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Polymorphism Summary

Polymorphism Concept

  • Ability of objects to take multiple forms
  • Method behavior depends on object's runtime type
  • Enables code reuse and flexibility
  • Example: 
    Shape s = new Circle(p, 1.0);  // Circle behaves as a Shape
s.getArea();  // calls Circle's getArea method

Dynamic Binding

  • Method calls resolved at runtime
  • Based on actual object type, not variable type
  • Also called late binding
  • Example: 
    void say(Object obj) {
  System.out.println(obj.toString());  // calls appropriate toString
}

Type Casting

  • Converting between supertype and subtype
  • Widening conversion (subtype to supertype):
    • Automatic
    • Always safe 
      Circle c = new Circle(p, 1.0);
Shape s = c;  // widening, no cast needed
  • Narrowing conversion (supertype to subtype):
    • Requires explicit cast
    • May cause runtime errors 
      Shape s = new Circle(p, 1.0);
Circle c = (Circle) s;  // narrowing, needs cast

instanceof Operator

  • Checks object's runtime type
  • Used before casting to prevent errors
  • Example: 
    if (obj instanceof Circle) {
  Circle c = (Circle) obj;  // safe cast
  // use c
}

Benefits of Polymorphism

  • Write more general code
  • Code works with future subclasses
  • Reduces code duplication
  • Example: 
    // Works with any Shape subclass
double totalArea(Shape[] shapes) {
  double total = 0;
  for (Shape s : shapes) {
    total += s.getArea();  // polymorphic call
  }
  return total;
}

Best Practices

  • Design for polymorphism when inheritance is used
  • Use most general type in declarations
  • Cast only when necessary
  • Always check with instanceof before casting
  • Document expected behavior of polymorphic methods