Overloading, Overriding, Runtime Types and Object Orientation - Encapsulation

• objects have both state (details about itself) and behaviour (what it can do)
• a software object maintains information about its state in variables
• what an object can do, its behaviour, is implemented with methods

• in Object-oriented programming (OOPs), an object is encapsulated when its variables and methods are combined into a single component
• encapsulation also involves access to an object; its interface
• a tightly encapsulated object hides all it's variables and provides public accessor methods ie the only way you can use the object is by invoking it's public methods

• encapsulation has two main benefits: (VA pg44)
  1. modularity
  2. maintainablity

Modularity

• because the object encapsulates all it's variables and the methods needed to make it work, it is a self-contained entity that can be maintained independently of other objects

Maintainability

• because the object hides it's implementation details behind a well-defined interface, the details can be changed without affecting other parts of the program

Example

class TestBook{
  public static void main(String[] args) {
    Book b1 = new Book();
    System.out.println(b1);
        
//  b1.title = "Java Programming Language";     
//  b1.author = "Ken Arnold and James Gosling"; 

   // must use accessor methods
   b1.setTitle("The Java Programming Language: Second Edition");
   b1.setAuthor("Ken Arnold and James Gosling");

// System.out.println(b1.title, b1.author);    
        
   System.out.println(" Title: " + b1.getTitle() );
   System.out.println("Author: " + b1.getAuthor() );        
    }
}

• In the example code, the instance variables title and author are private; they can only be accessed by their gettor and settor methods
• any attempt to directly set or get the variables produces a compile error

Example Code

• TestBook.java