Singleton Design Pattern

Rules :

1. Constructor should be private.
2. Private Static object of self should be declared in the class.
3. This object should be initialized in a getInstance method

Example :

public class SingletonDemo {
	private static SingletonDemo obj; //1. Make a object of self private and static
	
	private SingletonDemo(){ //2. create a private constructor
	}
	
	public static SingletonDemo getInstance() { //3. create a static method that returns the instantiated method
		if(obj==null) {
			obj = new SingletonDemo();
		}
		return obj;
	}
}

What are the difference between a static class and a singleton class?

• A static class can not be a top level class and can not implement interfaces where a singleton class can.
• All members of a static class are static but for a Singleton class it is not a requirement.
• A static class get initialized when it is loaded so it can not be lazily loaded where a singleton class can be lazily loaded.
• A static class object is stored in stack whereas singlton class object is stored in heap memory space.

How to prevent cloning of a singleton object?

Throw exception within the body of clone() method