Java Reference
In-Depth Information
Body
. The code must increment
nextID
so that the next
Body
object cre-
ated will get a unique identifier.
We then make an
earth
object in a similar fashion. This example builds a
solar system model. In this model, the sun is in the center, and
sun
's
or-
bits
field is
null
because it doesn't orbit anything. When we create and
initialize
earth
, we set its
orbits
field to
sun
. A
moon
object would have its
orbits
field set to
earth
. In a model of the galaxy, the
sun
would orbit
around the black hole presumed to be at the middle of the Milky Way.
You create objects using
new
, but you never delete them explicitly. The
virtual machine manages memory for you using
garbage collection,
which means that objects that you cannot possibly use any longer can
have their space automatically reclaimed by the virtual machine without
your intervention. If you no longer need an object you should cease re-
ferring to it. With local variables in methods this can be as simple as
returning from the methodwhen the method is no longer executing none
of its variables can possibly be used. More durable references, such as
fields of objects, can be set to
null
. Memory management is discussed
in more detail in
Chapter 17
.
Exercise 2.5
: Write a
main
method for your
Vehicle
class that creates a
few vehicles and prints their field values.
Exercise 2.6
: Write a
main
method for your
LinkedList
class that creates
a few objects of type
Vehicle
and places them into successive nodes in
the list.