Polymorphism:

• Ways the Language allows you to code routines with the same identifier but the actual parameters can be of different types.

• Polymorphism is a bad term to use.
• Helpful definitions:

"A function or operator is polymorphic is it has at least two possible types."¹
"A function or operator exhibits ad hoc polymorphism if it has at least two but only finitely many possible types."¹
"A function or operator exhibits universal polymorphism if it has infinitely many possible types"

Overloading

• When the same symbol translates to different code based on the (static) actual parameter types:
  
We say that the operator/function is overloaded.

• Which code is usually bound statically (before runtime)
  
• In most languages ‘+’ corresponds to different code i.e.:
3 + 4
Vs
3.6  +  4.1


• Java adds another meaning to ‘+’
“one” + “two”
• Ada and C++ allow users to overload operators.
• The interaction between overloading and coercion can be very tricky.
  
• C has no USER overloading -- operators or functions
• Java does not allow users to overload operators.
•  Overload Example from Ada

Dangerous Overloading of Operators

• C
z = &y  /* assign the address of y to x */
z = x & y /*assign the bit-wise AND of x and y */
• Fortran:
AVG = SUM/COUNT
( If SUM=20 and COUNT=30 are integers and AVG is a real variable. Then AVG will be 0.0)
• Pascal uses a different symbols for integer division and real division.  div vs ‘/’
• Ada makes the above statement illegal since the types do not match!

Inclusion Polymorphism  Also called subtype Polymorphism---

• A function or operator exhibits inclusion(subtype) polymorphism if one or more of its parameter type have subtypes.
• Ada, Pascal, Haskell, small talk, C++ and Java allow subtyping.

Object Oriented Programming Languages

• Encapsulation
  
• Inheritance
• Inclusion polymorphism
• Dynamic binding of function calls to function bodies (dynamic dispatching)

Object Oriented languages without CLASSES

• Prototype-based languages
  
• Self, JavaScript
• No constructors -- new objects are created from scratch by listing all methods
• Usually dynamically typed.
  
• Usually very small amount of encapsulation
  
• No static inheritance --  uses a dynamic  mechanism called delegation
  
• What is a Computer?
  

  Class Based - lists	Prototyped Base
  CPU keyboard Monitor	like an Apple Computer

Inclusion Polymorphism  --- Run-time dispatching

Parametric Polymorphism