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();        } }