Java Reference
In-Depth Information
Now programmers who want to use the body's identifier will invoke the
getID
method, which returns the value. There is no longer any way for
programmers to modify the identifierit has effectively become a read-
only value outside the class. It can be modified only by the internal
methods of the
Body
class.
Methods that regulate access to internal data are sometimes called
ac-
cessor methods.
Their use promotes encapsulation of the class's data.
Even if an application doesn't require fields to be read-only, making
fields private and adding methods to set and fetch them enables you to
add actions that may be needed in the future. If programmers can ac-
cess a class's fields directly, you have no control over the values they
will use or what happens when values are changed. Additionally, making
a field part of the contract of a class locks in the implementation of that
classyou can't change the implementation without forcing all clients to
be recompiled. For example, a future version of
Body
may want to look-
up the ID number in a database indexed by the body's name and not
actually store the ID number in the object at all. Such a change cannot
be made if
idNum
is accessible to clients. For these reasons, you will see
very few
public
or
protected
fields in subsequent examples in this topic.
Methods to get or set a value in an object's state are sometimes said to
define a
property
of that object. For example, the
Body
class's
getID
can
be said to define an
ID
property for
Body
objects that is retrieved by the
getID
method, and implemented by the
idNum
field. Some automatic sys-
tems, including those for the JavaBeans
™
component architecture, use
these conventions to provide automatic property manipulation systems;
orbits
fields to be properties by making them private and providing set
and get methods for them:
class Body {
private long idNum;
private String name = "<unnamed>";
private Body orbits = null;
private static long nextID = 0;
