Static and Dynamic Behavior

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Static and Dynamic Behavior

• Fall 2005 OOPD
• John Anthony

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Topics

• Static vs. Dynamic Typing
• Static Classes vs. Dynamic Classes
• Static vs. dynamic method Binding

Lets forget about inheritance and polymorphism
for a few moments.
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Static vs. Dynamic

• Static almost always means fixed or bound at
  compile time, and cannot thereafter be changed.
• (consider OO without polymorphism)
• Dynamic almost always means not fixed or bound
  until run time, and therefore can change during
  the course of execution
• (consider every variable as a polymorphic
  variable)

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Static vs. Dynamic Typing

• Statically Typed variables have a declared type
  which is fixed at compile time.(consider Java)
• Dynamically Typed types are associated with the
  values only. The variable is just a name.
• The variable can hold different types during the
  course of execution. (consider Smalltalk)

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Benefits and Drawbacks

• Static and Dynamically typed languages have
  existed as long as there have been programming
  languages. Arguments for and against
• Static typing allows better error detection, more
  work at compile time and hence faster execution
  time.
• Dynamic typing allows greater flexibility, easier
  to write (for example, no declaration
  statements).
• Both arguments have some validity, and hence both
  types of languages will continue to exist in the
  future.

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Remember the polymorphic variable???

• A variable (or reference) that can be attached to
  more than one type of object.

TeamMember member new TeamMember()
polymorphic variable
InnovationLabArchitect architect new
InnovationLabArchitect ()
non polymorphic variable
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Static Class vs. Dynamic Class

• In a statically typed language we say the class
  of the declaration is the static class. The
  static class is fixed at compile time and never
  changes.
• The dynamic class of a variable is the class
  associated with the value it currently holds.
  The dynamic class can change during the course of
  execution.

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Remember
Dynamic class
Static class
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Importance of Static Class

• In a statically typed object-oriented language,
  the legality of a message is determined at
  compile time, based on the static class.not the
  current dynamic receiver.

class Animal class Dog extends Animal
void bark() System.out.println("woof")
// will generate error since mammals don't
speakalthough //the value Dog could respond to
the message. Animal pet new Dog pet.bark()
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Reverse Polymorphism

• Polymorphism says we can assign a value from a
  child class to an instance of the parent class,
  but can this assignment then be reversed?
• There are two specific problems associated with
  the question of reverse polymorphism.
• The problem of identity - can I tell if a value
  declared as an instance of a parent class
  actually holds a value from a subclass.
  (instanceof in Java)
• The task of assignment - can I then assign the
  value from the parent class to a variable
  declared as the subclass. (casting)

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Static vs. Dynamic Method Binding

• Should the binding of a message be determined by
  the static class of a variable or the dynamic
  class of the value?
• Most OO languages use the static class to
  determine the legality of the message (compile
  time check).
• While the correct (method) implementation is
  determined by the dynamic class (run time).