Introducing the singleton - "The Single Java Object" - The Diigo Meta page

• 21 Sep 12
  

  shriniva
  • So to prevent direct instantiation, we create a  private default constructor, so that other classes can't create a new  instance.
  • But unless we add some further code, there'll be  absolutely no way to use the class. We want to prevent direct  instantiation, but we still need to allow a way to get a reference to an  instance of the singleton object.
  • We need to make sure that threads calling the getSingletonObject()  method don't cause problems
  • This prevents two threads from calling the  getSingletonObject() method at the same time.
  • So, to be absolutely positively 100%  certain that a singleton really is a singleton, we must add a clone()  method of our own, and throw a CloneNotSupportedException  if anyone dares try!
  • public Object clone()  throws CloneNotSupportedException  {  throw new CloneNotSupportedException();   // that'll teach 'em  }

  4 more annotations...

• 17 Sep 12
  

  Max Lee

  singleton

• 21 May 12
  

  Sohel Barua

  java singleton

• 05 Nov 11
  

  Hossein Kholghi

  java patterns tutorials

• 30 Nov 10
  

  Bhaskar Ghosh

  java singleton programming tutorial

• 29 Nov 10
  

  Rinav G

  A singleton is an object that cannot be instantiated. At first, that might seem counterintuitive - after all, we need an instance of an object before we can use it. Well yes a singleton can be created, but it can't be instantiated by developers - meaning that the singleton class has control over how it is created. The restriction on the singleton is that there can be only one instance of a singleton created by the Java Virtual Machine (JVM) - by prevent direct instantiation we can ensure that developers don't create a second copy.

  java singleton DesignPatterns

  • a design pattern is a template for  software development.
  • The purpose of the template is to define a particular behavior or technique that  can be used as a building block for the construction of software - to  solve universal problems that commonly face developers.
  • you're probably wondering what a singleton is
  • A  singleton is an object that cannot be instantiated. At first, that might  seem counterintuitive - after all, we need an instance of an object  before we can use it.
  • but it can't  be instantiated by developers
  • The restriction on the singleton is that  there can be only one instance of a singleton created by the Java  Virtual Machine (JVM) - by prevent direct instantiation we can ensure  that developers don't create a second copy.
  • So why would this be useful?
  • prevent others (ourselves included) from making copies  of it or creating new instances.
  • For example, a central configuration  object that stores setup information should have one and one only  instance - a global copy accessible from any part of the application,  including any threads that are running.
  • we provide a default constructor that is marked as private.
  • private SingletonObject()
  • So far so good. But unless we add some further code, there'll be  absolutely no way to use the class. We want to prevent direct  instantiation, but we still need to allow a way to get a reference to an  instance of the singleton object.
  • We need to provide an accessor method, that returns an instance of  the SingletonObject class but doesn't allow more than one copy to be  accessed.
  • SingletonObject
  • static
  • if (ref == null)  // it's ok, we can call this constructor  ref = new SingletonObject();  return ref;
  • static SingletonObject ref;
  • Preventing thread problems with your singleton
  • synchronized
  • they forget about cloning.
  • SingletonObject clone =   (SingletonObject) obj.clone();
  • clone()  method
  • java.lang.Object
  • By default, the clone() method  is marked as protected,
  • we must add a clone()  method of our own, and throw a CloneNotSupportedException  if anyone dares try!
  • public Object clone()  throws CloneNotSupportedException  {  throw new CloneNotSupportedException();   // that'll teach 'em  }

  24 more annotations...

• 18 Feb 10
  

  tsmaco

  public static SingletonObject getSingletonObject() { if (ref == null) // it's ok, we can call this constructor ref = new SingletonObject(); return ref; } private static SingletonObject ref;

  java singleton clone

• 06 Aug 09
  

  pldurrer

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  morningwinter

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  Java Tutorial Singleton OOP

• 18 Jun 07
  

  Franco Fernandes

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