Composition Summary

Composition Concept

• Building complex classes using simpler classes
• Models "HAS-A" relationship between classes
• Example:

  class Circle {
    private Point c;   // Circle HAS-A Point
    private double r;
  }
  

Benefits of Composition

• Builds layers of abstraction
• Encapsulates implementation details
• Makes code more modular
• Allows reuse of existing classes
• Simplifies complex class implementations

Example of Composition

// Building up from Point to Circle to Cylinder
class Point {
  private double x;
  private double y;
}

class Circle {
  private Point center;
  private double radius;
}

class Cylinder {
  private Circle base;
  private double height;
}

Reference Sharing Issues

• Objects can share references to components
• Changes to shared components affect all objects
• Example of problematic sharing:

  Point p = new Point(0, 0);
  Circle c1 = new Circle(p, 1);
  Circle c2 = new Circle(p, 4);
  p.moveTo(1, 1);  // affects both c1 and c2
  

Solutions to Reference Sharing

1. Avoid Sharing References

   Point p1 = new Point(0, 0);
   Circle c1 = new Circle(p1, 1);
   Point p2 = new Point(0, 0);
   Circle c2 = new Circle(p2, 4);
   

2. Create Defensive Copies

   class Circle {
     public Circle(Point c, double r) {
       this.c = new Point(c.getX(), c.getY());
       this.r = r;
     }
   }
   

Trade-offs

• Avoiding sharing:
  • Pros: Prevents unintended modifications
  • Cons: Uses more memory, less efficient
• Sharing references:
  • Pros: Memory efficient
  • Cons: Can lead to bugs, harder to maintain

Best Practices

• Consider carefully whether to share references
• Document sharing behavior clearly
• Use defensive copying when appropriate
• Consider making objects immutable
• Design classes to prevent accidental sharing