ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

How to implement Singleton Design Pattern in Java ?.

Updated on September 17, 2014

Introduction

Design Pattern is nothing but a solution to a common recurring problem in software design. It represent a pattern which solves a complex problem in software design. It is like, someone has already solved your problem and you have to just implement it accordingly. Singleton Design Pattern is one of such pattern which solves recurring problem in software design. Singleton Design Pattern is a pattern which limits instantiation of a of a class to one object. It helps in creating an instance of class only once. Trying to create multiple instance will be restricted and same object will be referenced.

Lets implement Singleton Design Pattern in Java through a video tutorial. Below is the video tutorial which gives walk through step by step in implementing Singleton Design Pattern. There is also complete Java source code provided below which explains it more clearly.

Video tutorial - How to implement Singleton Design Pattern in Java.

Java source code - How to implement Singleton Design Pattern in Java.

// In order to make a class singleton we have to follow two rules :
// 1. That it should have only single instance.
// 2. That this instance should be available through a global access.

// Steps to make a class singleton :

// Step 1 : Create a class which you want to make singleton. Here, we have created
// a class by name Circle which we want to make singleton.
public class Circle {

	// Step 2 : Create a private default constructor of class. Below is the Circle constructor
	// which has private access specifier. It should be made private so that no other class
	// outside Circle class can create instance of Circle.	
	private Circle() {

	}

	// Step 3 : Create instance variable of class Circle. This instance variable should be 
	// private and static. Assign it a instance of Circle class. 
	private static Circle circleInstance = new Circle();
	
	// Step 4 : Create a static accessor method which could always return back us a instance
	// of class created in step 3. 
	public static Circle getInstance() {
		return circleInstance;
	}
	
	// Step 5 : There should not be any method or constructor which can create instance of 
	// Circle class.
	
	// Step 6 : Create a integer variable say radius. Assign it value as 0.
	private int radius = 0;
	
	// Step 7 : Create getters and setters method of radius variable.
	public int getRadius() {
		return radius;
	}
	
	public void setRadius(int radius){
		this.radius = radius;
	}

}

Steps to implement -

In order to make a class singleton we have to follow two rules :
1. That it should have only single instance.
2. That this instance should be available through a global access.

Steps to make a class singleton :

Step 1 : Create a class which you want to make singleton. Here, we have created a class by name Circle which we want to make singleton.

Step 2 : Create a private default constructor of class. Below is the Circle constructor which has private access specifier. It should be made private so that no other class outside Circle class can create instance of Circle.

Step 3 : Create instance variable of class Circle. This instance variable should be private and static. Assign it a instance of Circle class.

Step 4 : Create a static accessor method which could always return back us a instance of class created in step 3.

Step 5 : There should not be any method or constructor which can create instance of Circle class.

Step 6 : Create a integer variable say radius. Assign it value as 0.

Step 7 : Create getters and setters method of radius variable.

Test class - SingletonPatternTest

// Step 1 : Create a application class to test Singleton Design Pattern. 
public class SingletonPatternTest {

	// Step 2 : Create a main method which will test Singleton nature of Circle
	// class.	
	public static void main(String[] args) {

		// Step 3 : Create instance of Circle say, circle1
		Circle circle1 = Circle.getInstance();
		
		// Step 4 : Set the radius of circle1 as 3. Initially it was 0.
		circle1.setRadius(3);
		
		// Step 5 : Print radius of circle1.
		System.out.println("Radius of circle1 : " + circle1.getRadius());
		
		// Step 6 : Create instance of Circle say, circle2
		Circle circle2 = Circle.getInstance();
		
		// Step 7 : Print radius of circle2. Instead of printing value as 0. It prints
		// value as 3. It proves no new instance was created. It prints 3 assigned
		// to circle1 instance.
		System.out.println("Radius of circle2 : " + circle2.getRadius());
	}

}

Steps to implement -

Step 1 : Create a application class to test Singleton Design Pattern.

Step 2 : Create a main method which will test Singleton nature of Circle class.

Step 3 : Create instance of Circle say, circle1.

Step 4 : Set the radius of circle1 as 3. Initially it was 0.

Step 5 : Print radius of circle1.

Step 6 : Create instance of Circle say, circle2.

Step 7 : Print radius of circle2. Instead of printing value as 0. It prints value as 3. It proves no new instance was created. It prints 3 assigned to circle1 instance.

Output of the program -

Output of the program - Explanation

In the code of test class "SingletonPatternTest" , there is a main method. This method is the starting point for execution of any Java program. Singleton Design Pattern limits creation or instantiation of a Java object to one. In the test class as soon as we create circle1 instance, that's it , no other instance can be created there after. After creation of circle1 instance, we set the radius to 3. When we create instance circle2 no new instance of class Circle is created. Here, getInstance method returns back the same instance to which circle1 is referring. The proof of Circle being a singleton class is the fact that when we call getRadius method on circle2 it prints 3. It means no new instance was created and it gets back instance which was created once in the memory.

Comments

    0 of 8192 characters used
    Post Comment

    No comments yet.

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, hubpages.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://hubpages.com/privacy-policy#gdpr

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)