Beginning Java Objects: Chapter 3
Objects and Classes
Three Distinguishing Features of an
Object-Oriented Programming Language
In order to be considered truly object oriented, a programming language must provide support for three
key mechanisms:
- (Programmer creation of) Abstract Data Types (ADTs)
- Inheritance
- Polymorphism
We've just learned about the first of these — ADTs — and will discuss the other two in chapters to follow.
Summary
In this chapter, we've learned that:
- An object is a software abstraction of a physical or conceptual real-world object.
- A class serves as a template for defining objects: specifically, a class defines:
- What data the object will house, known as an object's attributes.
- What behaviors or services an object will be able to perform, known as an object's
operations or methods.
- An object may then be thought of as a filled-in template, where attribute values have been
provided.
- Just as we can declare variables to be of simple built-in data types such as
int, float, and
char, we can also declare variables to be of an abstract data type, based on the classes that we
define. Such variables are known as reference variables, or simply references.
- When we create a new object (a process known as instantiation), we store a handle on that
object in a reference variable. We can then use that handle to communicate with the object.
- We can define attributes of a class 'A' to be references to objects belonging to another class
'B'. In doing so, we allow each object to encapsulate the information that rightfully belongs to
that object, but enable objects to share information by contacting one another whenever
necessary.
Exercises
1. From the perspective of an academic setting (and not necessarily the SRS case study), think
about what the appropriate attributes and methods of the following classes might be:
ClassroomDepartmentDegree
Which of the attributes of each of these classes might be declared as built-in Java data types,
and which might be declared as abstract data types? Explain your rationale.
2. For the problem area whose requirements you defined for exercise no. 3 in Chapter 2, list the
abstract data types/classes that you might need to model.
3. List the abstract data types/classes that you might need to model for the Conference Room
Reservation System discussed in Appendix B.
4. Would 'Color' make a good abstract data type? Why or why not?
New on the Java Boutique:
New Review:
Time Management Made Easy with the Quartz Enterprise Job Scheduler
Why not just use the Java timer API? This open source scheduling
API boasts simplicity, ease-of-integration, a well-rounded feature
set, and it's free!
New Applet:
Reverse Complement
Reverse Complement is a simple applet that converts DNA or RNA
sequences into three useful formats.
Elsewhere on internet.com:
WebDeveloper Java
Lots of Java information on webdeveloper.com
WDVL Java
Thorough Java resource at the Web Developer's Virtual Library.
ScriptSearch Java
Hundreds of free Java code files to download.
jGuru: Your View of the Java Universe
Customizable portal with online training, FAQs, regular news updates, and tutorials.
