CSci 581: Obj.-Oriented Design & Programming
Spring Semester 1999
Lecture Notes

A Second Look at Classes

This Web document was based on a set of "slides" created by the instructor for use in the Fall 1997 offering of the object-oriented design and programming course. That set was, in turn, based on a set of slides created by Timothy Budd to supplement chapter 20 of his textbook An Introduction to Object-Oriented Programming, Second Edition (Addison-Wesley, 1997).

What is a Class?

• A "cookie cutter" used for making clones of some object

• A generalization of a record

• A mechanism for variable scoping and encapsulation

• A special kind of type

• A special kind of object

Classes as Types

What is a type?

application programmer's view:

set of values with common attributes and operations

object-oriented view:

behavior specifications

compiler view:

set of syntactic constraints on expressions (uses)

semantics/verification view:

set of invariants that instances of the type must satisfy

systems programming view:

protective mechanism wrapped around the bits

implementation view:

description of how the bits are stored in memory

Implications of Classes for Types

The is-a relation produces some subtle issues

• Programmer analyzing program may not know final type of object (may be subclass)

• Inheritance only ensures that interface maintained, not other properties of execution

• Subclasses not guaranteed to preserve all aspects of parent (may take longer, perform actions in different sequence, etc).

• Testing of program, even proof of correctness, may be invalidated by replacement of methods

Classes as Objects

If a class is an object it must have responsibilities.

What are these responsibilities?

• Maintain information about the class (instance size, methods)

• Generate new copies of the class

• But, if a class is an object and every object is an instance of some class, what is the class of a class?

Classes as Objects in Java

• Root class of the inheritance hierarchy is class Object

• There is a "class object" associated with each class

• Parent class of all class objects is Class

• One instance of class Class for each class loaded onto the machine

• Method Object.getClass returns the class for the object -- instance of type Class

• If X is a class, them X.class is constant that refers to its Class object

• Class Class has several useful methods:

  • getName returns the name of the class as a string

  • forName loads the class with the fully qualified (including package) class name string and returns the class

  • newInstance creates a new instance of the class (applies the no-argument constructor)

  • getSuperclass returns the superclass of the class

  • other methods to return the interfaces, constructors, methods, fields, etc.

• Reflection API (java.lang.reflect package) provides additional capabilities

• Java Beans API (java.beans package) gives additional support for reusable components

Class Data

• Known as a class variable

  • static variable in Java

• Important problem: who initializes this value?

  • avoid failing to initialize

  • avoid multiple initialization

• Mechanism differs among languages

  • Java uses standard variable initializers for simple initialization

  • Java also uses static blocks for more complex initialization

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