- Visibility modifiers designate the accessibility of data members and methods
- Not syntactically required, but strongly recommended to use them
- Recommend always designate the data members and methods as public or private
- Object-oriented design philosophy
  - Instantiable class design: designing behavior of instances
  - Methods that other programmers using your class should be able to call are declared public
    - Client programmers: programmers using your classes
    - Client program: a program that uses your classes
  - Declare methods private if they involve internal details that should be hidden from client programmers
  - Information hiding: making internal details invisible when knowing how they work is unnecessary for properly using the class
  - Encapsulate: hide internal details, but use them to offer an interface for the classes' functionality
  - Encapsulation mechanism
    - Allows easier code modification
    - Changes to the internal workings don't affect client programs
  - Need data members to implement methods
    - Considered internal details of class because client programmers don't need to know about them to use the class
    - Declare data members private
  - Behavior of instances determined by public methods
  - Internal details hidden from client programmers implemented with private data members and private methods
- If declare an internal detail public, back door created that can break the functionality of the class
- Must declare data members private to ensure class integrity
  - Works because client programmers can only alter the data via public methods
- Class constants can sometimes be declared public in good style
  - Constant, so everyone already prohibited from modifying
  - Clean way to make certain characteristics of the instances known to the client programmers
  - Syntax for accessing public class data members
    <class name>.<class data member>
  - Very common in Java
- Distinguish private and public components in the class diagram
  - Plus symbol for public
  - Minus symbol for private

Quick Check
1.  No: client programs can't access private class data.
2.  It should be declared private, because other people don't need to know about it to interact with me!