Singleton classes

Singleton

• For any java class if we are allowed to create only one object then it is called singleton class.
• Runtime, BusinessDelegate, ServiceLoactor, etc.

Advantages

• If several people have same requirement then it is not recommended to create a separate object for every requirement.
• We have to create one object and we can reuse same object for every similar requirement so that performance and memory utilisation will be improved.
• This is the central idea of singleton classes.

How to create our own single classes

• We can create our own singleton classes, for this we have to use private constructor and private static variable and public factory method.
  • Approach 1

- Runtime class is internally implemented by using this approach.

- Approach 2 (Recommended approach)

• At any point of time for SingletonDemo class we can create only one object hence SingletonDemo class is a singleton class.

Another concept

• Suppose class is not final but we are not allowed to create child classes, how it is possible?
• Simple, declare every constructor as private.
• By using private constructors we can restrict child class creation
• Example
  • class P {
  • private P() {}
  • }
• For the above implementation of class P, it is impossible to create child class.