Singleton Pattern

Intent:

Make it impossible to create another instance of class (so, only one instance).  
And, provide a global point of access to it.

How to make a class Singleton?

1) Make the constructor private. Wait, a sec! Private constructor!!?? can we even have a constructor privates? Yes, we can.

2) Then, how do we create a object?

Allowing global access point ...
- We have a static method getInstance() that returns an instance of Singleton class.

 And, use Signleton.getInstance() not Singleton singleton = new Singleton(); singleton.getInstance();

to create an object.

3) How does getInstance finds the object?

- declare a private static member of the same type as of Singleton, private to avoid access from outside the class.
- getIntsance checks of there is already an instance and return that instance, else creates a new instance and stores it in the instance variable so that the same can be returned every-time a new instance is requested for.

That's it!! (There are some other issues in Java, details in other blog.)

Wait, But how does it look like?

Pseudo-code:

	class Signleton{
	private static Singleton singleton
	private Singleton(){
	//... do other intantiation stuff here..
	}
	public static Singleton getInstance(){
	if(singleton == null){
	this.singleton = new Singleton();
	}
	return singleton;
	}
	// other responsibities...
	..
	...
	}

view raw Singeton.java hosted with ❤ by GitHub

Examples:

• constants
• configuration
• cache 
• datasource