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Design Patterns (aka DP), Creational - Singleton Pattern

DP is a well-described solution to a common software problem. Its benefits:
  • Already defined to solve a problem.
  • Increase code reusability and robustness.
  • Faster devlopment and new developers in team can understand it easily
DP defined in to 3 categories:
  • Creational - Used to construct objects such that they can be decoupled from their implementing system.
  • Structural - Used to form large object structures between many disparate objects
  • Behavioral - Used to manage algorithms, relationships, and responsibilities between objects.

Creational:

  • Singleton - Singleton pattern restricts the instantiation of a class and ensures that only one instance of the class exists in the jvm.

We have different approaches for Singleton but all of these follow below bullets:

  • Private constructor
  • Private static variable of same class i.e. only instance of class.
  • Public static method of class that returns the instance.
A few points to think about before implementation:
  • You  want eager initialization or lazy initialization of object.
  • Exception handling of object if creation fails.
  • Thread safety
  • Reflection can break into this pattern. So, do you want to allow it or not.
  • Serialization can destroy this pattern.
Keeping above points in mind we can implement over pattern. Below code depicts 2 such implementations and rest depends upon you to implement it differently or choose any of the one detailed below.


package com.test.command.dp.creational.singleton;

public enum EnumSingleton {

       INSTANCE;
      
       public void aboutMe(){
              System.out.println("Dinesh Sachdev (Indore)");
       }
}

package com.test.command.dp.creational.singleton;

import java.io.Serializable;

public class SerializedSingleton implements Serializable {

       /**
        *
        */
       private static final long serialVersionUID = -1L;
      
       private SerializedSingleton(){}
      
       private static class SingletonHelper{
              private static final SerializedSingleton instance =
                           new SerializedSingleton();
             
       }
      
       public static SerializedSingleton getInstance(){
              return SingletonHelper.instance;
       }

       protected Object readResolve(){
              return getInstance();
       }

}



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