Object Creation in Java

This is one question I was not able to reply recently in an Interview. After the Interview I thought of a google search and decided to blog my finding..

As of now I found four different way of creating an Object in JAVA. if there is any other way kindly comment it.

A. Using new Keyword
B. Using Class.forName()
C. Using clone()
D. Using object deserialization.

A. Using new Keyword.

     This is the most common one and most of the time we end up using this.

Example:

Point originOne = new Point(23, 94);
Rectangle rectOne = new Rectangle(originOne, 100, 200);
Rectangle rectTwo = new Rectangle(50, 100);

Three Parts of the New Statement:

Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
Instantiation: The new keyword is a Java operator that creates the object.
Initialization: The new operator is followed by a call to a constructor, which initializes the new object

B. Using Class.forName()

    The java.lang.Class.forName(String className) method returns the Class object associated with the class or interface with the given string name.

Parameters : className.. This is fully qualified name of the desired class.

Return Value : This method returns the Class object for the class with the specified name.

Exception : 

LinkageError -- if the linkage fails.

ExceptionInInitializerError -- if the initialization provoked by this method fails.

ClassNotFoundException -- if the class cannot be located.

Example:

The following example shows the usage of java.lang.Class.forName() method.

package com.tutorialspoint;

import java.lang.*;

public class ClassDemo {

   public static void main(String[] args) {

     try {
        // returns the Class object for the class with the specified name
        Class cls = Class.forName("java.lang.ClassLoader");
         
        // returns the name and package of the class
        System.out.println("Class found = " + cls.getName());
        System.out.println("Package = " + cls.getPackage());
     }
     catch(ClassNotFoundException ex) {
        System.out.println(ex.toString());
     }
   }
}

Let us compile and run the above program, this will produce the following result:

Class found = java.lang.ClassLoader
Package = package java.lang, Java Platform API Specification, version 1.6

C. Using Clone()

Why use..

The clone() method saves the extra processing task for creating the exact copy of an object. If we perform it by using the new keyword, it will take a lot of processing to be performed that is why we use object cloning.

MyObject anotherObject = new MyObject(); 

MyObject object = anotherObject.clone();

Few words about object cloning :

The object cloning is a way to create exact copy of an object. For this purpose, clone() method of Object class is used to clone an object.

The java.lang.Cloneable interface must be implemented by the class whose object clone we want to create. If we don't implement Cloneable interface, clone() method generates CloneNotSupportedException.

D. Using object deserialization.

Object deserialization is nothing but creating an object from its serialized form.

ObjectInputStream inStream = new ObjectInputStream(anInputStream ); 

MyObject object = (MyObject) inStream.readObject();

Object Serialization and Deserialization: 

Object Serialization is a mechanism where an object can be represented as a sequence of bytes that includes the object's data as well as information about the object's type and the types of data stored in the object.

After a serialized object has been written into a file, it can be read from the file and deserialized that is, the type information and bytes that represent that represent the object and its data can be used to recreate the object in memory. Its JVM independent, meaning an object can be serialized on one platform and deserialized on an entirely different plateform.

The Two Stream Classes to serialize:

1. ObjectInputStream.
2. ObjectOutputStream.

Significant Method for Serialization and Deserialization :

The ObjectOutputStream class contains many write methods for writing various data types, but one method in particular stands out:

public final void writeObject(Object x) throws IOException

The above method serializes an Object and sends it to the output stream. Similarly, the ObjectInputStream class contains the following method for deserializing an object:

public final Object readObject() throws IOException, 
                                 ClassNotFoundException

This method retrieves the next Object out of the stream and deserializes it. The return value is Object, so you will need to cast it to its appropriate data type

Example :

Suppose that we have the following Employee class, which implements the Serializable interface:

public class Employee implements java.io.Serializable
{
   public String name;
   public String address;
   public transient int SSN;
   public int number;
   public void mailCheck()
   {
      System.out.println("Mailing a check to " + name
                           + " " + address);
   }
}

Notice that for a class to be serialized successfully, two conditions must be met:

The class must implement the java.io.Serializable interface.

All of the fields in the class must be serializable. If a field is not serializable, it must be marked transient.


If you are curious to know if a Java Standard Class is serializable or not, check the documentation for the class. The test is simple: If the class implements java.io.Serializable, then it is serializable; otherwise, it's not.

Serializing an Object:

The ObjectOutputStream class is used to serialize an Object. The following SerializeDemo program instantiates an Employee object and serializes it to a file.

When the program is done executing, a file named employee.ser is created. The program does not generate any output, but study the code and try to determine what the program is doing.

Note: When serializing an object to a file, the standard convention in Java is to give the file a .ser extension.

import java.io.*;

public class SerializeDemo
{
   public static void main(String [] args)
   {
      Employee e = new Employee();
      e.name = "Reyan Ali";
      e.address = "Phokka Kuan, Ambehta Peer";
      e.SSN = 11122333;
      e.number = 101;
      try
      {
         FileOutputStream fileOut =
         new FileOutputStream("/tmp/employee.ser");
         ObjectOutputStream out = new ObjectOutputStream(fileOut);
         out.writeObject(e);
         out.close();
         fileOut.close();
         System.out.printf("Serialized data is saved in /tmp/employee.ser");
      }catch(IOException i)
      {
          i.printStackTrace();
      }
   }
}

Deserializing an Object:

The following DeserializeDemo program deserializes the Employee object created in the SerializeDemo program. Study the program and try to determine its output:

import java.io.*;
public class DeserializeDemo
{
   public static void main(String [] args)
   {
      Employee e = null;
      try
      {
         FileInputStream fileIn = new FileInputStream("/tmp/employee.ser");
         ObjectInputStream in = new ObjectInputStream(fileIn);
         e = (Employee) in.readObject();
         in.close();
         fileIn.close();
      }catch(IOException i)
      {
         i.printStackTrace();
         return;
      }catch(ClassNotFoundException c)
      {
         System.out.println("Employee class not found");
         c.printStackTrace();
         return;
      }
      System.out.println("Deserialized Employee...");
      System.out.println("Name: " + e.name);
      System.out.println("Address: " + e.address);
      System.out.println("SSN: " + e.SSN);
      System.out.println("Number: " + e.number);
    }
}

Here are following important points to be noted:

The try/catch block tries to catch a ClassNotFoundException, which is declared by the readObject() method. For a JVM to be able to deserialize an object, it must be able to find the bytecode for the class. If the JVM can't find a class during the deserialization of an object, it throws a ClassNotFoundException.

Notice that the return value of readObject() is cast to an Employee reference.

The value of the SSN field was 11122333 when the object was serialized, but because the field is transient, this value was not sent to the output stream. The SSN field of the deserialized Employee object is 0.

More About Serialization and deserialization : http://www.tutorialspoint.com/java/java_serialization.htm

     Source
http://www.tutorialspoint.com/java/lang/class_forname.htm
https://javabeanz.wordpress.com/2007/09/13/different-ways-to-create-objects/
http://www.javatpoint.com/object-cloning