singleton vs static

[Harikrishna Gorrepati]

Hi, I have seen several postings in this forum but I did not get clarity on the differences between singleton and static method behavior in a class. I feel that whatever we can achieve using static method we can achieve with Singleton. If you can provide sample code, It would be really great. Let us make this thread as One Stop Shop for Singleton.

[Steve Fahlbusch]

Well to start off with, please provide your understanding of what static is/means and that singleton is/means and we can use that as a starting point.

[Manish Doomra]

My understanding for Singleton is:-
We are restricted to create only single instance of the class.

The following code depicts the framework of Singleton class:-

public class SingletonClass{

	// private static instance variable to hold the singleton instance of the class
	private static SingletonClass _singletonClass=null;
	
	// private constructor to ensure the object can't be created from anywhere outside the class
	private SingletonClass(){}
	
	public static SingletonClass getInstance(){
	
		if(_singletonClass==null){
			_singletonClass= new SingletonClass();
		}
		
		return _singletonClass;
	}
	
	void method1(){System.out.println("method1");}
	
	void method2(){System.out.println("method2");}
	
	
	public static void main(String args[]){
		
		SingletonClass singletonClass = SingletonClass.getInstance();
		// accessing method with the help of instance variable instead of Class variable
		singletonClass.method1();
		singletonClass.method2();
		
	}

}

while static member has a class level access, whereas in case of singleton the instance is merely static and restricted to get created only once.

Following code depicts the usage of static:-

public class StaticClass{

	static void method1(){System.out.println("method1");}
	
	static void method2(){System.out.println("method2");}
	
	
	public static void main(String args[]){
		
		// accessing method with the help of Class variable instead of instance variable
		StaticClass.method1();
		StaticClass.method2();
		
	}

}

Both produce the same output, but the method calling is different in both the cases.

Steve Fahlbusch wrote:Well to start off with, please provide your understanding of what static is/means and that singleton is/means and we can use that as a starting point.

[ujjawal rohra]

Well one difference is that an outer class can be singleton but an outer class can not be static.
That is, you can not make a singleton class by using static..

[Mohamed Sanaulla]

Harikrishna Gorrepati wrote:Hi, I have seen several postings in this forum but I did not get clarity on the differences between singleton and static method behavior in a class. I feel that whatever we can achieve using static method we can achieve with Singleton. If you can provide sample code, It would be really great. Let us make this thread as One Stop Shop for Singleton.

Static methods are not part of the instance. But singleton is an object. The restriction there is that only one object of a particular type can exist. I dont see why you are trying to compare Static methods with Singleton- which boils down to comparing methods and objects?

[Harikrishna Gorrepati]

Let me change the question, In what case(s), I should go for Singleton class instead of static class(I mean, static methods in a class) and in what cases I can for static classes instead of Singleton.

[Campbell Ritchie]

The two are completely different. You can find discussion of static in "beginning Java™", and a singleton is a design pattern whereby a certain number of objects can be created from a class. That "certain number" is usually 1; in fact the word "singleton" implies 1, but there are analogous patterns which allow exactly 2 instances for example.

You should not go on about "singleton or static". That is a bit like going on about "bicycles or cheese" A question without an answer.

[Campbell Ritchie]

I see Mohamed Sanaulla has also alluded to your confusion between "singleton or static".