SingleTon Pattern, how can we restrict in serialization to create multiple objects

SingleTon Pattern :

Questions will be anserwed in this post
What is Singleton
Why do we need SingleTon
How can a class behaves singleTon
How can we fail singleton concept through serialization, how can we solve it..

For a given class, if only one instance needs to be created, we will go for this pattern.
why is SingleTon needed :  there are some requirements, where only one class needs to be created, for example, database connection object,
for an application, only one database is needed, for this kind of situations, we go for singleTon.

 If we restrict constructor as private , no one can create object,
 instead provide one method to give instance, this method will create a object if it didnt exists, else creates object and returns it.
 public class SingleObject {

   //create an object of SingleObject
   private static SingleObject instance = new SingleObject();

   //make the constructor private so that this class cannot be
   //instantiated
   private SingleObject(){}

   //Get the only object available
   public static SingleObject getInstance(){
      return instance;
   }

   public void showMessage(){
      System.out.println("Hello World!");
   }
}





Through Serialization, lets say we need to serialize singleTon class, and deserialize n number of times , you can get n number of objects, which is failing singleTon principle, How can we restrict this

In order to make serialization and singleton work properly,we have to introduce readResolve() method in the singleton class.readResolve() method lets developer control what object should be returned  on deserialization.
For the current SingleObject singleton class,readResolve() method will look like this.


     /**
  * Special hook provided by serialization where developer can control what object needs to sent.
  * However this method is invoked on the new object instance created by de serialization process.
  * @return
  * @throws ObjectStreamException
  */
 private Object readResolve() throws ObjectStreamException{
  return INSTANCE;
 }
You need to implement readResolve method in your Singleton class. This small thing is used to override what serialization mechanism has created. What you return there will be used instead of data that came from serialization
 but readResolve will work properly only if all fields are transient,


The best way to do this is to use the enum singleton pattern:

public enum SingleObject {
  instance;
}


more details will be posted further

Different ways of creating an Object in Java

Here are the 5 different ways to create object in Java,
1. new Operator
2. Using Class.forName("class").newInstance()
3.Using de serialization  (by using ObjectInputstream readObject method)
4.using clone method
5.using newInstance method.

package com.test.objectcreation;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.ObjectInputStream;

public class Add implements Cloneable {
    public int number = 1;

    public int add(int a, int b) {
        return a + b;
    }

    public int change() {
        number = 2;
        return number;
    }

    public static void main(String args[]) throws InstantiationException, IllegalAccessException,
            CloneNotSupportedException, ClassNotFoundException, IOException {
        Add add = new Add(); // creation of object with new operator
        System.out.println(add.add(2, 3));

        Add a = Add.class.newInstance();// creation of object with newInstance
                                        // method
        // System.out.println(a.getName());
        // System.out.println(a.getMethod("add", "2","3"));
        System.out.println(a.add(2, 3));
        a.change();
        Add b = (Add) a.clone(); // creation of object using clone, Class needs to implement clonenable, otherwise you  will get clonenotsupportedException and you need to typecast to class to which you need to clone.
        System.out.println(b.add(2, 3));
        System.out.println(b.toString());
        System.out.println("static int in object a :" + a.number);
        System.out.println(a.toString());
        System.out.println("static int in object b (cloned object) :" + b.number);
        FileInputStream fileIn = new FileInputStream("/E/Add.txt");
        ObjectInputStream in = new ObjectInputStream(fileIn);// here reading serializable object which stored in add.txt file, we are creating Add  classinstance,used mostly for deserialization.

        Add e = (Add) in.readObject();// reading object and type casting.
        in.close();
        fileIn.close();

        Class s = Class.forName("com.test.objectcreation.Add");// using forName
        Add obj = (Add) s.newInstance(); // we are creating object from Object s
        System.out.println(obj.hashCode());
    }

}

Difference between Statement and Prepared Statement

Statement:

When you use normal statements compilation of statement or
parsing of statement will happen every time. This is time
consuming when u have multiple queries to execute.

Statement s = conn.createStatement();
s.executeQuery();

Prepared Statement:
In prepared statement SQL query will be compiled or parsed
for the very first time and kept in some kind of pool. When
u execute one more time the same statement with different
values it saves the parsing time.

The PreparedStatement is a slightly more powerful version of a Statement, and should always be at least as quick and easy to handle as a Statement.

The PreparedStatement may be parametrized.
Prepared statement is precompiled statement it is used when
we want one query to be executed n no. of times.
Prepared Statement is Pre-compiled class , but Statement is not.
So in PreparedStatement the execution will be faster.

SQL select * from employee where employeeID=?,empName=?;
PreparedStatement ps = conn.PreparedStatement();
ps.execute(1,aru);
ps.execute(2,arya);

Callable Statement:

Callable statements are used to execute stored procedures
similar to java functions.

Object Class functions in java

Object Class
Object Class is the Super class for all classes in JAVA.The Object class defines the basic state and behavior that all objects must have, such as the ability to compare oneself to another object, to convert to a string, to wait on a condition variable, to notify other objects that a condition variable has changed, and to return the object's class.
The Object Class Functions:
equals:
This function is used to compare two objects whether they have equivalent values are not.
It returns true if they are equal else returns false.
The getClass() Method:
The getClass method is a final method (cannot be overridden)
This method returns a Class object.With this class you can get the information of object like name, its super class, and the names of the interfaces that it implements.
use of the getClass method is to create a new instance of a class without knowing what the class is at compile time. This sample method creates a new instance of the same class as obj which can be any class that inherits from Object (which means that it could be any class):
Object createNewInstanceOf(Object obj) {
return obj.getClass().newInstance();
}
The toString Method
Object's toString method returns a String representation of the object. You can use toString to display an object.
The Object class also provides five methods that are critical when writing multithreaded Java programs:
• notify
• notifyAll
• wait (three versions)
hashCode()
Returns a hash code value for the object.
finalize()
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.

System.out.println

Some people dont know what is System,out,println in System.out.println.
As per my Knowledge,

System is a class in java which is final class(Final will stop inheritance)

out is a static final PrintStream object that is in System Class
And println is a function in PrintStream class
Since out is a static object with out creating object we can access
directly
As
System.out.println();

Overloading And Overriding Concepts

Method Overloading:
Method overloading in Java is nothing but two or more functions having same function name but with different parameters and signatures.

This can be implemented by compile time polymorphism


Method Overriding:


Method overriding in java is nothing but the function present in super class is overrides the function in sub class in other way add additional functionality to the function in super class
One thing to be noted down is
the function names in super class and sub class are same with signatures also..
Method overriding can be implemented by runtime polymorphism.