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Visibility
in Detailed Design


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Overview
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  • The Concept
  • Types of Visibility
  • Notation
  • Rules of Thumb
  • Visibility in Java
Visibility
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  • 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
Types of Visibility
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Block Entity is (created and) accessible within a block of code
Subprogram Entity is accessible within a subprogram (e.g., local variables; parameters passed by value
Referential Visibility is temporarily provided to another entity (e.g., parameters passed by reference)
Class/File Entity is visible to all entities within a class/file (e.g., private variables in Java; static variables in C)
Types of Visibility (cont.)
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Generalized Entity is accessible to its class and all extensions (e.g., protected variables in Java)
Group Entity is accessible to all members of a pre-defined group (e.g., package visibility in Java, COMMON blocks in FORTRAN)
Complete Entity is visible everywhere (e.g., public variables in Java; global variables)
Notation
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  • UML Notation:
    • Public +
    • Private -
    • Protected #
  • Other Notation:
    • Public images/rose_public.gif
    • Private images/rose_private.gif
    • Protected images/rose_protected.gif
    • Implemented (or Package) images/rose_implemented.gif
Rules of Thumb
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  • Avoid the use of global variables
  • Instance variables (i.e., attributes) should be protected/private
  • Utility functions should be protected/private
Visibility in Java
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  • Public
    • public int length;
    • public void paint(Graphics g){ }
    • public class String{ }
    Expand
  • Private
    • private int size;
    • private void recalculate(){ }
    Expand
  • Protected
    • protected String title;
    • protected void layout(){ }
    Expand
  • Package
    • class Converters { }
    Expand
Visibility in Java (cont.)
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An Example

javaexamples/datahiding/Node.java
 
javaexamples/datahiding/Queue.java
 
javaexamples/datahiding/Example.java
 

javaexamples/datahiding/WillNotCompile.java

 

There's Always More to Learn
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