|
/*
The Design Patterns Java Companion
Copyright (C) 1998, by James W. Cooper
IBM Thomas J. Watson Research Center
*/
class Printer {
//this is a prototype for a printer-spooler class
//such that only one instance can ever exist
static boolean instance_flag = false; //true if 1 instance
public Printer() throws SingletonException {
if (instance_flag)
throw new SingletonException("Only one printer allowed");
else
instance_flag = true; //set flag for 1 instance
System.out.println("printer opened");
}
public void finalize() {
instance_flag = false;
}
}
class SingletonException extends RuntimeException {
//new exception type for singleton classes
public SingletonException() {
super();
}
public SingletonException(String s) {
super(s);
}
}
public class SinglePrinter {
static public void main(String argv[]) {
Printer pr1, pr2;
//open one printer--this should always work
System.out.println("Opening one printer");
try {
pr1 = new Printer();
} catch (SingletonException e) {
System.out.println(e.getMessage());
}
//try to open another printer --should fail
System.out.println("Opening two printers");
try {
pr2 = new Printer();
} catch (SingletonException e) {
System.out.println(e.getMessage());
}
}
}
|
