• The Concept
• Types of Visibility
• Notation
• Rules of Thumb
• Visibility in Java

• The Concept:
  • The ability of one entity to "see" (i.e., have access to) another
• Use:
  • Information Hiding
  • Reduce Coupling
• Time Dependence:
  • In principle, visibility can change over time

• UML Notation:
  • Public +
  • Private -
  • Protected #
• Other Notation:
  • Public
  • Private
  • Protected
  • Implemented (or Package)

• Avoid the use of global variables
• Instance variables (i.e., attributes) should be protected/private
• Utility functions should be protected/private

• Public
• public int length;
• public void paint(Graphics g){ }
• public class String{ }
• Private
• private int size;
• private void recalculate(){ }
• Protected
• protected String title;
• protected void layout(){ }
• Package
• class Converters { }

An Example

javaexamples/datahiding/Node.java

package datahiding;


/**
 * A Node (containing a String) in a Queue
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 */
public class Node
{
    // These attributes have package visibility
    String    value;
    Node      next;
}
        

javaexamples/datahiding/Queue.java

package datahiding;


/**
 * A Queue (of String objects)
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 */
public class Queue
{
    private Node back, front;


    /**
     * Construct a new (empty) Queue
     */
    public Queue()
    {
       front = null;
    }


    /**
     * Pop a String off of the front of this Queue
     *
     * @return  The String on the front of this Queue
     */
    public String pop()
    {
       String  value;


       if (front != null) 
       {
          value = front.value;
          front = front.next;
       } 
       else
       {
          value = null;
       }
       return value;
    }


    /**
     * Push a String onto the back of this Queue
     *
     * @param s   The String to push
     */
    public void push(String s)
    {
       Node temp;

       temp = new Node();
       temp.value = s;
       temp.next = null;

       if (front == null)
       {
          front = temp;
       }
       else
       {
          back.next = temp;
       }

       back = temp;
    }
}
        

javaexamples/datahiding/Example.java

package datahiding;

/**
 * An example that uses a simple Queue
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 */
public class Example
{


    public static void main(String[] args)
    {
       Queue            q;
       String           poppedOff, userInput;


       q = new Queue();
       q.push("I");
       q.push("love");
       q.push("CS349");


       System.out.println("\n\nHere's what was on the Queue:\n");

       while ((poppedOff = q.pop()) != null)
       {
          System.out.println(poppedOff);
       }
    }

}
        

import datahiding.Node;


/**
 * An example that illustrates package visibility
 *
 * Note: This class is not part of the example2 package.
 *       As a result, Node.value is not visible to this
 *       class (since it has package visibility).
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 */
public class WillNotCompile
{
    Node   n = new Node();    
    String s = n.value;

}