Occasional Lecture Notes
last change: Friday, Sept. 10, 2004
Notes
from Thursday, Sept. 9, 2004
Application Programs vs. Software Components
(reusable classes)
- The user of an application program is generally an
end user, a person who's using the program to solve their problem, and
whose main concern is that the program
- does what it's supposed to do, and
- behaves predictably no matter what the input.
- The user of a reusable class is not an end user but
another programmer.
- We document application programs with user manuals;
we document classes with a list of methods, including preconditions and
postconditions for each method.
- cs1.Keyboard and java.lang.Math are software
components.
Writing Your Own ADT Classes
- Sets of reusable classes are the basis for all modern
software development, regardless of language or operating system.
- An Abstract Data Type (ADT)
class provides the necessary support for a data structure: the ability
to instantiate objects of the class, and a set of methods to manipulate
the objects.
- An object has state (a set of data values, sometimes
called fields or attributes) and behavior (a set of methods). The
methods act on the data values, which changes the state.
- The data values are usually private, so that a
calling program can't access them directly but must call methods.
- The methods are usually public, so a calling program
can use them. Sometimes there are also private methods; why would we
want these?
Characteristics of Data Values and Methods
- Public methods and data values are visible and
available to any program that imports the class; private methods and
data values are "invisible" outside the class itself and can be
directly referred to only within the class.
- A static method or data value is associated with the
class; a non-static one is associated with each object. That is, each
object has its own "copy" of the method or data value.
- A value-returning method is one that returns a value
to the calling program; a void method is one that does not return a
value. At this time we'll consider both value-returning and void
methods.
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