Encapsulation Summary

Composite Data Type

• Groups primitive types together into new types
• Gives the group a name for future reference
• Abstracts away implementation details
• Examples: complex numbers, 2D points, vectors

Class and Object

• Class: Bundles composite data type with associated functions
• Components:
  • Fields (also called states, attributes, or properties)
  • Methods (functions that operate on the fields)
• Objects: Instances of a class
• Example:

  class Circle {
    private double x;
    private double y;
    private double r;
  
    double getArea() {
      return 3.141592653589793 * r * r;
    }
  }
  

Object Creation and Access

• Objects created using new keyword
• Access fields and methods using dot notation
• Example:

  Circle c = new Circle();
  c.r = 10;
  c.getArea();
  

Object-Oriented Programming Principles

• Program consists of interacting objects
• Objects contain:
  • Data (fields)
  • Operations (methods)
• Natural modeling of real-world objects
• Models:
  • Nouns as classes/objects
  • Properties/relationships as fields
  • Actions as methods

Reference Types in Java

• Classes are reference types
• Variables store references to objects, not objects themselves
• Multiple variables can reference same object
• Example of sharing:

  Circle c1 = new Circle();
  Circle c2 = c1;  // c1 and c2 share same object
  c1.r = 10;       // affects both c1 and c2
  

Special Reference Value: null

• Default value for uninitialized reference variables
• Attempting to use null reference causes NullPointerException
• Must initialize reference variables before use
• Example:

  Circle c1;           // c1 is null
  c1.r = 10;          // NullPointerException
  

Good Design Practices

• Keep related data and methods together
• Hide implementation details
• Model real-world relationships appropriately
• Initialize references before use
• Be careful with shared references