SingletonPattern.java

import java.io.*;

class BasicSingleton implements Serializable
{
  // cannot new this class, however
  // * instance can be created deliberately (reflection)
  // * instance can be created accidentally (serialization)
  private BasicSingleton() {
    System.out.println("Singleton is initializing");
  }

  private static final BasicSingleton INSTANCE = new BasicSingleton();

  private int value = 0;

  public int getValue()
  {
    return value;
  }
  public void setValue(int value)
  {
    this.value = value;
  }

  // required for correct serialization
  // readResolve is used for _replacing_ the object read from the stream

//  protected Object readResolve()
//  {
//    return INSTANCE;
//  }

  // generated getter
  public static BasicSingleton getInstance()
  {
    return INSTANCE;
  }
}

class BasicSingletonDemo
{
  static void saveToFile(BasicSingleton singleton, String filename)
    throws Exception
  {
    try (FileOutputStream fileOut = new FileOutputStream(filename);
         ObjectOutputStream out = new ObjectOutputStream(fileOut))
    {
      out.writeObject(singleton);
    }
  }

  static BasicSingleton readFromFile(String filename)
    throws Exception
  {
    try (FileInputStream fileIn = new FileInputStream(filename);
         ObjectInputStream in = new ObjectInputStream(fileIn) )
    {
      return (BasicSingleton)in.readObject();
    }
  }

  public static void main(String[] args) throws Exception
  {
    BasicSingleton singleton = BasicSingleton.getInstance();
    singleton.setValue(111);

    String filename = "singleton.bin";
    saveToFile(singleton, filename);

    singleton.setValue(222);

    BasicSingleton singleton2 = readFromFile(filename);

    System.out.println(singleton == singleton2);

    System.out.println(singleton.getValue());
    System.out.println(singleton2.getValue());
  }
}