Review:    accessibility modifiers

  • So far, we have learned 3 types of accessibility modifiers: 

      Modifier     Accessed    Accssed       Access 
  on members   from the    from the      from a
  in a class   same class  same package	 diff. package
 ===================================================================
  public           OK         OK            OK
  default          OK         OK            No
  private          OK	      No            No

  • The private accessibility modifier was discussed in Chapter 18.5 in the text book and my material is here:

  • The default accessibility modifier was discussed in Chapter 9.8 in the text book and my material is here:

  • Often it is desirable to allow subclasses to access data fields or methods defined in the superclass, but not to allow nonsubclasses to access these data fields and methods.

"Closeness" of association of code in Java classes

  • To understand the accessibility modifiers you need to understand the degree of closeness (= trust) of programs written by different people

  • The degree of closeness (= trust) of classes in a Java program:

      1. Code inside a class is usually written by the same programmer (or at most a few programmers)

      2. Code inside a package is usually written by the same team of programmer (or at most a few programmers)

      3. Code inside a different package can be written by anyone...

  • We will now learn about the protected accessibility modifier

  • The protected modifier will allow subclasses (defined by anyone) to access data fields or methods defined in the superclass,

The protected accessibility modifier

  • Syntax to define a member with protected accessibility: 

        protected memberDefinition;

  • Where can you access a member with protected accibilitity:

    1. From inside a method in the same class (closest association)

    2. From inside a method in the same package (2nd closest association)

    3. From inside a method in a subclass defined outside the package (new!)

The protected accessibility modifier

  • Accesibility modifier diagram that we have learned so far: 

      Modifier     Accessed    Accssed       Access            Access 
  on members   from the    from the      from subclass in  from a
  in a class   same class  same package	 diff. package     diff. package
 =================================================================== 
  public           OK         OK             OK               OK
  protected
  default          OK         OK	     No		      No
  private          OK	      No	     No               No

  • Accesibility modifier diagram with the protected accessibility modifier: 

      Modifier     Accessed    Accssed       Access            Access 
  on members   from the    from the      from subclass in  from a
  in a class   same class  same package	 diff. package     diff. package
 =================================================================== 
  public           OK         OK             OK               OK
  protected        OK         OK             OK               No
  default          OK         OK	     No		      No
  private          OK	      No	     No               No

Example that show protected members can be accessed from a method in the same package 

package Geometrics;

public class myProg
{
    public static void main(String[] args)
    {
        Circle1 c = new Circle1("red", 2);
        System.out.println(c.radius);
    }
}


   /* --- No compile error --- */














 
package Geometrics;

public class Circle1 extends GeometricObject1
{
    protected double radius;

    public Circle1(String col, double r)
    {
        super(col);
        radius = r;
    }

    public double getRadius()         
    { 
        return radius;
    }

    public double getArea()
    {
        return 3.14159*radius*radius;
    }

    public String toString()
    {
        return "Color = " + getColor() + ":"
                + "radius = " + radius;
    }
}

Example that show protected members can be accessed from a subclass in a different package 


import Geometrics.*;

public class NewCircle extends Circle1
{
    NewCircle()
    {
        super("red", 1);
    }
    
    public void accessInheritedProtected()
    {
        System.out.println(radius);
    }
}

   /* --- No compile error --- */











 
package Geometrics;

public class Circle1 extends GeometricObject1
{
    protected double radius;

    public Circle1(String col, double r)
    {
        super(col);
        radius = r;
    }

    public double getRadius()         
    { 
        return radius;
    }

    public double getArea()
    {
        return 3.14159*radius*radius;
    }

    public String toString()
    {
        return "Color = " + getColor() + ":"
                + "radius = " + radius;
    }
}

Summary

  • A class can be use in 2 different ways:

      1. for creating instances of the class (with new)

      2. for defining subclasses by extending the class

  • Assign accessibility to members in a class as follows:

      • private if a member is not to be used from outside the class.

      • "default" if a member is not to be used from outside the package.

      • public if a member to be used by any user

      • protected if a member is intended for the extenders of the class but not for all users