Wed Dec 26 16:00:00 GMT 2001

SingletonPattern

//: c01:SingletonPattern.java // The Singleton design pattern: you can // never instantiate more than one. // Since this isn't inherited from a Cloneable // base class and cloneability isn't added, // making it final prevents cloneability from // being added through inheritance: final class Singleton { private static Singleton s = new Singleton(47); private int i; private Singleton(int x) { i = x; } public static Singleton getReference() { return s; } public int getValue() { return i; } public void setValue(int x) { i = x; } } public class SingletonPattern { public static void main(String[] args) { Singleton s = Singleton.getReference(); System.out.println(s.getValue()); Singleton s2 = Singleton.getReference(); s2.setValue(9); System.out.println(s.getValue()); try { // Can't do this: compile-time error. // Singleton s3 = (Singleton)s2.clone(); } catch(Exception e) { e.printStackTrace(System.err); } } } ///:~


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