// References https://www.digitalocean.com/community/tutorials/java-singleton-design-pattern-best-practices-examples

//https://www.geeksforgeeks.org/java-singleton-design-pattern-practices-examples/

import java.io.*;

import java.lang.reflect.Constructor;

import java.lang.reflect.InvocationTargetException;

//Before jumping into pattern just explain what is lazy loading and eager loading

// Mainly this class used to show violations using serializable and reflection.

public class Main {

public static void main(String[] args) throws IOException, ClassNotFoundException, InvocationTargetException, InstantiationException, IllegalAccessException {

//exampleSerialization();

exampleReflection();

}

private static void exampleSerialization() throws IOException, ClassNotFoundException {

LazySingleton lazySingleton = LazySingleton.getInstance();

ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream("object.obj"));

objectOutputStream.writeObject(lazySingleton);

objectOutputStream.close();

ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("object.obj"));

LazySingleton deserializedLazy = (LazySingleton) objectInputStream.readObject();

objectInputStream.close();

System.out.println("Object 1 :" + lazySingleton.hashCode());

System.out.println("Object 2 :" + deserializedLazy.hashCode());

SerializableSingleton serializableSingleton = SerializableSingleton.getInstance();

ObjectOutputStream objectOutputStream2 = new ObjectOutputStream(new FileOutputStream("object1.obj"));

objectOutputStream2.writeObject(serializableSingleton);

objectOutputStream2.close();

ObjectInputStream objectInputStream2 = new ObjectInputStream(new FileInputStream("object1.obj"));

SerializableSingleton deserializedInstance = (SerializableSingleton) objectInputStream2.readObject();

objectInputStream2.close();

System.out.println("SerializableSingleton Object 1 :" + serializableSingleton.hashCode());

System.out.println(" SerializableSingleton Object 2 :" + deserializedInstance.hashCode());

}

private static void exampleReflection() throws InvocationTargetException, InstantiationException, IllegalAccessException {

Constructor[] constructors = LazySingleton.class.getDeclaredConstructors();

//Knowing only one constructor taking it using index

Constructor constructor = constructors[0];

constructor.setAccessible(true);

LazySingleton lazySingleton = (LazySingleton) constructor.newInstance();

LazySingleton instance = LazySingleton.getInstance();

System.out.println("Reflected hashcode singleton :"+lazySingleton.hashCode());

System.out.println("Singleton instance : "+ instance.hashCode());

//Solution to this is go by enum

EnumSingleton.INSTANCE.doSomething();

}

}