to which the methods and 
fields belong. private methods and fields are not visible within subclasses and are not inherited
by subclasses. So, the private access specifier is opposite to the public access specifier. 
It is mostly used for encapsulation: data are hidden within the class and accessor methods are 
provided. An example, in which the position of the upper-left corner of a square can be set or
obtained by accessor methods, but individual coordinates are not accessible to the user.


public class Square {   // public class
  private double x, y   // private (encapsulated) instance variables

  public setCorner(int x, int y) {  // setting values of private fields
    this.x = x;
    this.y = y;
  }

  public getCorner() {  // setting values of private fields
    return Point(x, y);
  }
}


Summary of Access Specifiers



Access Modifiers


              Same Class	Same Package	Subclass	Other packages
public                 	Y	Y	Y	Y
protected	        Y	Y	Y	N
default         	Y	Y	N	N
private	                Y	N	N	N