Information Hiding Summary

Breaking Abstraction Barrier

• Direct access to internal fields breaks abstraction
• Example problem:

  Circle c = new Circle();
  c.r = 10;  // directly modifying radius
  

• Makes code dependent on implementation details
• Changes to implementation can break client code
• Reduces code maintainability
• As an implementer, expose as few fields/methods as possible
• As a client, follow the behavior as stated in the specification such as Java API

Access Modifiers

• Java provides access control through modifiers:
  • private: Only accessible within the class
  • public: Accessible from anywhere
• Default access (no modifier) exists but not covered; it is package-private
• Access control enforced by compiler at compile time
• Different objects of the same class can access each other's private fields and methods within the class
• Example:

  class Circle {
    private double x;  // hidden from outside
    private double y;
    private double r;
  
    public double getArea() {  // accessible from outside
      return 3.141592653589793 * r * r;
    }
  }
  
  // Testing
  Circle c = new Circle();
  c.r = 10; // compilation error
  

Access Modifier Summary Table

Access From	private	public
Inside class	Yes	Yes
Outside class	No	Yes

Constructors

• Behavior of a constructor:
  • Allocate memory for all fields and assign the reference to this
  • Invoke the constructor function, passing the keyword this implicitly
  • Once the constructor is done, return the reference pointed to by this
• Special methods for clients to initialize objects
• Same name as class
• No return type
• Called automatically with new
• Example:

  class Circle {
    private double x;
    private double y;
    private double r;
  
    public Circle(double x, double y, double r) {
      this.x = x;
      this.y = y;
      this.r = r;
    }
  }
  

Default Constructor

• Provided by Java if no constructor is defined
• Takes no parameters
• Empty body
• Example:

  Circle() {
  } // no parameters and no code written for the body
  

• Not provided if any constructor is defined

The this Keyword

• Reference to current object
• Used to:
  • Distinguish between parameters and fields
  • Access object's own members
• Example:

  this.x = x;  // field x = parameter x
  

• Makes code more explicit and readable
• Helps avoid naming conflicts