Java Reference
In-Depth Information
public
public class
class
EnumList
EnumList
{
enum
enum
State
{
ON
,
OFF
,
UNKNOWN
}
public
public static
static
void
void
main
(
String
[]
args
) {
for
for
(
State i
:
State
.
values
()) {
System
.
out
.
println
(
i
);
}
}
}
The output of the
EnumList
program is, of course:
ON
OFF
UNKNOWN
Enforcing the Singleton Pattern
Problem
You want to be sure there is only one instance of your class in a given Java Virtual Machine.
Solution
Make your class enforce the Singleton Pattern (see page 127 of the topic
Design Patterns
),
primarily by having only a private constructor(s).
Discussion
It is often useful to ensure that only one instance of a class gets created, usually to funnel all
requests for some resource through a single point. An example of a Singleton from the stand-
ard API is
java.lang.Runtime
; you cannot create instances of
Runtime
, you simply ask for
a reference by calling the static method
Runtime.getRuntime()
. Singleton is also an ex-
ample of a design pattern that can be easily implemented.
The easiest implementation consists of a private constructor and a field to hold its result, and
a static accessor method with a name like
getInstance()
.
